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New Paper: A machine-learning photometric classifier for massive stars in nearby galaxies I. The method

This is the first paper that results from my work with the ASSESS team over the last years. It focuses on the development of a machine-learning photometric classifier to characterize massive stars originating from IR (Spitzer) catalogs, which will help us understand the episodic mass loss. The first paper presents the method and the multiple test we performed to understand its capabilities and limitations. Now we proceed with the derivation of the catalogs and their analysis.


A machine-learning photometric classifier for massive stars in nearby galaxies I. The method

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

Context. 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. Moreover, episodic mass loss in evolved massive stars is not included in the models while the importance of its role in the evolution of massive stars is currently undetermined.
Aims. 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.
Methods. 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 which helps with foreground source detection. We grouped them in 7 classes (Blue, Red, Yellow, B[e] supergiants, Luminous Blue Variables, Wolf-Rayet, and outliers, e.g. QSOs and background galaxies). As this training set is highly imbalanced, we implemented synthetic data generation to populate the underrepresented classes and improve separation by undersampling the majority class. We built an ensemble classifier utilizing color indices as features. The probabilities from three machine-learning algorithms (Support Vector Classification, Random Forests, Multi-layer Perceptron) were combined to obtain the final classification.
Results. The overall weighted balanced accuracy of the classifier is ∼ 83%. Red supergiants are always recovered at ∼ 94%. Blue and Yellow supergiants, B[e] supergiants, and background galaxies achieve ∼ 50 − 80%. Wolf-Rayet sources are detected at ∼ 45% while Luminous Blue Variables are recovered at ∼ 30% from one method mainly. This is primarily due to the small sample sizes of these classes. In addition, the mixing of spectral types, as there are no strict boundaries in the features space (color indices) 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 ∼ 70%. This discrepancy is attributed to the different metallicity and extinction effects of their host galaxies. Motivated by the presence of missing values we investigated the impact of missing data imputation using simple replacement with mean values and an iterative imputor, which proved to be more capable. We also investigated the feature importance to find that r − i and y − [3.6] were the most important, although different classes are sensitive to different features (with potential improvement with additional features).
Conclusions. The prediction capability of the classifier is limited by the available number of sources per class (which corresponds to the sampling of their feature space), 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, as well as at lower metallicities (with some accuracy loss), making it an excellent tool for accentuating interesting objects and prioritizing targets for observations.

The confusion matrix for 54 sources without missing values in the three galaxies (IC 1613, WLM, and Sextans A). We achieve an overall accuracy of ~70%, and we notice that the largest confusion occurs between BSG and YSG. The overall difference in the accuracy compared to that obtained with the M31 and M33 sample is attributed to the photometric errors, and the effect of metallicity and extinction in these galaxies.

arXiv: 2203.08125

New paper: exploring the outbursts of ρ Cas from visual observations

This is a paper that I finally managed to complete. Starting back in 2016 we looked into the light curves for ρ Cas to identify potential correlations with its latest outburst in 2013, but not all data made it through the final paper (Kraus et al. 2019). Given this first analysis and the fact that visual observations cover almost a century of star’s behavior, we continued the study and we looked into the four distinct outbursts. The result is even more interesting as there is a clear trend of shorter and more frequent outbursts, as if ρ Cas is bouncing against the Yellow Void.


Bouncing against the Yellow Void — exploring the outbursts of ρ Cas from visual observations

Grigoris Maravelias and Michaela Kraus

Massive stars are rare but of paramount importance for their immediate environment and their host galaxies. They lose mass from their birth through strong stellar winds up to their spectacular end of their lives as supernovae. The mass loss changes as they evolve and in some phases it becomes episodic or displays outburst activity. One such phase is the Yellow Hypergiants, in which they experience outbursts due to their pulsations and atmosphere instabilities. This is depicted in photometry as a decrease in their apparent magnitude. The object ρ Cassiopeia (Cas) is a bright and well known variable star that has experienced four major outbursts over the last century, with the most recent one detected in 2013. We derived the light curves from both visual and digital observations and we show that with some processing and a small correction (∼0.2 mag) for the visual the two curves match. This highlights the importance of visual observations both because of the accuracy we can obtain and because they fully cover the historic activity (only the last two of the four outbursts are well covered by digital observations) with a homogeneous approach. By fitting the outburst profiles from visual observations we derive the duration of each outburst. We notice a decreasing trend in the duration, as well as shorter intervals between the outbursts. This activity indicates that ρ Cas may be preparing to pass to the next evolutionary phase.

