maravelias.info

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.

IAUS 366, 2022 (NASA/ADS link)

Finally after two years of pandemic and two postponements of the IAU Symposium 361: “Massive Stars Near and Far” it was held in Ballyconnell, Ireland in the week of 8-13 May 2022.

It was really a great opportunity. Not only I managed to meet in-person people and collaborators that I met through zoom meetings but actually it was the first time (since 2020) to meet also my colleagues working in the ASSESS project (while some of them for the first time!).

The event was of course a great place to present our latest results. I got the privilege to have a talk about our latest results from the machine-learning classifier we have developed (see this post).

• Talk: “Introducing the ASSESS project: Episodic Mass Loss in Evolved Massive Stars – Key to Understanding the Explosive Early Universe” – Alceste Bonanos
• Talk: “Using machine-learning to investigate the populations of dusty evolved stars in various metallicities” – Grigoris Maravelias
• Poster: “Progenitors and companions of stripped-envelope supernovae” – Manos Zapartas
• Poster: “Properties of dusty Red Supergiant stars in the Magellanic Clouds” – Stephan de Wit
• Poster: “ASSESSing evolved massive stars in NGC 6822 and IC 10” – Gonzalo Munoz Sanchez

The week 1-5 of November 2021, I (virtually) participated to the IAU Symposium 366 on the origin of outflows in evolved stars. I had the opportunity to present our recently submitted work on a photometric machine-learning classifier.

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

I have also contributed to the following works:

Although the summer has finished long ago, only now I got some time to update on summer activities, i.e. a number of conferences I attended and contributed to.

Since 2018, I have been working in an automated classifier for massive stars in nearby galaxies, using photometric datasets. These have been produced by my colleagues within the ASSESS team, an ERC project led by Alceste Bonanos at the National Observatory of Athens, and I have been responsible to develop a machine learning method to achieve this. We have made a lot of progress and we have reached to the point that the results are almost final (working now on the Maravelias et al. paper). So, this work has been presented in:

1. A poster presentation at the Supernova Remnants II, Chania, Greece, 3-8 June 2019,
   as “Identifying massive stars in nearby galaxies, in a smart way”
2. A talk, done by Frank Tramper due to my unavailability to attend the 14th Hellenic Astronomical Conference, Volos, Greece, 8-11 July 2019,
   as “Automated classification of massive stars in nearby galaxies”
3. A talk at the Computational Intelligence in Remote Sensing and Astrophysics, FORTH workshop, Heraklion, Greece, 17-19 July 2019,
   as “An automated classifier of massive stars in nearby galaxies”
4. A remote talk for the ASTROSTAT 2nd Consortium meeting, Boston, USA, 18-19 July 2019,
   as “Towards an automated classifier of massive stars in nearby galaxies”

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

Abstract:
Current photometric surveys can provide us with multiwavelength measurements for a vast numbers of stars in many nearby galaxies. Although the majority of these stars are evolved luminous stars (e.g. Wolf-Rayet, Blue/Yellow/Red Supergiants), we lack an accurate spectral classification, due to the demands that spectroscopy faces at these distances and for this number of stars. What we can do instead is to take advantage of machine learning algorithms (such as Support Vector Machines, Random Forests, Convolutional Neural Networks) to build an automated classifier based on a large multi-wavelength photometric catalog. We have compiled such a catalog based on optical (e.g. Pan-STARRS, OGLE) and IR (e.g. 2MASS, Spitzer) surveys, combined with astrometric information from the GAIA mission. We have also gathered spectroscopic samples of massive stars for a number of nearby galaxies (e.g. the Magellanic Clouds, M31, M33) and by using our algorithm we have achieved a success ratio of more than 80% for the training and test samples. By applying the fully trained algorithm to the available photometric datasets, we can uncover previously unclassified sources, which will become our prime candidates for spectroscopic follow-up aiming to confirm their nature and our approach.

Also Ming has presented his work in a couple of conferences:

1. As a poster presentation at the Supernova Remnants II, Chania, Greece, 3-8 June 2019,

“Evolved Massive Stars at Low-metallicity: 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, Stephan A. S. de Wit

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 datasets. 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. 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 masses for the RSGs population may be as low as 7 or even 6 M⊙, 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.

