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New paper: Environments of evolved massive stars – evidence for episodic mass ejections

A proceedings paper from IAUS 366 that took place virtually back in October 2021 (for which I had another poster contribution) was finally published at the end of 2022. It summarizes a collective work led by Michaela on B[e] Supergiants and Yellow Hypergiants, two massive star phases where we observe episodic mass loss.

Environments of evolved massive stars: evidence for episodic mass ejections

M. Kraus, L. S. Cidale, M. L. Arias, A. F. Torres, I. Kolka, G. Maravelias, D. H. Nickeler, W. Glatzel and T. Liimets

The post-main sequence evolutionary path of massive stars comprises various transition phases, in which the stars shed large amounts of material into their environments. Our studies focus on two of them: B[e] supergiants and yellow hypergiants, for which we investigate the structure and dynamics within their environments. We find that each B[e] supergiant is surrounded by a unique set of rings or arc-like structures. These structures are either stable over time or they display high variability, including expansion and dilution. In contrast, yellow hypergiants are embedded in multiple shells of gas and dust. These objects are famous for their outburst activity. Moreover, the dynamics in their extended atmospheres imply an enhanced pulsation activity prior to outburst. The physical mechanism(s) leading to episodic mass ejections in these two types of stars is still uncertain. We propose that strange-mode instabilities, excited in the inflated envelopes of these objects, play a significant role.

Figure 1. Real parts (= pulsation periods, left panel) and the imaginary parts (right panel) of
the eigenfrequencies, which are normalized to the global free-fall time. Positive imaginary parts
correspond to damped modes, and negative ones to unstable modes. The computations have
been performed for T eff = 7000 K and log L/L  = 5.7, matching the observed values of ρ Cas.

IAUS 366, 2022 (NASA/ADS link)

New paper: Using machine learning to investigate the populations of dusty evolved stars in various metallicities

This is actually a preview of what will follow after the first paper of the machine-learning classifier. We put it into action to get predictions for a number of galaxies and we start exploring the results. Of more interest is the fractions of the populations with metallicity, although a more detailed study is needed to take care of all caveats.

Using machine learning to investigate the populations of dusty evolved stars in various metallicities

Grigoris Maravelias, Alceste Z. Bonanos, Frank Tramper, Stephan de Wit, Ming Yang, Paolo Bonfini, Emmanuel Zapartas, Konstantinos Antoniadis, Evangelia Christodoulou, Gonzalo Muñoz-Sanchez

Mass loss is a key property to understand stellar evolution and in particular for low-metallicity environments. Our knowledge has improved dramatically over the last decades both for single and binary evolutionary models. However, episodic mass loss although definitely present observationally, is not included in the models, while its role is currently undetermined. A major hindrance is the lack of large enough samples of classified stars. We attempted to address this by applying an ensemble machine-learning approach using color indices (from IR/Spitzer and optical/Pan-STARRS photometry) as features and combining the probabilities from three different algorithms. We trained on M31 and M33 sources with known spectral classification, which we grouped into Blue/Yellow/Red/B[e] Supergiants, Luminous Blue Variables, classical Wolf-Rayet and background galaxies/AGNs. We then applied the classifier to about one million Spitzer point sources from 25 nearby galaxies, spanning a range of metallicites (1/15 to ∼3 Z⊙). Equipped with spectral classifications we investigated the occurrence of these populations with metallicity.

The fractions, of the predicted class members over the total sample size for each galaxy, with metallicity.

arXiv: 2209.06303

EAS 2021 poster contributions

Three poster contributions during EAS 2021 with the following … statistics: all of them on massive stars,  two within the framework of the ASSESS project, and two on machine-learning applications.

1. Applying machine-learning methods to build a photometric classifier for massive stars in nearby galaxies

Grigoris Maravelias, Alceste Bonanos, Frank Tramper, Stephan de Wit, Ming Yang, Paolo Bonfini

Mass loss is a key parameter in the evolution of massive stars. Despite the recent progress in the theoretical understanding of how stars lose mass, discrepancies between theory and observations still hold. Even worse, episodic mass loss in evolved massive stars is not included in the models while the importance of its role in the evolution os massive stars is currently undetermined. A major hindrance to determining the role of episodic mass loss is the lack of large samples of classified stars. Given the recent availability of extensive photometric catalogs from various surveys spanning a range of metallicity environments, we aim to remedy the situation by applying machine learning techniques to these catalogs.We compiled a large catalog of known massive stars in M31 and M33, using IR (Spitzer) and optical (Pan-STARRS) photometry, as well as Gaia astrometric information. We grouped them in 7 classes (Blue, Red, Yellow, B[e] supergiants, Luminous Blue Variables, Wolf-Rayet, and outliers, e.g. QSO’s and background galaxies). Using this catalog as a training set, we built an ensemble classifier utilizing color indices as features. The probabilities from three machine-learning algorithms (Support Vector Classification, Random Forests, Neural Networks) are combined to obtain the final classifications. The overall performance of the classifier is ~87%. Highly populated (Red/Blue/Yellow Supergiants) and well-defined classes (B[e] Supergiants) have a high recovery rate between ~98-74%. On the contrary, Wolf-Rayet sources are detected at ~20% while Luminous Blue Variables are almost non-existent. The is mainly due to the small sample sizes of these classes, although M31 and M33 have spectral classifications for several massive stars (about 2500). In addition, the mixing of spectral types, as there are no strict boundaries in the features space (color indexes) between those classes, complicates the classification. In an independent application of the classifier to other galaxies (IC 1613, WLM, Sextans A) we obtained an overall accuracy of ~71% despite the missing values on their features (which we replace with averaged values from the training sample). This approach results only in a few percent difference, with the remaining discrepancy attributed to the different metallicity environments of their host galaxies. The classifier’s prediction capability is only limited by the available number of sources per class, reflecting the rarity of these objects and the possible physical links between these massive star phases. Our methodology is also efficient in correctly classifying sources with missing data and at lower metallicities, making it an excellent tool for spotting interesting objects and prioritizing targets for observations. Future spectroscopic observations will offer a test-bed of its actual performance along with opportunities for improvement.

For more see this k-poster (submitted for SS32: Machine Learning and Visualisation in Data Intensive Era ).