Figure 3.The duration of each outburst (dots) with time(using the minimum dates as identified from the fitting process). There is a trend of shorter outbursts with time (linear model indicated with the violet dashed line). They also seem to occur more frequently, as it is indicated by the time difference between the outbursts (violet arrows).

arXiv: 2112.13158

Contributions to the 15th Hellenic Astronomical Conference

During the 15th Hellenic Astronomical Conference I had the opportunity to present advances in two major topics:

1. Applying machine-learning methods to build a photometric classifier for massive stars in nearby galaxies”  (talk)
2. “Bouncing against the Yellow Void – the case of rho Cas” (poster)

I have also contributed to the following works:

by E. Kyritsis on “A new automated tool for the spectral classification of OB stars”
by Stephan de Wit on “Spectral analysis of evolved massive stars in the SMC and LMC”

New Paper: Evolved Massive Stars at Low-metallicity III. A Source Catalog for the Large Magellanic Cloud

Evolved Massive Stars at Low-metallicity III. A Source Catalog for the Large Magellanic Cloud

Ming Yang, Alceste Z. Bonanos, Biwei Jiang, Jian Gao, Panagiotis Gavras, Grigoris Maravelias, Shu Wang, Xiao-Dian Chen, Man I Lam, Yi Ren, Frank Tramper, Zoi T. Spetsieri

We present a clean, magnitude-limited (IRAC1 or WISE115.0 mag) multiwavelength source catalog for the LMC. The catalog was built upon crossmatching (1′′) and deblending (3′′) between the SEIP source list and Gaia DR2, with strict constraints on the Gaia astrometric solution to remove the foreground contamination. The catalog contains 197,004 targets in 52 different bands including 2 ultraviolet, 21 optical, and 29 infrared bands. Additional information about radial velocities and spectral/photometric classifications were collected from the literature. 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. After applying modified magnitude and color cuts based on previous studies, we identify and rank 2,974 RSG, 508 YSG, and 4,786 BSG candidates in the LMC in six CMDs. The comparison between the CMDs of the LMC and 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, AGB, and RGs) are located, which is likely due to the effect of metallicity and SFH. Further quantitative comparison of colors of massive star candidates in equal absolute magnitude bins suggests that, there is basically no difference for the BSG candidates, but large discrepancy for the RSG candidates as 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, and also the age effect. The Teff of massive star populations are also derived from reddening-free color of (JKS)0. The Teff ranges are 3500<Teff<5000 K for RSG population, 5000<Teff<8000 K for YSG population, and Teff>8000 K for BSG population, with larger uncertainties towards the hotter stars.

New paper: Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the Small Magellanic Cloud

Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the Small Magellanic Cloud

Ming Yang, Alceste Z. Bonanos, Bi-Wei Jiang, Jian Gao, Panagiotis Gavras, Grigoris Maravelias, Shu Wang, Xiao-Dian Chen, Frank Tramper, Yi Ren, Zoi T. Spetsieri, Meng-Yao Xue

We present the most comprehensive RSG sample for the SMC up to now, including 1,239 RSG candidates. The initial sample is derived based on a source catalog for the SMC with conservative ranking. Additional spectroscopic RSGs are retrieved from the literature, as well as RSG candidates selected from the inspection of CMDs. We estimate that there are in total ∼ 1,800 or more RSGs in the SMC. We purify the sample by studying the infrared CMDs and the variability of the objects, though there is still an ambiguity between AGBs and RSGs. There are much less RSGs candidates (∼4%) showing PAH emission features compared to the Milky Way and LMC (∼15%). The MIR variability of RSG sample increases with luminosity. We separate the RSG sample into two subsamples (“risky” and “safe”) and identify one M5e AGB star in the “risky” subsample. Most of the targets with large variability are also the bright ones with large MLR. Some targets show excessive dust emission, which may be related to previous episodic mass loss events. We also roughly estimate the total gas and dust budget produced by entire RSG population as ∼1.9(+2.4/−1.1)×10−6 M⊙/yr in the most conservative case. Based on the MIST models, we derive a linear relation between Teff and observed J−KS color with reddening correction for the RSG sample. By using a constant bolometric correction and this relation, the Geneva evolutionary model is compared with our RSG sample, showing a good agreement and a lower initial mass limit of ∼7 M⊙ for the RSG population. Finally, we compare the RSG sample in the SMC and the LMC. Despite the incompleteness of LMC sample in the faint end, the result indicates that the LMC sample always shows redder color (except for the IRAC1−IRAC2 and WISE1−WISE2 colors due to CO absorption) and larger variability than the SMC sample.

