The distribution and ages of star clusters in the Small Magellanic Cloud: Constraints on the interaction history of the Magellanic Clouds

T. Bitsakis, R. A. Gonzalez-Lopezlira, P. Bonfini, G. Bruzual, G. Maravelias, D. Zaritsky, S. Charlot, V. H. Ramirez-Siordia

We present a new study of the spatial distribution and ages of the star clusters in the Small Magellanic Cloud (SMC). To detect and estimate the ages of the star clusters we rely on the new fully-automated method developed by Bitsakis et al. (2017). Our code detects 1319 star clusters in the central 18 deg2 of the SMC we surveyed (1108 of which have never been reported before). The age distribution of those clusters suggests enhanced cluster formation around 240 Myr ago. It also implies significant differences in the cluster distribution of the bar with respect to the rest of the galaxy, with the younger clusters being predominantly located in the bar. Having used the same set-up, and data from the same surveys as for our previous study of the LMC, we are able to robustly compare the cluster properties between the two galaxies. Our results suggest that the bulk of the clusters in both galaxies were formed approximately 300 Myr ago, probably during a direct collision between the two galaxies. On the other hand, the locations of the young (≤50 Myr) clusters in both Magellanic Clouds, found where their bars join the HI arms, suggest that cluster formation in those regions is a result of internal dynamical processes. Finally, we discuss the potential causes of the apparent outside-in quenching of cluster formation that we observe in the SMC. Our findings are consistent with an evolutionary scheme where the interactions between the Magellanic Clouds constitute the major mechanism driving their overall evolution.

2018 ApJ, 853, 104 / NASA/ADS / arXiv: 1712.04974

This is a follow-up paper from the initial work on the Large Magellanic Cloud.

The first neutron star merger

By grigoris | Filed in Astronomy

A new era, of multi-messenger astronomy (combining gravitational waves, particles, photons), has been born today. LIGO/VIRGO plus 70 teams around the world announced the first detection of colliding neutron stars, which means that a gravitational wave detection (GW170817) triggered the observations using almost everything available (from gamma rays to radio wavelengths) that identify and follow its electromagnetic counterpart.

I feel very fortunate that I participated in the above exciting event, along with Thodoris Bitsakis and Carlson Adams, by obtaining spectroscopic and photometric observations from the Clay telescope (at Las Campanas Observatory), a few days after the event. These are included in the following papers published in Science:

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis

M. R. Drout, A. L. Piro, B. J. Shappee, C. D. Kilpatrick, J. D. Simon, C. Contreras, D. A. Coulter, R. J. Foley, M. R. Siebert, N. Morrell, K. Boutsia, F. Di Mille, T. W.-S. Holoien, D. Kasen, J. A. Kollmeier, B. F. Madore, A. J. Monson, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, T. C. Beers, R. A. Bernstein, T. Bitsakis, A. Campillay, T. T. Hansen, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, C. Moni Bidin, J. L. Prieto, K. C. Rasmussen, C. Rojas-Bravo, A. L. Strom, N. Ulloa, J. Vargas-González, Z. Wan, D. D. Whitten

On 17 August 2017, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the universe.

2017, Sci, 358, 1570 / NASA/ADS / Science 16 Oct 2017, eaaq0049

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger

B. J. Shappee, J. D. Simon, M. R. Drout, A. L. Piro, N. Morrell, J. L. Prieto, D. Kasen, T. W.-S. Holoien, J. A. Kollmeier, D. D. Kelson, D. A. Coulter, R. J. Foley, C. D. Kilpatrick, M. R. Siebert, B. F. Madore, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, R. A. Bernstein, T. Bitsakis, K. Boutsia, J. R. Bravo, F. Di Mille, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, V. M. Placco, G. Prieto, Z. Wan

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from 11000(+3400,-900) Kto 9300(+300,-300) K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.

2017, Sci, 358, 1574 / NASA/ADS / Science 16 Oct 2017, eaaq0186

[1] About GW170817

The last day at Valparaiso

By grigoris | Filed in Astronomy

I think it had everything:

– waking up early to go to the bank,
– then go to the university and finalize some aspects of my talk (as a final contribution),
– go for lunch with the whole group,
– return to the university,
– to say goodbye to people,
– do the presentation,
– materialize a long-postponed discussion,
– show/discuss a script,
– buy some small gifts on the way home,
– where I just dropped my things,
– to go to an outreach talk (in spanish),
– to continue for the last drinks with all the friends from the university,
– say farewell to all,
– return home to take a shower
– and rest a bit,
– to finally think of today
– and write this post.