2. As a talk at the ESO workshop “A synoptic view of the Magellanic Clouds: VMC, Gaia and beyond”, Garching near Munich, Germany, September 9-13, 2019

“Evolved Massive Stars and Red Supergiant Stars in the Magellanic Clouds”

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, and Stephan de Wit

We present an ongoing investigation of infrared properties, variabilities, and mass loss rate (MLR) of evolved massive stars in the Magellanic Clouds, especially the red supergiant stars (RSGs). For the LMC, 744 RSGs compiled from the literature are identified and analysed by using the color-magnitude diagram (CMD), spectral energy distribution (SED) and mid-infrared (MIR) variability, based on 12 bands of near-infrared (NIR) to MIR co-added data from 2MASS, Spitzer and WISE, and ∼6.6 yr of MIR time-series data collected by the ALLWISE and NEOWISE-R projects. The results show that there is a relatively tight and positive correlation between the brightness, MIR variability, MLR, and the warm dust or continuum, where both the variability and the luminosity may be important for the MLR. The identified RSG sample has been compared with the theoretical evolutionary models and shown that the discrepancy between observation and evolutionary models can be mitigated by considering both variability and extinction. For the SMC, we present a relatively clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog with 45,466 targets in total, intending to build an anchor for the future studies, especially the massive stars at low-metallicity. It contains data in 50 different bands including 21 optical and 29 infrared bands, retrieved from SEIP, VMC, IRSF, AKARI, Heritage, Gaia, SkyMapper, NSC, Massey et al. (2002), and GALEX, ranging from the ultraviolet to the far-infrared. Additionally, radial velocities and spectral classifications are collected from the literature, as well as the infrared and optical variability information derived from WISE, SAGE-Var, VMC, IRSF, Gaia, NSC, and OGLE. The catalog is essentially built upon a 1” crossmatching and a 3” deblending between the Spitzer Enhanced Imaging Products (SEIP) source list and Gaia Data Release 2 (DR2) photometric data. Further constraints on the proper motions and parallaxes from Gaia DR2 allow us to remove the foreground contamination. We estimate that about 99.5% of the targets in our catalog are likely to be the genuine members of the SMC. By using the evolutionary tracks and synthetic photometry from MESA Isochrones & Stellar Tracks and the theoretical J−Ks color cuts, we identify 1,405 red supergiant, 217 yellow supergiant and 1,369 blue supergiant candidates in the SMC in five different CMDs. We rank the candidates based on the intersection of the different CMDs. A comparison between the models and observational data shows that, the lower limit of the RSGs population may reach to 7 or even 6M⊙, making RSGs an unique population connecting the evolved massive and intermediate stars, since stars with initial mass around 6 to 8M⊙ are thought to go through a second dredge-up to become asymptotic giant branch stars. We encourage the interested reader to further exploit the potential of our catalog, including, but not limited to, massive stars, supernova progenitors, star formation history and stellar population. Detailed analysis and comparison of RSGs in the LMC and SMC may be also presented depending on the progress of the investigation.

During a session – from a galaxy far far away – at the Supernova Remnants II conference in Chania, Greece 2019 (photo/editing by A. Manousakis).

Working and supporting the Supernova Remnants II conference in Chania, Greece 2019 (photo by A. Manousakis).

Auckland

The Massive Stars meeting of 2016 takes place in Auckland, in New Zealand. I was fortunate enough to be given the opportunity to present two works, one poster and one talk during the splinter session on X-rays from massive stars. I have been working hard to finish them, producing the final results almost a day before my presentation. Fortunately, everything went fine and then since Tuesday I am enjoying the conference more relaxed!

1. “The circumstellar environment of B[e] Supergiants” | nzstars2016-poster (pdf)

G. Maravelias, M. Kraus, L. Cidale, M. L. Arias, A. Aret, M. Borges Fernandes

Abstract: Massive stars affect strongly the insterstellar medium through their intense stellar winds and their rich chemically processed material as they evolve. This interaction becomes substantial in short-lived transition phases of massive stars (e.g. B[e] Supergiants, Luminous Blue Variables, Yellow Hypergiants) in which mass-loss is more enhanced and usually eruptive. A complex environment, combining atomic, molecular and dust regions, is formed around these stars. In particular, the circumstellar environment of B[e] Supergiants is not well understood. To address that, we have initiated a campaign to investigate these environments for a sample of Galactic and Magellanic Cloud sources. Using high-resolution optical and near-infrared spectra (using MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively), we examine a set of emission features ([OI], [CaII], CO bandheads) to trace their physical conditions and kinematics in their formation regions. We find that the B[e] Supergiants are surrounded by a series of single and/or multiple equatorial rings, of different physical conditions (temperature, density), a probable result of previous mass-loss events. In many cases the CO forms very close to the star, while we notice also an alternate mixing of densities and temperatures (which give rise to the different emission features) along the equatorial plane.