arXiv.org: 2005.10108

New paper: Evolved Massive Stars at Low-metallicity I. A Source Catalog for the Small Magellanic Cloud

Evolved Massive Stars at Low-metallicity I. A Source Catalog for the Small Magellanic Cloud

Ming Yang, Alceste Z. Bonanos, Bi-Wei Jiang, Jian Gao, Panagiotis Gavras, Grigoris Maravelias, Yi Ren, Shu Wang, Meng-Yao Xue, Frank Tramper, Zoi T. Spetsieri, Ektoras Pouliasis

We present a clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog for the SMC with 45,466 targets in total, with the purpose of building an anchor for future studies, especially for the massive star populations at low-metallicity. The catalog contains data in 50 different bands including 21 optical and 29 infrared bands, ranging from the ultraviolet to the far-infrared. Additionally, radial velocities and spectral classifications were collected from the literature, as well as infrared and optical variability statistics were retrieved from different projects. The catalog was essentially built upon a 1′′ crossmatching and a 3′′ deblending between the SEIP source list and Gaia DR2 photometric data. Further constraints on the proper motions and parallaxes from Gaia DR2 allowed us to remove the foreground contamination. We estimated that about 99.5\% of the targets in our catalog were most likely genuine members of the SMC. By using the evolutionary tracks and synthetic photometry from MIST and the theoretical J−KS color cuts, we identified 1,405 RSG, 217 YSG and 1,369 BSG candidates in the SMC in five different CMDs, where attention should also be paid to the incompleteness of our sample. We ranked the candidates based on the intersection of different CMDs. A comparison between the models and observational data shows that the lower limit of initial mass for the RSGs population may be as low as 7 or even 6 M⊙ and the RSG is well separated from the AGB population even at faint magnitude, making RSGs a unique population connecting the evolved massive and intermediate stars, since stars with initial mass around 6 to 8 M⊙ are thought to go through a second dredge-up to become AGBs. We encourage the interested reader to further exploit the potential of our catalog

arXiv.org: 1907.06717

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).

New paper on Magellanic Supergiants – Disk tracing for B[e] supergiants in the Magellanic Clouds

Disk tracing for B[e] supergiants in the Magellanic Clouds

G. Maravelias, M. Kraus, A. Aret

B[e] supergiants are evolved massive stars with a complex circumstellar environment. A number of important emission features probe the structure and the kinematics of the circumstellar material. In our survey of Magellanic Cloud B[e] supergiants we focus on the [OI] and [CaII] emission lines, which we identified in four more objects.

arXiv:1507.08443

Closest O type stars

Reading the book of F.D. Seward and P.A. Charles, “Exploring the X-ray Universe”, I found out some really massive and luminous O type stars that are close by and can be pointed out easily at the night sky:

star distance (kpc) magnitude (V) spectral type
ζ Pup 0.44 2.3 O4If
δ Ori 0.5 2.2 O9.5II
θ Ori C (*) 0.5 5.1 O6V
ζ Ori 0.5 1.6 O9.7Ib
η Car (*) 2.6 -1 to 7 LBV

(*): binary systems

How big a star can be?

It seems that this question has no final answer yet. Although the recent models were claiming that no star bigger than 100-150 solar masses can exist, there is evidence that a star close to 300 solar masses exists in the Large Magellanic Cloud. One star, R136a1 (from the young cluster RMC 136a or R136), is considered to have a mass ~265 solar masses (with a birthweight close to 320 solar masses).

More on ESO’s announcement (eso1030) & the paper (2010arXiv1007.3284C / Crowther P.A. et al, The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150Msolar stellar mass limit, MNRAS, 408, p 731, 2010)