Definitely not the most typical day, but it isn’t after all. It is strange as I remember my first days in Valpo, almost 9 months before. It was short but nevertheless a very full experience and I loved it all. I have only a couple of days to spend at Santiago before the final departure on Thursday, to head to Greece. A process ends but another starts.

For lunch with the group (clockwise from left): Ignacio Araya, Michel Cure, Lydia Cidale, Maxi Haucke, Catalina Arcos, Alex Gormaz, and me (I do not smile because I really couldn’t keep my eyes open due to the Sun!)
[Photo by Ignacio]

A memory from Valparaiso.
Gracias a tod@s por todo!
[Photo by Michel]

A short talk for IFA-UV Monday meetings

By grigoris | Filed in Astronomy

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.

Extrapolating for the new post-doc position

By grigoris | Filed in Astronomy

The idea of this post came to me on my second or third day in Chile when I noticed its flag on some boats. As today is my last day at work, perhaps it is the best (and last) opportunity to make a post about it. (So, practically it is either now or never.)

My previous post-doc position was at Ondrejov village, very close to Prague, Czech Republic. My current position is at Valparaiso of Chile. Now let us place their flags next to each other :

Czech Republic Chile

Taking into account the places also we (at least I) notice some similarities:
I. Both countries start with “C”: Czech Republic and Chile.
II. Each flag is split in three regions with the same colors, i.e. blue on the left, white on top and red at the bottom. [The small white star in Chile’s flag is not statistically important compared to the whole flag pattern.]
III. Both places are close to an UNESCO monument city, i.e. Prague and Valparaiso.
IV. There should be an appropriate workplace in Astronomy (e.g. the Astronomical Institute of the Czech Academy of Sciences, the Universidad de Valparaiso).

So, given that I have been to post-doc positions in places with those characteristics, then it is reasonable (what??!!??!) to extrapolate my future track. Let’s see where can I be next.

First, countries starting with “C”:
1. There is Cuba with a slightly different like flag, with alternate blue and white stripes. Havana is an UNESCO heritage city. But there are no job offers in Astronomy. So it is 2.712/4.000 criteria satisfied.

2. Next, there is Croatia, with a different style of flag, with stripes (from top to bottom: red, white, blue) and a big coat of arms. As far as UNESCO cities there is the famous city of Dubrovnik and Trogir, but again no Astronomical institute nearby (by the way I found this historic overview by Garaj 1999, ARSE Conf, 44). In this case we are at 2.187/4.000 criteria satisfied.

With these two countries we actually run out of “C” countries. Then we are left to explore similar flags among other countries.
3. Philippines has almost the same flag to Czech Republic but with only a slight color rearrangement between the triangle (white) and the top stripe (blue). [There is a small symbol within the white triangle but we consider that its significance is rather low compared to the importance of style and colors – similar to what we assumed for Chile’s flag.] There is the historic town of Vigan, but again no astronomical opportunities around this area. So approximately at 2.123/4.000 criteria satisfied.

And then we have practically run out of options, since all other flags have either similar styles but different colors (e.g. Jordan,Sudan), or they have the same colors but with total different styles, such as vertical (e.g. France, Thailand), horizontal stripes (e.g. Russia, Paraguay), and totally different patterns (e.g. USA, UK). So, even though we can find UNESCO cities and astronomical facilities in those countries, these score at most 2.111/4.000.

Thus, we (i.e. I) conclude that there is not an appropriate place (with a score more than 3.455/4.000) that I could continue as a post-doc based on my track so far. In lack of such an option I will have to move to Heraklion (Crete, Greece) up to further changes in the countries that will allow me to reconsider my options!


Proper acknowledgments

By grigoris | Filed in Astronomy

From a friend with a similar interest in martial arts. He noticed the following acknowledgment (by Zeng & Sasselov 2013, PASP, 125, 227):

We acknowledge partial support for this work by NASA cooperative agreement NNX09AJ50A (Kepler Mission science team).
We would like to thank Michail Petaev and Stein Jacobsen for their valuable comments and suggestions. This research is supported by the National Nuclear Security Administration under the High Energy Density Laboratory Plasmas through DOE grant # DE-FG52-09NA29549 to S. B. Jacobsen (PI) with Harvard University. This research is the authors’ views and not those of the DOE.
Li Zeng would like to thank Professor Pingyuan Li, Li Zeng’s grandfather, in the Department of Mathematics at Chongqing University, for giving Li Zeng important spiritual support and guidance on research. The guidance includes research strategy and approach, methods of solving differential equations and other numeric methods, etc.
Li Zeng would also like to give special thanks to Master Anlin Wang. Master Wang is a Traditional Chinese Kung Fu Master and World Champion. He is also a practitioner and realizer of Traditional Chinese Philosophy of Tao Te Ching, which is the ancient oriental wisdom to study the relation between the universe, nature and humanity. Valuable inspirations were obtained through discussion of Tao Te Ching with Master Wang as well as Qigong cultivation with him.