2. A new automated tool for the spectral classification of OB stars

E. Kyritsis, G. Maravelias, A. Zezas, P. Bonfini, K. Kovlakas, P. Reig

As more and more large spectroscopic surveys become available, an automated approach in spectral classification becomes necessary. Due to the importance of the massive stars it is of paramount importance to identify the phenomenological parameters of these stars (e.g., the spectral type ) which can be used as proxies to their physical parameters (e.g mass, temperature).
In this work, we use the Random Forest (RF) algorithm to develop a tool for automated spectral classification of the OB-type stars into their sub-types. We use the regular RF algorithm, the Probabilistic RF (PRF) which is an extension of RF that incorporates uncertainties, and we introduce the KDE – RF method which is a combination of the Kernel-Density Estimation and the RF algorithm. We train the algorithms on the Equivalent Width (EW) of characteristic absorption lines measured in the spectra from large Galactic (LAMOST, GOSSS) and extragalactic surveys (2dF, VFTS) with available spectral-type classification. By following an adaptive binning approach we group the labels of these data on 11 sub-types within the range O3-B9. We examined which of the characteristic spectral lines (features) are more important to use based on a number of feature selection methods and we searched for the optimal hyper-parameters of the classifiers, to achieve the best performance.
From the feature screening process, we find 13 spectral lines as the optimal number of features. We find that the overall accuracy score is ~ 76 % with similar results across all approaches, with our KDE – RF being slightly lower at ~ 73 %. In addition, we show that our optimized RF model can reach an overall accuracy score of ~ 85 % in the ideal case of robust measurement of the weakest characteristic spectral lines. We apply our model in other observational data sets providing examples of potential application of our classifier on real science cases. We find that it performs well for both single massive stars and for the companion massive stars in Be X-ray Binaries, especially for data with S/N in the range 50-300. Furthermore, we present an alternative model for lower quality data S/N < 25 based on a reduced feature-set classification scheme, including only the strongest spectral lines.
The similarity in the performances of our models indicates the robustness and the reliability of the RF algorithm when used for spectral classification of early-type stars. This is strengthened also by the fact that we are working with real-world data and not with simulations. In addition, the approach presented in this work is very fast and applicable to products from different surveys in terms of quality (e.g different resolutions) and of different formats (e.g., absolute or normalized flux).

For more see this k-poster (submitted for S16: Massive stars: birth, rotation, and chemical evolution).

3. Evolved massive stars in the Magellanic Clouds

Ming Yang, Alceste Bonanos, Biwei Jiang, Jian Gao, Panagiotis Gavras, Grigoris Maravelias, Man I Lam, Shu Wang, Xiaodian Chen, Yi Ren, Frank Tramper, Zoi Spetsieri

We present two clean, magnitude-limited (IRAC1 or WISE1≤15.0 mag) multiwavelength source catalogs for the Large and Small Magellanic Cloud (LMC and SMC). The catalogs were built by crossmatching (1”) and deblending (3”) between the source list of Spitzer Enhanced Imaging Products (SEIP) and Gaia Data Release 2 (DR2), with strict constraints on the Gaia astrometric solution in order to remove the foreground contamination. It is estimated that about 99.5% of the targets in our catalog are most likely genuine members of the LMC and SMC. The LMC catalog contains 197,004 targets in 52 different bands, while SMC catalog including contains 45,466 targets in 50 different bands, ranging from the ultraviolet to the far-infrared. Additional information about radial velocities and spectral and photometric classifications were collected from the literature. For the LMC, we compare our sample with the sample from Gaia Collaboration et al. (2018), indicating that the bright end of our sample is mostly comprised of blue helium-burning stars (BHeBs) and red HeBs with inevitable contamination of main sequence stars at the blue end. For the SMC, by using the evolutionary tracks and synthetic photometry from MESA Isochrones & Stellar Tracks and the theoretical J-Ks color cuts, we identified and ranked 1,405 red supergiant (RSG), 217 yellow supergiant (YSG), and 1,369 blue supergiant (BSG) candidates in the SMC in five different color-magnitude diagrams (CMDs), where attention should also be paid to the incompleteness of our sample. For the LMC, due to the problems with models, we applied modified magnitude and color cuts based on previous studies, and identified and ranked 2,974 RSG, 508 YSG, and 4,786 BSG candidates in the LMC in six CMDs. The comparison between the CMDs from the two catalogs of the LMC SMC indicates that the most distinct difference appears at the bright red end of the optical and near-infrared CMDs, where the cool evolved stars (e.g., RSGs, asymptotic giant branch stars, and red giant stars) are located, which is likely due to the effect of metallicity and star formation history. A further quantitative comparison of colors of massive star candidates in equal absolute magnitude bins suggests that there is essentially no difference for the BSG candidates, but a large discrepancy for the RSG candidates since LMC targets are redder than the SMC ones, which may be due to the combined effect of metallicity on both spectral type and mass-loss rate as well as the age effect. The effective temperatures (Teff) of massive star populations are also derived from reddening-free color of (J-Ks). The Teff ranges are 3500≤Teff≤5000 K for an RSG population, 5000≤Teff≤8000 K for a YSG population, and Teff≥8000 K for a BSG population, with larger uncertainties toward the hotter stars.

For more see this k-poster (submitted for S16: Massive stars: birth, rotation, and chemical evolution).

Notes on the density of the disk/rings around B[e] supergiants

In Zickgraf et al. 1989 (A&A, 220, 206) there is a comment on the density of the disk around the supergiant LHA 115-S 18. In Table 1 they provide (following Waters 1986) the mean particle density at r=R* for a number of sources: 3.2 x 1012, 2.6 x 1012, 4.6 x 1012, 1.7 x 1012 cm-3 for sources S 18, S 12, S 134, and R 126, respectively. Having calculated a maximum disk radius of 300R*, they calculate a mean density of few 109 cm-3. These values are consistent with both estimates from IR emission and by McGregor et al. (1988), who independently found densities of about 109 cm-3 from the CO first overtone emission in S 12, S 134, and R 66.

New paper on the circumstellar environment of galactic B[e] supergiants

Finally, after some years of work, it has been accepted for publication in MNRAS.

