Massive evolved stars


The Luminous Blue Variable eta Carinae. [source, credit: Nathan Smith (University of California, Berkeley), and NASA.]

Massive stars (larger than 8 solar masses) affect strongly the insterstellar medium of their host galaxies through intense stellar winds, that transfer mass, momentum and energy, and enrich the interstellar medium 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. Even though the current stellar models can follow the observed stellar evolution not everything is certain yet (e.g. B[e] Supergiants are not predicted). In order to acquire a better understanding of these evolutionary phases we investigate the mass-loss history of these stellar populations in our Galaxy and the Magellanic Clouds.

We study their complex circumstellar material (which combines atomic, molecular and dust regions of different temperatures and densities) and how they formed. Moreover, photometric/spectroscopic variability can probable indicate mass-ejection mechanisms, a possible formation scenario. We also look into different phases (e.g. B[e] Supergiants and Yellow Hypergiants) to identify potential evolutionary links. For our research we are using state-of-the-art high-resolution spectrographs in the optical (e.g. FEROS/ESO) and infrared (e.g. SINFONI and CRIRES/ESO, Phoenix/GEMINI), as well as slit spectrographs (e.g. the 2m Perek telescope at Ondřejov observatory).

> Aret, A.; Kolka, I.; Kraus, M.; Maravelias, G. 2017, “The Yellow Hypergiant – B[e] Supergiant Connection”, (ASPC, 510, 162)
> Aret, A.; Kolka, I.; Kraus, M.; Maravelias, G. 2017, “A New Outburst of the Yellow Hypergiant Star ρ Cas”, (ASPC, 508, 357)
> Maravelias, G.; Kraus, M.; Aret, A.; Cidale, L.; Arias, M. L.; Borges Fernandes, M. 2017, “B[e] Supergiants’ Circumstellar Environment: Disks or Rings?”, (ASPC, 508, 213)
> Kraus, M.; Cidale, L. S.; Arias, M. L.; Maravelias, G.; Nickeler, D. H.; Torres, A. F.; Borges Fernandes, M.; Aret, A.; Curé, M.; Vallverdú, R.; Barbá, R. H. 2016, “Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73”, (A&A,593A,112)
> Maravelias, G.; Kraus, M.; Aret, A. 2015, “Disk Tracing for B[e] Supergiants in the Magellanic Clouds”, (EAS, 71, 229)

High-Mass X-ray Binaries


Artist’s view of a High-Mass X-ray Binary [source/credit: ESA, NASA, and Felix Mirabel.

High-Mass X-ray Binaries (HMXBs) consist of massive, early-type (O or B) stars and compact objects (usually a neutron star). In a such binary, the massive star loses material either through a decretion equatorial disk (Be/X-ray Binaries;BeXRBs) or through strong stellar winds (supergiant X-ray Binaries;sgXRBs), which results into accretion around the compact object. This leads to persistent or transiert X-ray sources depending on the available material and the geometry of the orbit. Thus, HMXBs provide valuable information regarding the physics of compact stars and stellar evolution.

We study these populations in our Galaxy and the Magellanic Clouds, using both small telescopes (e.g. the 1.3m Skinakas) as well as larger facilities (e.g. the 4m AngloAustralian, the 8.2m ESO telescopes) which provide access to multi-object/slit spectroscopy.

> Maravelias, G.; Zezas, A.; Antoniou, V.; Hatzidimitriou, D.; Haberl, F. 2017, “Hα imaging for BeXRBs in the Small Magellanic Cloud”, (arXiv:1702.04606)
> Maravelias, G.; Zezas, A.; Antoniou, V.; Hatzidimitriou, D. 2014, “Optical spectra of 5 new Be/X-ray Binaries in the Small Magellanic Cloud and the link of the supergiant B[e] star LHA 115-S 18 with an X-ray source”, (MNRAS, 438, 2005)


Artist’s impression of a rocky planet orbiting a red dwarf. Credit: David A. Aguilar (CfA)

Artist’s impression of a rocky planet orbiting a red dwarf. [source/paper, credit: David A. Aguilar (CfA) ]

The SEAWOLF project (Search for Exoplanets by Analysis of WASP Optical Lightcurves and Follow-up; led by Eric Gaidos,University of Hawaii, USA) is a search for transiting Neptune- to Jupiter-size planets among nearby late K and early M dwarf stars, using a network of 1-2m class telescopes. These stars are of high interest (and under-examined by established Doppler and wide-field transit surveys which favor brighter, early-type stars) because it is easier to determine both the mass and the radius of a planet. Statistics of these planet populations versus parent star masses can test theories of planet formation, and compare the results with the Kepler mission and the MEarth survey.

Our contribution to this project includes a large number of follow-up observations with the 1.3m Skinakas telescope, studies on high-accuracy photometric techniques, and development of data analysis pipeline.

> Gaidos, E.; Anderson, D. R.; Lépine, S.; Colón, K. D.; Maravelias, G.; Narita, N.; Chang, E.; Beyer, J.; Fukui, A.; Armstrong, J. D.; Zezas, A.; Fulton, B. J.; Mann, A. W.; West, R. G.; Faedi, F. 2014, “Trawling for transits in a sea of noise: A Search for Exoplanets by Analysis of WASP Optical Lightcurves and Follow-up (SEAWOLF)”, (MNRAS, 437, 3133)

Pro-Am collaborations

Artist’s view of epsilon Aurigae [source, credit: NASA/JPL-Caltech]

The last two decades have revolutionized the availability of “hardware” (telescopes, CCD and video cameras) for Amateur Astronomers, which combined with software and learning skills can offer invaluable help to Professional Astronomers. Thus, Pro-Am collaborations are common today and gain ground quickly.

I am interested in these collaborations, both as an organizer and as a participant.
Particularly, I contribute to the areas of meteors and variable stars, by establishing observing methods, organizing campaigns, collecting and analyzing data. Last but not least, there is a continuous on-going effort to publicize these events as broadly as possible to motivate more Amateurs into these collaborations.

Since 2003 I have been appointed the position of Director of the Meteor and the Variable Stars Sections of the Hellenic Amateur Astronomy Association.

> Kardasis, E.; Rogers, J. H.; Orton, G.; Delcroix, M.; Christou, A.; Foulkes, M.; Yanamandra-Fisher, P.; Jacquesson, M.; Maravelias, G. 2015, “The need for Professional-Amateur collaborations in studies of Jupiter and Saturn”, (arXiv:1503.07878)
> Kloppenborg, B. K.; Pieri, R.; Eggenstein, H.-B.; Maravelias, G.; Pearson, T. 2012, “A Demonstration of Accurate Wide-field V-band Photometry Using a Consumer-grade DSLR Camera”, (JAVSO, 40, 815)
> Maravelias, G.; Kardasis, E.; Strikis, I.-M.; Georgalas, B.; Koutoulaki, M. 2012, “Report From the e Aurigae Campaign in Greece”, (JAVSO, 40, 679)
> G. Maravelias 2012, “First results on video meteors from Crete, Greece”, (Proc. of the International Meteor Conference 2011, p. 94)
> G. Maravelias 2011, “Hellenic Amateur Astronomy Association’s activities: Preliminary results on Perseids 2010”, (Proc. of the International Meteor Conference 2010, p. 45)