Personally I find it amazing because it shows the human side of the researchers. Our work does not depend only on the funding scheme! We should be/feel free to credit appropriately what we think.
Plus, I personally sympathize the sentences of the last paragraph …

Resolving the circumstellar environment of the Galactic B[e] supergiant star MWC 137 from large to small scales

Michaela Kraus, Tiina Liimets, Cristina E. Cappa, Lydia S. Cidale, Dieter H. Nickeler, Nicolas U. Duronea, Maria L. Arias, Diah S. Gunawan, Mary E. Oksala, Marcelo Borges Fernandes, Grigoris Maravelias, Michel Cure, Miguel Santander-Garcia

The Galactic object MWC 137 was suggested to belong to the group of B[e] supergiants. However, with its large-scale optical bipolar ring nebula and the high velocity jet and knots, it is a rather atypical representative of this class. We performed multi-wavelength observations spreading from the optical to the radio regime. Based on optical imaging and long-slit spectroscopic data we found that the northern parts of the large-scale nebula are predominantly blue-shifted, while the southern regions appear mostly red-shifted. We developed a geometrical model consisting of two double-cones. While various observational features can be approximated with such a scenario, the observed velocity pattern is more complex. Using near-infrared integral-field unit spectroscopy we studied the hot molecular gas in the close vicinity of the star. The emission from the hot CO gas arises in a small-scale disk revolving around the star on Keplerian orbits. While the disk itself cannot be spatially resolved, its emission is reflected by dust arranged in arc-like structures and clumps surrounding MWC 137 on small scales. In the radio regime we mapped the cold molecular gas in the outskirts of the optical nebula. We found that large amounts of cool molecular gas and warm dust embrace the optical nebula in the east, south and west. No cold gas or dust were detected in the north and north-western regions. Despite the new insights on the nebula kinematics gained from our studies, the real formation scenario of the large-scale nebula remains an open issue.

2017, AJ, 154, 186 / NASA/ADS / 1709.06439

Tough days the last days

By grigoris | Filed in Astronomy

A couple of hectic weeks finished on last Friday. After two trips to La Serena to observe from the magnificent facilities at CTIO and LCO (posts still pending since there is an overwhelming amount of photos and videos!), I had an intense week to finalize the paper I am preparing (for some looong time now).

At the same time and during the last week I had to prepare an invited review talk on “The circumstellar structures around B[e] supergiants” at the meeting “Massive Stars in Transition Phases” (Tôravere, Estonia, 11-15 Sep, 2017). I participated remotely that made me to get up at 4am each day, due to the time difference between Estonia and Chile (6 hours).

That alone wouldn’t have been so much of a problem if it wasn’t for a Marie Curie fellowship proposal, with a deadline on the same dates! Thankfully, I had most of all these ready to some extend, but still some fine (or more) tuning was needed.

Preparing for the remote talk: Three monitors, a laptop, and a desktop. Overkill? Possbile, but I split the different functions and it worked great.

After a few days of rest I am back to work, as I have only two and half weeks left before I leave Chile. And there are quite a few things to do …

Linking classical to dwarf novae

By grigoris | Filed in Astronomy

In a recent Nature (Shara et al. 2017, Nature, 548, 558) letter, the authors found that a current dwarf nova is actually the Nova Scorpii AD 1437, recorded by Korean astronomers at that time.

What is interesting is that they managed to trace both the dwarf nova’s position and the center of the observed gas shell back to their positions in 1437. Adding the information derived from the Korean records (and that is interesting to read) they show that the Nova Scorpii 1437 is the source of the current shell and that the dwarf nova in the region was the Nova. This shows that a classical nova, whose eruptions are due to the fusion and explosion of hydrogen accreted around its surface, can display dwarf nova behavior, i.e. eruptions due to instabilities in the accretion disk, providing a link between the two types of novae.

An IFA-UV seminar talk

By grigoris | Filed in Astronomy

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 ?”

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.