Resolving the kinematics of the disks around Galactic B[e] supergiants

Grigoris Maravelias, Michaela Kraus, Lydia S. Cidale, Marcelo Borges Fernandes, Maria L. Arias, Michel Curé, Georgios Vasilopoulos

B[e] Supergiants are luminous evolved massive stars. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dusty regions usually found in rings or disk-like structures. For a better comprehension of the mechanisms behind the formation of these rings, detailed knowledge about their structure and dynamics is essential. To address that, we obtained high-resolution optical and near-infrared spectra for 8 selected Galactic B[e] Supergiants, for which CO emission has been detected. Assuming Keplerian rotation for the disk, we combine the kinematics obtained from the CO bands in the near-IR with those obtained by fitting the forbidden emission [OI] λ5577, [OI] λλ6300,6363, and [CaII] λλ7291,7323 lines in the optical to probe the disk structure. We find that the emission originates from multiple ring structures around all B[e] Supergiants, with each one of them displaying a unique combination of rings regardless of whether the object is part of a binary system. The confirmed binaries display spectroscopic variations of their line intensities and profiles as well as photometric variability, whereas the ring structures around the single stars are stable.

arXiv.org: 1807.00796

Figure 12 from the paper: A cartoon illustration of the disk-structures as derived from our analysis. We represent the [OI] λ5577 line as *[OI]*, the [OI] λλ6300, 6363 doublet as [OI], and the [CaII] λλ7291, 7323 as [CaII]. The arrows above the rings symbolize the typical ring-widths and are given in km/s. (For more details on the data used and references see Table 3. Note that the relative structures and sizes are not in scale.

The (paper-)story of HD 87643

A summary for HD 87643 (or MWC 198, Hen 3-365, V640 Car, … see also Simbad). That is a great number of papers (similar to the post for GG Car).

