maravelias.info

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.

June 2015 has been quite busy as I participated in two conferences:

1. The Physics of Evolved Stars (Nice, France; June 8-12, 2015) – poster contribution

   Title: “Disk tracing for B[e] supergiants in the Magellanic Clouds”

   Abstract: “The B[e] supergiants are an important short-lived transition phase in the life of massive stars, in which enhanced mass-loss leads to a complex circumstellar environment containing atomic, molecular and dust regions of different temperatures and densities. A number of important emission features probe the structure and the kinematics of the circumstellar material. We focus on the [OI] and [CaII] emission lines in comparison to our previous work (Aret et al. 2012), which we further extend by doubling the number of B[e] supergiants studied.”

   In this work we (myself, Michaela Kraus, and Anna Aret) presented our recent results obtained from FEROS observations on Magellanic B[e] Supergiants. We have identified the [CaII] lines in another 4 sources and along with other important disk tracers we show their spectra for the first time. Moreover, for a few number of sources we investigated their spectral variability and some results are under way.

   A couple of photos of our (mine and Anna’s) posters follow:

   

   

2. The 12th Hellenic Astronomical Conference (Thessaloniki, Greece; June 28 – July 2, 2015) – oral contribution

   Title: “Tracing the disks around B[e] supergiants in the Magellanic Clouds”

   Abstract: “Massive stars affect strongly the insterstellar medium through their intense stellar winds, which transfer momentum and energy to the interstellar medium and enrich it with 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. Since these phases are not well-understood and not predicted accurately by theory observations are needed in order to understand the complex circumstellar environment around these stars.
   In particular, B[e] supergiants are often surrounded by rings or disk-like structures, combining atomic, molecular and dust regions of different temperatures and densities. Using high-resolution optical spectra, obtained with the FEROS instrument mounted at the MPG/ESO 2.2m telescope, we examine a number of important emission features which probe the structure and the kinematics of their circumstellar environment. We investigate the [OI] and [CaII] emission lines in comparison to our previous work (Aret et al. 2012), which we further extend by doubling the number of B[e]SG studied in the Magellanic Clouds.”

   Well, even though it is almost the same subject I did have the opportunity to discuss it with a totally different audience. Moreover, we had time to work on some kinematical modelling of the line profiles for which I presented some preliminary results.

   There are no photos to follow (even though I carried my camera and there were so many smart phones!).

On Thursday 14th of May 2015 I had the unique opportunity to give a talk about Astronomy at the high school I graduated from in Aigaleo (Athens, Greece). As I have moved away from Aigaleo since 2007 and my visits in Athens are scarce the least, it is difficult to think of such an event (which should account for the other side’s interest too!).

An invitation was sent to the Hellenic Amateur Astronomy Association (HAAA) to ask for the possibility to organize an astronomical event at the 1st high school of Aigaleo. Astronomy was one of the supplementary courses at the high school to be chosen freely by students. However, it is one of the courses discarded recently after major changes in the curriculum. Fortunately, there are still (some) professors who spend extra hours with their students to offer them this opportunity (outside their work schedule and the curriculum of course!). The HAAAA tries to help bridge this gap by providing experienced outreach speakers and telescopes for public observing.

[There is not enough space and actually it is outside of the scope of this post to describe the activities of the Hellenic Amateur Astronomy Association (perhaps I should make one? in the mean time take a look at the poster of Voutyras et al., “10 Years of Developing Outreach Techniques and Best Practice by the Hellenic Amateur Astronomy Association”, European Planetary Science Congress 2013, held 8-13 September in London, UK, but astronomical events for the public and students are routinely organized, always including at least a talk and observation through telescope].

However, due to many other obligations at the time the event was difficult to organize. Moreover, the professor and the students were unwilling to postpone the whole event for later this year, since they had worked on the topic so far and the academic year was reaching its end. As I was planning to visit Crete a few days later (than the initial date they proposed) I though that it would be a great opportunity for me to give this talk! And since I had a connecting flight in Athens perhaps I could change my flight to Crete for the next day and spend more time at the event. After some mail exchanges the school agreed to change the day so it was left to me to decide if I could attend it or not. However, changing the ticket proved to be more expensive than what I had paid already for! In that case another speaker should be found.

But … would I spend 3 hours (doing nothing…) at the airport while the event would took place anyhow at my school? Of course not! So, a radical decision was made: I would ask from a friend to come and get me from the airport, head down to Aigaleo (~40 min by motorbike), spend about an hour there, and return me again at the airport (hopefully) on time to catch the plane for Crete!

Manos Kardasis (a friend with who I have shared many “astronomical adventures”) was the “volunteer” to become the “taxi” motorbike. I arrived at the Athens airport at the 19:45 and he was waiting to pick me up and drive to Aigaleo. Around 20:25 we reached the school. Fortunately, all students were there and after the necessary introductions with the professor we managed to gather all people inside the room within 10 min (probably a record time for such events!), as I could not spend more than an hour there. It was about 20:35 when I started my talk.

I began by presenting a short bio of myself, not due to any selfish reason, but mainly to show them and stress the fact that I have graduated from the very same school (something they didn’t know beforehand!). That of course raised some cheers by the students. I went on to speak about Astronomy and more specifically about “Massive Stars … and some interested cases”.

It is usual in most public talks of this kind to present more general subjects (like our Solar system). I had decided to speak about what I am currently working on (such as the B[e] supergiants and the High-Mass X-ray Binaries), how they are connected with massive stars, and why the latter are important in our understanding of stellar and galactic evolution. And yes, I did included images that are more appropriate for professional audience (only a few though!). I know that this was too much information to be understood by high school students but, at the time, I though that I could give them just a taste of real scientific aspects (and not just pretty images or graphics). After all, these students do not have often opportunities to listen to and, moreover, to interact with scientists.

I planned for a short talk (~20-30 min) to allow more time for questions and interaction with the students, which was actually the part that I was anticipating. And indeed it was the most interesting with questions ranging from which professors I had when I was in school (obviously we had many in common!), up to Hawking radiation and stellar evolution simulations (!).

The time passed quickly and by 21:30 the telescopes were ready outside, waiting for the students, while we had to ride back to the airport. Even though the bike’s temperature alarm was on, we fortunately managed to get to the airport on time (around 22:10), so I was able to pass security, go to gate and relax a bit before my flight (at 22:55).

Even though it was indeed a very tight plan I really enjoyed it. It was great that I returned back to my school (after 17 years !) to make a presentation on what I am currently working on as an astronomer. Even more important I am pleased by the fact that I was able to give back something to a place that I know well that these opportunities are rare.

I am deeply indebted to Manos Kardasis for his unconditional commitment to help with the materialization of this plan (and for the photos also!), and Manos Vourliotis who organized the event.