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Seminar talk at NOA

As I am waiting my flight to Heraklion, I thought that I should post the talk that I gave earlier today at the National Observatory of Athens, Greece:

“B[e] Supergiants: a missing part from the massive star puzzle?”

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. Consequently, 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. Moreover, their possible link to other phases has not been fully uncovered yet, which is crucial given the importance of massive stars for Stellar Astrophysics and their influence on their host galaxies. Starting from an observational point of view, I will provide an overview of the B[e] Supergiant population and discuss the latest results.

A short talk for IFA-UV Monday meetings

Every Monday the IFA-UV organizes a meeting that starts with a short (or longer) presentation of 20 mins. The whole process requires the active participation by the students to present the speaker, control, the discussion, while there is also a feedback given by the audience to the speaker. This is a great opportunity for the students to help them gain experience especially when they have to present. However, the topics (and the stage) is open to everyone. And this Monday was my turn to present a short talk on:

“Searching for Hα counterparts of Be/X-ray binaries in the Small Magellanic Cloud”

Abstract: The Small Magellanic Cloud (SMC) hosts a large number of high-mass X-ray binaries, and in particular of Be/X-ray Binaries (BeXRBs; neutron stars orbiting OBe-type stars), offering a unique laboratory to address the effect of metalicity. One key property of their optical companion is Hα in emission, which makes them bright sources when observed through a narrow-band Hα filter. We performed a survey of the SMC Bar and Wing regions using wide-field cameras (WFI@MPG/ESO and MOSAIC@CTIO/Blanco) in order to identify the counterparts of the sources detected in our XMM-Newton survey of the same area. 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 the fraction of OBe/OB stars to be 13% down to this limit, and by investigating this fraction as a function of the brightness of the stars we deduce that Hα excess peaks at the O9-B2 spectral range. Using the most up-to-date numbers of SMC BeXRBs we find their fraction over their parent population to be ~0.002-0.025 BeXRBs/OBe, a direct measurement of their formation rate.

After the talks it comes another important step which is the wine and cheese ceremony, accompanying the informal discussions.

An IFA-UV seminar talk

Yesterday, I had finally the opportunity to give a seminar talk at the Instituto de Física y Astronomía de la Universidad de Valparaíso, where I am currently based (and only for about a month more …)

“What have we learned from observations of B[e] Supergiants ?”

Abstract:
B[e] Supergiants are a rare phase among the massive stars, displaying a complex circumstellar environment. However, they may provide an important link to other phases (e.g. the Yellow Hypergiants, the Luminous Blue Variables). Given the importance of massive stars for Stellar Astrophysics and their influence on their host galaxies it is critical to understand their evolution. Starting from an observational point of view, I will provide an overview of the B[e] Supergiants and discuss the latest results.

The 2016 Massive Stars meeting in New Zealand

Auckland

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).

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!

Me and

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].


bla

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.

Tartu Observatory talk

On Wednesday, May 11, I gave a talk at Tartu Observatory (Estonia), where I am spending a few weeks as a visitor.
Of course this is by no means something extraordinary. However, it is interesting that they could record the talk so it is available for others to see (or, most probable, for my own use).

So, the link is here:
Grigoris Maravelias: “The circumstellar environment of B[e] Supergiants – disks or rings?”

The circumstellar environment of B[e] Supergiants – disks or rings?

Massive stars affect strongly their environment 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. However, these phases are not well-understood, such as the lack of B[e] Supergiants predicted from stellar evolution theory. In order to improve our knowledge for the particular class of B[e] Supergiants we have initiated a campaign to study them with high-resolution optical and near-infrared spectroscopy. This tool allow us to investigate their complex circumstellar environment, consisting of a combination of atomic, molecular and dust regions of different temperatures and densities. We use the strategic [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.

Initial Source: Tartu Observtory / seminars

Back to (my) school!

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.

Some shots during the presentation

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”.

back2mySchool-4

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.

back2mySchool-1

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 (!).

back2mySchool-3

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.

Talk at the AsU Summer Interdepartmental Meeting 2015

Every three months there is a meeting among the departments of the Astronomical Institute. Each director presents some news in short, followed by a talk from a member of that department. At the Summer Interdepartmental Meeting of this year (held on Monday, 1st of June, 2015, in Ondrejov) I was “representing” the Stellar department, talking about:

“High-Mass X-ray Binaries in the Small Magellanic Cloud”

The abstract reads:
High-Mass X-ray Binaries (HMXBs) are a phase in the life of some binary stellar systems that consist of a compact object (black hole or neutron star) and a massive companion (an early OB-type star). Their X-ray emission is powered by the infall of matter, provided my the massive companion, into the strong gravitational field of the compact star. The Small Magellanic Cloud (SMC) is a powerhouse of HMXB production (almost 100 systems), and due to its proximity we are able to investigate individual sources. However, we haven’t yet fully characterize its HMXB population. To address that we have initiated wide spectroscopic and Halpha imaging campaigns. I will discuss our results and how the SMC HMXB population compares with that of our Galaxy.

UPDATE 17 July 2015: A summary of my talk has been published in Czech at the site of the Astronomical Institute, and it can be found at:
Astronomický ústav AV ČR: “Pokroky ve výzkumu ve Stelárním oddělení: Studium superhmotných rentgenových dvojhvězd”.