  • 1909 /
    Object of the Henry Draper catalog.
  • 1933 / Merrill, P. W.; Burwell, C. G. 1933 (ApJ, 78, 87)
    First reference as a Be star.
  • 1968 / Hiltner, W. A.; Stephenson, C. B.; Sanduleak, N. 1968 (ApL, 2, 153H)
    They identified Balmer lines with their absorption borders to be similar to those of P Cygni. They identified also FeII lines in emission. They argued that the star has changed significantly from a P Cygni star (in 1909) and as a emission line star with WR characteristics (referring to Münch Luis 1953; Boletin los Obs. de Ton. y Tae., No 8, 27) to a nova like object, similar to Nova Delphini 1967.
  • 1971 / Crampton, D. 1971 (AJ, 76, 260)
    Identifying it within an HII region (KCW 47).
  • 1972 / van den Bergh, S. 1972 (PASP, 84, 594)
    A nebula is identified. A suggestion of a B2 V star with Av=2.5 mad and Vo=6.0, at a distance of 500 pc is proposed.
  • 1973 / Allen, D. A. 1973 (MNRAS, 161, 145), Sanduleak, N.; Stephenson, C. B. 1973 (ApJ, 185, 899)
    Allen: H,K,L ir observations show excess which is accounted to “reradiation from circumstellar dust clouds” (the star belongs to Dn category, which means “D”ust and presence of “n”ebulosity). See also section 4 (explanation). Sanduleak: Identified as a peculiar Be star.
  • 1974 / Stephenson, C. B. 1974 (ApJ, 191, 685), Albers, H. 1974 (ApJ, 189, 463), Swings J. P. 1974 (A&A, 34, 333)
    Stephenson: A nova-like nature is proposed. There is “a diffuse absorption spectrum violet-displaced from the corresponding emission lines by many hundreds of km/s”. In Hiltner+ 1968 that was ~850 km/s while in the current work is ~900 km/s. Moreover, the P Cygni absorption, present in 1968, is gone in 1973 (current work). Other than that the spectrum is similar to Hiltner’s, but very different than the original observation in HD catalog (1909). Albers: Identification of the CaII triplet and the OI 8446 lines (with a strong presence). Swings: Similar spectrum to GG Car. Balmer lines with P Cygni profiles, FeII (double-peaked with ~30km/s) and [FeII] (diffuse) lines present.
  • 1976 / Williams, P. M.; Beattie, D. H.; Stewart, J. M. 1976 (Obs, 96, 184), Allen, D. A. 1976 (MNRAS, 174P, 29), Henize, K. G. 1976 (ApJS, 30, 491), Allen, D. A.; Swings, J. P. 1976 (A&A, 47, 293)
    Williams: First SED for HD 87643. Mid-r observations indicative of a dust shell presence. Allen: Deriving reddening parameters: Teff = 11000 K, Av=1.9 mag, B(B-V)/Av = 0.35. Henize: Catalog of emission line stars, object 365 (or MWC 198). Comments: “Hβ is seen in emission and a P Cygni profile is suspected at Hα. Classified as P Cyg in HDE and as W-R by Münch (1953). Mount Stromlo slit spectra show a peculiar nova-like spectrum (Henize 1962; Carlson 1968). A direct plate with the Mount Stromlo 74 inch (1.9 m) reflector shows the star to be centered in a peculiar nebula (Henize 1962). Recent observations of this star have been reported by Hiltner, Stephenson, and Sanduleak (1968) and by van den Bergh (1972).” Allen+Swings: Member of Group 2 of peculiar Be stars with IR excess (see paper for more).
  • 1979 / 1979 / Carlson, E. D.; Henize, K. G. 1979 (VA, 23, 213)
    Included in the catalog of southern peculiar emission-line stars. [Note: I couldn’t access the paper though!]
  • 1981 / Surdej, A.; Surdej, J.; Swings, J. P.; Wamsteker, W. 1981 (A&A, 93, 285)
    The Reflection Nebula Surrounding HD87643: Imaging of HD 87643 shows a filamentary structured nebula (already mentioned in previous works) but more careful examination of Hα, [OIII], [SII] images showed that the nebula is not emitting, and the authors conclude towards a reflection nature of the nebula (at about 1 kpc). A spectrum of this nebula reveals strong P Cygni profiles (from Hβ to Hε) and many diffuse FeII lines. Expansion velocity ~1200 km/s
  • 1982 / Barbier, R.; Swings, J. P. 1982 (IAUS, 98, 103), de Freitas Pacheco, J. A.; Gilra, D. P.; Pottasch, S. R. 1982 (A&A, 108, 111)
    Barbier: Polarization measurements (in UBV). de Freitas Pacheco: Using UV spectra they estimated the mass loss ~7×107 M/year, evidence of a strong wind. See also the explanation of line formation (physics).
  • 1983 / Surdej, J.; Swings, J. P. 1983 (A&A, 117, 359)
    A non-isotropic and/or homogeneous environment is suggested based also in their previous work (Surdej+ 1981).
  • 1985 / de Freitas Pacheco, J. A.; Faria Lopes, D.; Landaberry, S. C.; Selvelli, P. L. 1985 (A&A, 152, 101)
    High-resolution UV spectra revealed a large number of lines. They discuss the excitation mechanism for FeII and provide a sketch of the circumstellar environment with: regions of ionized elements closer to the star, a region of neutral H, and a cold shell. The presence of a strong low ionization wind is mentioned. The wind velocity is estimated to ~800 km/s, with a mass loss of ~107 M/year, and the NaI doublet suggests a distance of 2 kpc.
  • 1986 / Olnon, F. M.; Raimond, E.; Neugebauer, G.; van Duinen, R. J.; Habing, H. J.; Aumann, H. H.; Beintema, D. A.; Boggess, N.; Borgman, J.; Clegg, P. E.; Gillett, F. C.; Hauser, M. G.; Houck, J. R.; Jennings, R. E.; de Jong, T.; Low, F. J.; Marsden, P. L.; Pottasch, S. R.; Soifer, B. T.; Walker, R. G.; Emerson, J. P.; Rowan-Robinson, M.; Wesselius, P. R.; Baud, B.; Beichman, C. A.; Gautier, T. N.; Harris, S.; Miley, G. K.; Young, E. 1986 (A&AS, 65, 607)
    Identified in the IRAS catalog.
  • 1987 / Volk, K.; Kwok, S. 1987 (ApJ, 315, 654)
    Identifying dust at 10 micron (~162 K).
  • 1988 / Shore, S. N.; Sanduleak, N.; Brown, D. N.; Sonneborn, G.; Bopp, B. W.; Robinson, C. R. 1988 (ESASP, 281a, 417), McGregor, P. J.; Hyland, A. R.; Hillier, D. J. 1988 (ApJ, 324, 1071)
    Shore: Similarities between Galactic and Magallanic Cloud stars, ie. HD 87643 (Hen3-365) to S22 in LMC. McGregor: IR spectra with HI lines present, but no first-overtone CO emission (in contrast to others lines GG Car, CPD-529243, CPD-572874). Estimating E(B-V)=1.0, 2.5 kpc, Teff~16000 K, L=4.9 L. Hot (T(K-L)=1190 K) and cold (T(25-60)=125 K) dust present for this star (they give other properties like radii and masses).
  • 1989 / Gaylard, M. J.; West, M. E.; Whitelock, P. A.; Cohen, R. J. 1989 (MNRAS, 236, 247)
    IR measurements and detection of OH.
  • 1990 / Deguchi, S.; Nakada, Y.; Sahai, R. 1990 (A&A, 230, 339), Shore, S. N.; Brown, D. N.; Bopp, B. W.; Robinson, C. R.; Sanduleak, N.; Feldman, P. D. 1990 (ApJS, 73, 461)
    Deguchi: No detection of CO or SiO in radio observations (HD 87643 is referred as SAO 237672). Shore: UV spectra for HD 87643, with P Cygni profiles. They assign a tentative B3 type with Teff~15000 K, L=3.7 L. Discussing the shell and how it affects the optical photometry. Identifying ~10% variation in the Fe II lines. Note from Section IIIb: “Van den Bergh (1972) attempted to determine the distance to He 3-365, assuming it to be a B2 V star. The derived distance of 0.5 kpc is certainly an underestimate. De Freitas Pacheco, Gilra, and Pottasch (1982) have analyzed the optical and UV pectrum of this star, concluding that it is a B supergiant at a distance of about 2-3 kpc. In a subsequent paper, de Freitas Pacheco, Gilra, and Pottasch (1985) refer to it as a B[e] star with a cool wind. They reported Mg II P Cygni profiles at high dispersion and suggested that the mass-loss rate is about M~lO-7 M/yr. Surdej and Swings (1983) detected a reflection nebula in the vicinity of the star. MHH88 report the detection of hot dust emission.”
  • 1992 / Gnedin, Yu. N.; Kiselev, N. N.; Pogodin, M. A.; Rosenbush, A. E.; Rosenbush, V. K. 1992 (SvAL, 18, 182), Lopes, D. F.; Damineli Neto, A.; de Freitas Pacheco, J. A. 1992 (A&A, 261, 482)
    Gndedin: A small polarization ~0.6% was observed, but without any variation reported. Lopes:Optical and ir spectroscopy, and short discussion of the the observed lines/features. Extracting physical properties: B3I, E(B-V)=1.3, d= 2.9 kpc, Mv=-7.6, Mbol=-8.9, M=34 M, Vwind=1300 km/s, MassLoss=1.2×10-5 M/year (a good physics paper also!).
  • 1994 / The, P. S.; de Winter, D.; Perez, M. R. 1994 (A&AS, 104, 315)