A photo of my poster (not very well illuminated).

2. “Hα imaging for BeXBs in the Small Magellanic Cloud” | nzstars2016-talk (pdf)

G. Maravelias, A. Zezas, V. Antoniou, D. Hatzidimitriou, F. Haberl

High-Mass X-ray Binaries consist of an early-type (OB) massive star and a compact object (neutron star or black hole), which accretes matter from the massive star either through strong stellar winds and/or Roche-lobe overflow in supergiant systems or through an equatorial decretion disk in, non-supergiant, OBe stars (Be X-ray Binaries;BeXBs). Due to these disks the BeXBs display strong Balmer line emission in their optical spectra. At the same time they are among the brightest sources when observed with narrow-band Ηα imaging. The Small Magellanic Cloud (SMC) hosts a large number of BeXBs (almost 100) and offers a unique laboratory to examine these sources with a homogenous and consistent approach. Driven by this, we performed an Hα survey of the SMC (covering both the Bar and the Wing regions) using wide-field cameras (WFI at the MPG/ESO 2m, and MOSAIC at the CTIO/Blanco 4m telescopes). We obtained broad-band R and narrow-band Hα photometry, and identified ~10000 Hα emission sources down to a sensitivity limit of 18.7 mag (equivalent to ∼B8 type Main Sequence stars). We find that OBe stars (the parent population of BeXBs) are 13% of the total OB star population in the SMC down to 18.7 mag, and by investigating this fraction as a function of the brightness of the stars we deduce that Hα excess peaks at spectral range O9-B2. Using the most up-to-date numbers of BeXBs in the SMC we find their fraction with respect to the OBe stars to be in the range ∼ 0.5 − 1.4 × 10^(−3) BeXB/OBe, a direct measurement of the formation rate of BeXBs in the SMC.

Conferences are always a great place to meet old friends and make new ones. The two following photos show exactly that!

The 2016 Ondrejov AsU post-doc Alumni they find each other at the other side of the Earth! From left to right: GM, Anthony Herve, and Mary Oksala [CC-BY-SA-NC].

Meeting my new colleagues at the University of Valparaiso, Chile. From left to right: Ignacio Araya, Catalina Arcos, Alex Gormaz-Matamala, and Michel Cure. Taken from level 51 of Auckland’s Sky Tower, at 186 m high[CC-BY-SA-NC].

UPDATE 22/02/2017: The proceedings paper on “Hα imaging for BeXBs in the Small Magellanic Cloud” has been uploaded at arXiv:1702.04606.

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).

The very short and compact meeting of the Royal Astronomical Society “Bridging the gap: from massive stars to supernovae” was held in Kavli Royal Society Centre of Chicheley Hall, in the English countryside of Buckinghamshire (June 1-2). Even though it lasted two days only, a number of excellent talks was delivered discussing the connection or the gap (depending on the perspective) between the massive stars and supernovae, proving that we know some things but a lot has to be done.

I had the opportunity to present a poster (pdf file) entitled: “B[e] Supergiants’ circumstellar environment: disks or rings?” with the following abstract:

Extreme mass-loss activity characterize some phases of evolution of massive stars (such as Luminous Blue Variables, Yellow Hypergiants, B[e] Supergiants), which strongly affects the stars and their circumstellar environment almost before they become Supernovae. Currently, our understanding of these phases it not well-established, such as the lack of B[e] Supergiants (B[e]SGs) predicted from stellar evolution theory. In order to improve our knowledge for the particular class of B[e]SGs we have initiated a campaign to investigate the structure of their circumstellar material, which consists of a complex combination of atomic, molecular and dust regions of different temperatures and densities. 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 examine 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 in a series of single and/or multiple equatorial rings, a probable result of previous mass-loss events.

My poster displayed.

A view of Chicheley Hall.