    Object refer to the catalog as “extreme emission line object”
  • 1998 / Lamers, H. J. G. L. M.; Zickgraf, F.-J.; de Winter, D.; Houziaux, L.; Zorec, J. 1998 (A&AS, 131, 401), Oudmaijer, R. D.; Proga, D.; Drew, J. E.; de Winter, D. 1998 (MNRAS, 300, 170)
    Lamers: It is not considered a definite SG since “show some characteristics of pre-main sequence stars (e.g. forbidden emission lines, evidence for disks, nebulosity) but there is no evidence for infall.” They provide a B4II[e] type classification for HD 87643. Noting: “HD 87643 is embedded in a reflecting nebula (Henize 1962, van den Bergh 1972, Surdej et al. 1981). The star was originally classified as a P Cygni type star and even a nova-like star (Carlson & Henize 1979). From IUE spectra McGregor et al. (1988a) and Shore et al. (1990) assigned an effective temperature of 15000 K to HD 87643. A distinctive peculiarity of HD 87643 is the presence of a strong, low ionization wind (de Freitas Pacheco et al. 1985, L ́opes et al. 1992). P Cygni line profiles are observed in many species, indicating outflows up to 1400 km s −1 (Surdej et al. 1981, Shore et al. 1990) and 1800 km s −1 (Oudmaijer et al. 1998). The estimated bolometric luminosity indicates that this star should be a bright giant (Zorec 1998; Oudmaijer et al. 1998).”. Yudin: Polarimteric observations show an increase of the polarization (compared to Barbier & Swings 1982 and Gnedin 1992) to ~1%, as well as some variability within a few days, consistent with HAeBe stars with Algol-like minima. Oudmaijer: A focused study on HD 87643, with high-resolution spectroscopy and medium-resolution spectropolarimetric data. Good intro. Imaging show nebulosity in V, R but not in Ha and [SII] (consistent with Surdej et al. 1981). Suggesting an evolved SG nature, with an “optically thick disk where irradiation pressure is sufficient to power mass loss from both the star and the disk”. Three different line-forming regions at ~1800 km/s (Ha and HeI), ~150km/s (H and FeII lines), and ~40km/s (forbidden lines, FeI, weaker FeII lines), with a disk-like structure. Commenting on the fainting magnitude of the system.
  • 1999 / Oudmaijer, R. D.; Drew, J. E. 1999 (MNRAS, 305, 166), Voors, R. H. M. 1999 (Ph.D. Thesis, Universiteit Utrecht, The Netherlands)
    Oudmaijer:Hα spectropolarimetry “indicates that the polarization profile can be best reproduced with a circumstellar disc that is both rotating and expanding”. Voors: Proposed a circumbinary disk.
  • 2000 / Valenti, J. A.; Johns-Krull, C. M.; Linsky, J. L. 2000 (ApJS, 129, 399), Clark, J. S.; Miroshnichenko, A. S.; Larionov, V. M.; Lyuty, V. M.; Hynes, R. I.; Pooley, G. G.; Coe, M. J.; McCollough, M.; Dieters, S.; Efimov, Yu. S.; Fabregat, J.; Goranskii, V. P.; Haswell, C. A.; Metlova, N. V.; Robinson, E. L.; Roche, P.; Shenavrin, V. I.; Welsh, W. F. 2000 (A&A, 356, 50)
    Valenti: Treated as Herbig Ae/Be star (id 89). A spectral type of B3.5 is given and a Av=1.88 mag. Clark: “HD 87643 (which faded by ∼1 mag since the 1960’s), and MWC 342 (∆V∼0.6 mag). Of these, only HD 87643 showed a long term trend in colour with brightness, becoming redder as it faded (Miroshnichenko 1998).” (Miroshnichenko A.S., 1998, In: Jaschek C., Hubert A.M. (eds.), B[e] stars, Kluwer Academic Publishers, p. 145)
  • 2001 / de Winter, D.; van den Ancker, M. E.; Maira, A.; Thé, P. S.; Djie, H. R. E. Tjin A.; Redondo, I.; Eiroa, C.; Molster, F. J. 2001 (A&A, 380, 609)
    Previously unpublished photometry from data “obtained between 1978 and 1997 in the Walraven (WULBV), Johnson/Cousins (UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometric systems”.
  • 2003 / Zickgraf, F.-J. 2003 (A&A, 408, 257)
    Detailed work on the spectral features observed on many galactic objects, including HD 87643 (simply put… must see paper!).
  • 2005 / Greaves, J. 2005 (IBVS, 5699, 18), Cool, Richard J.; Howell, Steve B.; Peña, Maria; Adamson, Andy J.; Thompson, Robert R. 2005 (PASP, 117, 462)
    Greaves: Assigning an SDOR type of variability for HD 87643 (V=8.68 – 9.83mag). Cool: They show UV spectra for HD 87643 as a comparison but they say they have been published previously (de Freitas Pacheco et al. 1982). The star is considered a similar source to the ones studied (iron stars) and the discussion about the nature is interesting. E.g. “hot Be star with an evolved late‐type secondary. The hydrogen emission features arise in the hot wind from the Be star, while the corresponding P‐Cygni absorption lines are produced from dense material in the expanding, radiation‐driven wind around each system.”
  • 2006 / Baines, D.; Oudmaijer, R. D.; Porter, J. M.; Pozzo, M. 2006 (MNRAS, 367, 737)
    They treat HD 87643 are a Herbig Ae/Be star (although its supergiant nature is not excluded, as it stands out from all others), plus they do not detect binarity. Hα emission is considered as an outflow, which consists of a high- and low-velocity components.
  • 2007 / Groh, J. H.; Damineli, A.; Jablonski, F. 2007 (A&A, 465, 993)
    Comment: “HD 87643 (Fig. 11) is thought to be an evolved B[e] star (Oudmaijer et al. 1998) and appears to be related to the LBV class, although the link is not clear yet. This object has an optical spectrum dominated by emission lines of Fe II and P Cygni profiles in the Balmer series, together with low excitation forbidden lines. The spectrum is produced by a fast polar wind combined with a slow disk wind (Oudmaijer et al. 1998). The ultraviolet and optical spectra of HD 87643 was previously discussed by de Freitas Pacheco et al. (1985,1982), who reported a strong spectral line variability. HD 87643 has a bright reflection nebula, which was analyzed by Surdej et al. (1981) and Surdej & Swings (1983). Its 2001 near infrared spectrum shows prominent Fe II, CI lines and Pa γ emission, compatible with the presence of a cold wind. He I 10 830 A presents a P-Cygni profile, with a weak emission and a strong absorption that goes up to -1750 km s-1, which is probably formed in the fast polar wind.”
  • 2007 / Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. 2007 (IBVS, 5863, 1)
    Included in the catalog of Variable stars (as V0640 Car).
  • 2009 / Millour, F.; Chesneau, O.; Borges Fernandes, M.; Meilland, A.; Mars, G.; Benoist, C.; Thiébaut, E.; Stee, P.; Hofmann, K.-H.; Baron, F.; Young, J.; Bendjoya, P.; Carciofi, A.; Domiciano de Souza, A.; Driebe, T.; Jankov, S.; Kervella, P.; Petrov, R. G.; Robbe-Dubois, S.; Vakili, F.; Waters, L. B. F. M.; Weigelt, G. 2009 (A&A, 507, 317), Kraus, M. 2009 (A&A, 494, 253)
    Millour: Just read this paper! 🙂 They identify a companion (much fainter). Their separation is ~51 AU, and the corresponding orbital period 20-50 years. The structure of the nebula may be the result of periastron passages. The suggested picture is: i. a giant/supergiant with a dusty disk (contributes mainly to the IR emission, which originates from the inner and hotter rim of the disk, 2.5-3 AU), ii. a much fainter companion star (probably not hot) embedded in a dusty envelope, iii. a cooler circumbinary envelope. Comment on the continuous fainting of the system.Kraus: Simple commenting on HD 87643 in Table A.1 with some physical parameters and further references.
  • 2011 / Carmona, A.; van der Plas, G.; van den Ancker, M. E.; Audard, M.; Waters, L. B. F. M.; Fedele, D.; Acke, B.; Pantin, E. 2011 (A&A, 533A, 39)
    They include HD 87643 as a Herbig Ae/Be object, although they mention its controversial nature. They do not detect any H2 (in IR spectra of R~90000 resolution CRIRES data)
  • 2015 / Menu, J.; van Boekel, R.; Henning, Th.; Leinert, Ch.; Waelkens, C.; Waters, L. B. F. M. 2015 (A&A, 581A, 107)
    They treat HD 87643 as a Herbig Be star. They find that T=Tsub( R/Rsub)^q with q=0.72 (for the temperature of the molecular disk at 10.7 um) and a half-light radius of 37.3 mas corresponding to 56 au. However, they do not detect any H2 (as found in other pre-main sequence objects) or report any gap presence, although a number of objects studied do show gaps.

And the list is not exhaustive.

The (paper-)story of Hen 3-298

Similarly to the post for GG Car, I though that it is better to keep a track of all papers I find for my sample. As far as Hen 3-298 (Simbad) the results are not that many!

  • 1966 / Wray, J. D. 1966(PhD thesis, Northwestern University)
    First detection as an Hα emission star.
  • 1976 / Henize, K. G. 1976 (ApJS, 30, 491)
    Also included in this catalog for Hα emission stars in the Southern Skies.
  • 1986 / Olnon, F. M.; Raimond, E.; Neugebauer, G.; van Duinen, R. J.; Habing, H. J.; Aumann, H. H.; Beintema, D. A.; Boggess, N.; Borgman, J.; Clegg, P. E.; Gillett, F. C.; Hauser, M. G.; Houck, J. R.; Jennings, R. E.; de Jong, T.; Low, F. J.; Marsden, P. L.; Pottasch, S. R.; Soifer, B. T.; Walker, R. G.; Emerson, J. P.; Rowan-Robinson, M.; Wesselius, P. R.; Baud, B.; Beichman, C. A.; Gautier, T. N.; Harris, S.; Miley, G. K.; Young, E. 1986 (A&A, 65, 607)
    Detected as an IR excess source.
  • 1994 / The, P. S.; de Winter, D.; Perez, M. R. 1994 (A&AS, 104, 315)
    Included in this catalog as an emission line star.
  • 2005 / Miroshnichenko, A. S.; Bjorkman, K. S.; Grosso, M.; Hinkle, K.; Levato, H.; Marang, F. 2005 (A&A, 436, 653)
    The first dedicated study of Hen 3-298. They identified double-peaked emission lines, such as Hα with a stronger red peak, and the [CaII] doublet. [OI] lines were found single-peaked. They also detected CO features in emission and estimated the temperature ~2000 K, suggesting that originates from a dust free zone, inside but close to the dusty disk. Moreover, they estimate log L/L∼ 5.1 and a spectral type of no earlier than B3, at a distance of 3-4.5 kpc. Suggesting a supergiant nature.
  • 2013 / Oksala, M. E.; Kraus, M.; Cidale, L. S.; Muratore, M. F.; Borges Fernandes, M. 2013 (A&A, 558A, 170)
    Part of the IR survey. Detecting and studying the CO features (also estimating ~2000 K), and estimating the ratio 12CO/13CO=20±5 (consistent with supergiants; Kraus 2009).

The (paper-)story of GG Car

As I was diving into the literature of GG Car (aka HD 94878 or CPD-59 2855 or MWC 215 or … for more see Simbad!) I got overwhelmed of the number of papers and how back it goes. I only wanted to make a summary (for a forthcoming paper) but since I spent so much time and effort to read through the literature I though to make some kind of a timeline with some notes.

  • 1896 / (yea … since then!) Pickering, E. C. 1896 (Astron. Nachr., 141, 169), Pickering, E. C. & Fleming, W. P. 1896 (ApJ, 4, 142)
    First reference of GG Car as a star with a peculiar spectrum and one that resembles η Car.
  • 1916 / Cannon, A. J. & Pickering, E. C. 1916 (An. Har., 76, 19)
    GG Car is found under P Cygni type stars. All these stars are characterized by strong Balmer lines and look similar to β Lyr.
  • 1930 / Kruytbosch, W. E. 1930 (BAN, 6, 11)
    Identifying a period of 31.043 d using the minima of the light curve, from photographic plates. A binary system with two variable stars is proposed.
  • 1933 / Greenstein, N. K. 1938 (BHarO, 908, 25)
    The main conclusion from this work is a re-determination of the period to 62.07 d, based on the argument that two following minima were not the same.
  • 1950 / Thackeray, A. D. 1950 (MNRAS, 110, 524)
    In a search for southern stars related with nebulosity, nothing is found for GG Car.
  • 1955 / Smith, H. J. 1955 (PhD thesis, Harvard University)
    A slit spectrum revealed H and FeII lines, without any absorption lines or P Cygni profiles detected.
    [Note: I couldn’t check this reference myself and the text is borrowed from Herbig 1960, and others who also cite this thesis later on.]
  • 1960 / Herbig, G. H. 1960 (ApJS, 4, 337)
    Mentioned on the catalog, but only refers to the works by Smith 1955 and Thackeray 1950.
  • 1973 / Allen, D. A. 1973 (MNRAS, 161, 145)
    GG Car is found to exhibit an infrared excess (JHK measurements).
  • 1974 / Swings, J. P. 1974 (A&A, 34, 333); Albers, H. 1974 (ApJ,189,463)
    Swings: Spectroscopic observations (from ESO, La Silla, Chile) revealed [FeII] and [OI] emission lines, P Cygni profiles in the Balmer lines, and double-peaked profiles in FeII, which suggest a “thin equatorial ring rotating around the object”. Albers: GG Car displays two of the CaII triplet lines and a strong OI λ8446 line in emission (see Table 3).
  • 1976 / Allen, D. A. & Swings, J. P. 1976 (A&A, 47, 293), Henize 1976 (ApJS, 30, 491)
    Included in both of these catalogs: in Group 2 of Allen & Swings’ work as a peculiar Be star with infrared excess, and as an emission-line star in Henize’s work.
  • 1977 / Klare, G. & Neckel, T. (A&AS, 27, 215)
    UBVΗβ and polarization measurements for a catalog of southern OB stars, including GG Car.
  • 1979 / Carlson, E. D. & Henize, K. G. 1979 (VA, 23, 213)
    Included in the catalog of southern peculiar emission-line stars. [Note: I couldn’t access the paper though!]
  • 1980 / Cohen, M. & Barlow, M. J. 1980 (ApJ, 238, 585)
    Mid-infrared survey which identified GG Car with excess.
  • 1981 / Hernandez, C. A.; Sahade, J.; Lopez, L.; Thackeray, A. D. 1981 (PASP, 93, 747)
    There is a great introduction (that perhaps inspired this post also!). They describe the observables of GG Car and they give a period of 31.03 d, derived from spectroscopic observations.
  • 1982 / Bouchet, P. & Swings, J. P. 1982 (IAUS, 98, 241); Barbier, R.; Swings, J. P. 1982 (IAUS, 98, 103)
    Bouchet: Near infrared (JHK) observations and variability search. Barbier: Polarization measurements (in UBV).
  • 1984 / Gosset, E.; Surdej, J.; Swings, J. P. 1984 (A&AS, 55, 411)
    Optical photometry (UBV) and period determination (two periods at 31.020 and 62.039 d found).
  • 1985 / Gosset, E.; Hutsemekers, D.; Swings, J. P.; Surdej, J. 1985 (A&A, 153, 71)
    Great introduction. Minor summary of spectral properties. Binary solution from radial velocity measurements.
  • 1987 / Brandi, E. & Gosset, E. 1987 (A&AS, 68, 283)
    The ultraviolet spectrum and light curve for GG Car, which was found similar to the optical light curve.
  • 1988 / McGregor, P. J.; Hyland, A. R.; Hillier, D. J. 1988 (ApJ, 324, 1071)
    Infrared spectra of GG Car, exhibiting H, He, Fe lines along with CO bandhead in emission. Detailed measurements of the various features, and extraction of physical parameters for all systems (very good paper).
  • 1992 / Lopes, D. F.; Damineli Neto, A.; de Freitas Pacheco, J. A. 1992 (A&A, 261, 482), Gnedin, Yu. N.; Kiselev, N. N.; Pogodin, M. A.; Rosenbush, A. E.; Rosenbush, V. K. 1992 (SvAL, 18, 182)
    Lopes: Optical and infrared spectra. The presence of HeI in emission suggested a B0-B2 spectral classification. Various physical parameters are given: EQW(NaI)=1.18 Å, E(B-V)=1.1 mag, distance=2.5 kpc, Mv=-6.3 mag, Mb=-8.2 mag, M=25 M, mass-loss rate=5.7 M/year, V=670 km/s (good paper on physics also). Gnedin: Polarimetric observations on southern classical and peculiar Be stars.
  • 1996 / Morris, P. W.; Eenens, P. R. J.; Hanson, M. M.; Conti, P. S.; Blum, R. D. 1996 (ApJ, 470, 597)
    Infrared spectroscopy showing: HeI, FeII, MgII, HI, CO features, and first indication of variability (in infrared).
  • 2004 / Machado, M. A.; de Araújo, F. X.; de Faria Lopes, D.; Pereira, C. B. 2004 (RMxAC, 20, 239)
    Optical spectral variability of GG Car.
  • 2007 / Groh, J. H.; Damineli, A.; Jablonski, F. 2007 (A&A, 465, 993)
    Infrared spectra displaying FeII, MgII, CI, HeI 10830 Å, and Pa γ in emission.
  • 2009 / Pereyra, A.; de Araújo, F. X.; Magalhães, A. M.; Borges Fernandes, M.; Domiciano de Souza, A. 2009 (A&A, 508, 1337) Kraus, M 2009 (A&A, 494, 253)
    Pereyra: A good introduction with some more references on previous works on polarization. Spectropolarimetric data around Hα are presented for the first time for GG Car, helping constraining the geometry of the system. Kraus: The 13CO enrichment in B[e]SGs, which hints their evolved nature. References to other works that identify 13CO features in GG Car (with a 12CO/13CO less than 10).
  • 2010 / Borges Fernandes, M. 2010 (RMxAC, 38, 98)
    Interferometric observations of B[e]SGs including G Car. An inclination angle of 50°-60° at a distance of 1 kpc is stated.
  • 2012 / Marchiano, P.; Brandi, E.; Muratore, M. F.; Quiroga, C.; Ferrer, O. E.; García, L. G. 2012 (A&A, 540, 91)
    An introduction with many references about the binary nature of GG Car and variability in general. Identifying the orbital parameters for the binary (with spectroscopic data) and extracting physical parameters for the star and the environment: eccentricity = 0.28, period = 31.033 d, mass ratio = 2.2 with Mprimarysin3i=18 M and Msecondarysin3i=8 M, Teff = 23000 K, logg=3, E(B-V)=0.39 mag, inclination angle between 54°-72°. A gaseous and dusty envelope is assumed (see Table 4 for details physical properties). Even though the calculations were performed for a single star the a (of two similar-B-type stars) would still fit the observed properties, if the second contributes les than 10% of the primary flux. A distance of 5 kpc was estimated using two different methods.
  • 2013 / Kraus, M.; Oksala, M. E.; Nickeler, D. H.; Muratore, M. F.; Borges Fernandes, M.; Aret, A.; Cidale, L. S.; de Wit, W. J. 2013 (A&A, 549, 28)
    The most updated summary (introduction) for the star. Infrared high-resolution spectra revealed the structure of the dusty disk (through the detailed analysis of the CO profiles). A Tco = 3200 K leads to a detached and circumbinary disk. Its motion is consistent with a Keplerian rotation (at 80 km/s). A ratio of 12CO/13CO=15±5 confirms the evolved nature of GG Car. less than 10). Two scenarios for the disk formation are discussed, including the possibility of a non-conservative Roche lobe overflow or that the accumulation of material has been performed gradually through a classical Be phase. Although none can be excluded, the second case is only slightly favored, due to the extreme conditions needed for first scenario.
  • 2015 / Domiciano de Souza, A.; Borges Fernandes, M.; Carciofi, A. C.; Chesneau, O. 2015 (IAUS, 307, 291)
    Mid-infrared interferometric observations (MIDI/ESO-VLTI), showing an inclination angle of ~60° (consistent with Marchiano et al. 2012). The central star is modeled as a B-type star with Teff~20000 K, R~10 R, L~104 L, which can describe the observed data but not all lines (as they put it: “However, they are not a perfect representation of the circumstellar environment of GG Car”. They do find that the dust is formed in a compact ring much closer to the start than expected (≤100 R).

UPDATE 5/5/2017: Adding Albers 1974.
UPDATE 7/5/2017: Adding Barbier & Swings 1982.

New paper: The yellow hypergiant – B[e] supergiant connection

Our most recent work comes as a proceedings paper for the conference “Stars: from collapse to collapse” (Special Astrophysical Observatory, Nizhnij Arkhyz, Karachai-Cherkessian Republic, Russia, 3-7 October 2016):

The yellow hypergiant – B[e] supergiant connection

A. Aret, M. Kraus, I. Kolka, G. Maravelias

B[e] supergiants and yellow hypergiants share a number of common properties regarding their circumstellar environments. Using the forbidden [O I] and [Ca II] lines as disk tracers, we suggest the presence of a Keplerian disk or ring around the yellow hypergiant V509 Cas and confirm the pole-on inner disk around V1302 Aql. These findings indicate a change in mass-loss behavior from spherical in cooler yellow hypergiants to axisymmetric in the hotter ones during the passage through the Yellow Void. The accumulation of material in the equatorial plane reminds of the disks of B[e] supergiants, supporting the suggestion that yellow hypergiants might appear as B[e] supergiants after they reach the blue edge of the yellow instability domain.

arXiv: 1611.06044

The B[e] stars conference in Prague

During the last week I was traveling from Ondrejov forth and back to the “The B[e] Phenomenon: Forty Years of Studies”, in Prague (27 June – 1 July 2016). It was a nice conference with many interesting talks, and fruitful discussions. Most importantly, I met some old friends and made new ones! I was fortunate enough to contribute to this conference with a number of works that follow.

1. “B[e] Supergiants’ circumstellar environment: disks or rings?”
G. Maravelias, M. Kraus, A. Aret, L. Cidale, M. L. Arias, M. Borges Fernandes

B[e] Supergiants are a phase in the evolution of some massive stars for which we have observational evidence but no predictions by any stellar evolution model. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dust regions usually found in rings or disk-like structures. However, the structure and the formation of this circumstellar environment is not well-understood, which means that further investigation is needed. To address that, we obtained high-resolution optical and near-infrared spectra (using MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively) for a number of Galactic B[e]SGs. We examined the [OI] and [CaII] emission lines and the CO bandheads to probe the structure and the kinematics of their formation regions. We find that these emission lines form either in a single or in multiple equatorial rings, a probable result of previous mass-loss events.

    link to site | local file

2. “Similarities in the structure of the circumstellar environments of B[e] supergiants and yellow hypergiants”
A. Aret, I. Kolka, M. Kraus, G. Maravelias

Despite their different evolutionary phases, B[e] supergiants and yellow hypergiants share
a number of common properties regarding their circumstellar environments. Both types of stars experience phases of strongly enhanced mass-loss, and the released material accumulates in (multiple) shells, bipolar nebulae, and/or disk-like structures, often veiling the central object. Moreover, the physical conditions in the envelopes of these stars are ideal for molecule and dust condensation. While the enhanced mass-loss and eruptions in yellow hypergiants are probably caused by an increased pulsation activity, the physical mechanism leading to the formation of the dense winds and Keplerian disks observed in B[e] supergiants is still unclear. Recently, we performed an optical spectroscopic survey of a large sample of Galactic emission-line stars in diverse evolutionary states. This survey was aimed at identifying characteristic emission features that help to study the structure and kinematics of the circumstellar environments of different types of evolved massive stars, including several yellow hypergiants and a number of B[e] stars in different evolutionary phases. Motivated by the results from previous studies, we focused on the strategic forbidden emission lines of [OI] and
[CaII], which are considered as ideal tracers for circumstellar disks. Interestingly, we identified both sets of lines in most of the yellow hypergiants in our sample, while from the B[e] star sample only the supergiants displayed these features. This indicates that the physical conditions in the environments of both types of stars (yellow hypergiants and B[e] supergiants) could be similar. In particular, the double-peaked emission lines of [CaII] observed in the yellow hypergiants of earlier spectral type suggest that these stars possibly possess a dense circumstellar ring or disk-like structure alike their hotter B[e] supergiant counterparts.

    link to site | local file

3. “Clumpy molecular structures revolving the B[e] supergiant MWC 137”
M. Kraus, L. S. Cidale, T. Liimets, D. S. Gunawan, C. E. Cappa, M. E. Oksala, M. L. Arias, G. Maravelias, M. Borges Fernandes, M. Cure

The Galactic object MWC 137 is a peculiar early-type star surrounded by the optical nebula Sh 2-266 (80″ × 60″) of unclear origin. The large-scale structure seen in Hα images suggests that Sh 2-266 is a ring nebula probably produced by the interaction of the stellar winds with the ambient medium, with a possible bipolar outflow perpendicular to the ring/disk plane. A collimated outflow with several knots was indeed recently detected in the light of the [N II] 6583 line. Moreover, near-infrared spectroscopic observations displayed intense, kinematically broadened CO band emission in both isotopes 12CO and 13CO. The observed enrichment in 13CO implies that MWC 137 is an evolved object. This result combined with the high luminosity of the star suggests that it belongs to the group of B[e] supergiants. To investigate the physical conditions and spatial distribution of the hot molecular gas we obtained K-band IFU observations with the ESO/SINFONI spectrograph in its high spatial resolution mode in two different seasons. In addition, to map the cold molecular gas regions, we collected molecular line observations in the sub-mm range with APEX. We find that the molecular gas is distributed on multiple clumpy ring structures. These rings are more or less perpendicular to the jet axis, and the material is revolving the central object on (quasi-)Keplerian orbits.

    link to site | local file

4. “A new outburst of the yellow hypergiant star ρ Cas”
A. Aret, M. Kraus, I. Kolka, G. Maravelias

Yellow hypergiants are massive stars that have passed through the red-supergiant phase and evolve back bluewards in the Hertzsprung-Russell diagram. It has been suggested that these stars may be evolving toward the B[e] supergiant phase. Such a possible evolutionary link should be investigated.
In 2011, we started to monitor spectroscopically several yellow hypergiants using the Ondrejov 2m telescope. The aim of this campaign is to track and to study their mass ejection phases. One of the objects we monitor is ρ Cas. This star is famous for its historical and recent outbursts, during which the star develops TiO bands in a cool, optically thick wind with a very brief but high mass-loss rate (3 × 10−2 M in 200 days). Each outburst is accompanied by a drop in the light curve of more than one magnitude. At least three such outbursts were recorded for ρ Cas: 1945-1947, 1985-1986, and 2000-2001. Our spectroscopic data show that during 2013, another outburst occurred, which is obvious from the development of pronounced TiO bands. Also many atmospheric lines characteristic for a later spectral type appear. Moreover, the photometric light curve displays a drop by about 0.6 mag during the same period. While the total mass loss connected with this recent outburst was probably less violent, the decrease of the time interval between the outbursts might indicate that ρ Cas is preparing for its passage through the Yellow Void region towards the hot side of the Hertzsprung-Russell diagram.

    link to site | local file

UPDATE 6 Aug 2016: As the organizers have uploaded all talks and posters at their website, I also added here the corresponding links and files.

UPDATE 10 Oct 2016: You can find the proceedings paper on arXiv: 1610.00607 (Maravelias et al.)

UPDATE 24 Oct 2016: You can find Kraus et al. proceedings paper on arXiv: 1610.05596

UPDATE 15 Nov 2016: Aret et al. proceedings papers became also available in arXiv: arXiv: 1611.04490 (on similarities between B[e]SGs and YHGs and arXiv: 1611.04493 (on YHG rho Cas).