On the candidate Thorne Zytkow Object HV 2112: foreground S star.

1 Jan

Thorne and Zytkow and others hypothesised that, when a neutron star sinks inside another star, a stable star like object would be formed.  For decades astronomers have been trying to unambiguously detect this exotic object. Several suggestions were made. By far the most promising candidate appeared to be a red luminous star located in the southern sky HV 2112, as pointed out by Emily Levesque. The star shows evidence for having a strange chemical composition and, if indeed located in the Small Magelanic Cloud, would have been extraordinarily bright.

We show that the object has a large proper motion, meaning that it should be located much closer in the fore ground.  If this measurement is robust, it would exclude its TZO nature.  To explain the abundance patterns we propose a standard binary scenario for extrinsic S stars. Here, the Mo and Rb came from the wind of a former AGB binary companion, Li is made in situ by the star itself as it rises to it’s own AGB phase.  The Ca and K composition are naturally explained by this scenario where the star is part of the very old galactic Halo, where alpha enhanced compositions are normal.

800,000 sources in the Hubble Tarantula Treasury Project (HTTP)

19 Nov

The Tarantula nebula is a region of extreme star Screen Shot 2015-11-19 at 9.23.16 AMformation located in a satellite galaxy of our milky way. Sabbi and the HTTP team present the results from a treasure survey conducted with the Hubble Space Telescoope identifying the brightness, colors and positions of 800,000 sources, going down to subsolar masses. For this Hubble took images in 8 different filters from the near ultra violet to the near infrared

The HTTP catalogue is the richest sample of intermediate and low mass pre-main sequence candidates. It allows to study how star formation has been developing through the region and the dual role stellar feedback in quenching and triggering star formation.  The results have been accepted for publication in ApJS.

Hubble Tarantula Treasury Project. III. Photometric Catalog and Resulting Constraints on the Progression of Star Formation in the 30 Doradus Region by E. Sabbi, D.J. Lennon, J. Anderson, M. Cignoni, R.P. van der Marel, D. Zaritsky, G. de Marchi, N. Panagia, D.A. Gouliermis, E.K. Grebel, J.S. Gallager III, L.J. Smith, H. Sana, A. Aloisi, M. Tosi1, C.J. Evans, H. Arab, M. Boyer, S.E. de Mink, K. Gordon, A.M. Koekemoer, S.S. Larsen, J.E. Ryon, P. Zeidler


Controversial Age Spreads in Intermediate-Age Starclusters

31 Oct

Screen Shot 2015-10-31 at 1.28.49 AMMost star clusters at an intermediate age (1-2 Gyr) in the Magellanic Clouds show a puzzling feature in their color-magnitude diagrams. The main sequence turn-off of these clusters is much broader than  expected.  One (highly-debated) interpretation of this feature is that age spreads of the order 200-500 Myr exist within individual clusters.

We analyze  12 clusters that show an extended turn-off using data taken with the Hubble Telescope. We fit the star formation history of the turn-off  and the red clump independently with two different models.  In most of the cases, the age spreads inferred from the red clumps are smaller than the ones resulting from the turn-off region. The width of the main sequence turn-off feature is correlated with the age of the clusters in a way which would be unexplained in the “age spread” interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off.

F. Niederhofer, N. Bastian, V. Kozhurina-Platais, M. Hilker, S. E. de Mink, I. Cabrera-Ziri, C. Li, B. Ercolano, “Controversial Age Spreads from the Main Sequence Turn-Off and Red Clump in Intermediate-Age Clusters in the LMC”, accepted for publication in A&A

Final Kiss of Two Stars Heading for Catastrophe (ESO press release)

21 Oct


Using ESO’s Very Large Telescope, an international team of astronomers have found the hottest and most massive double star with components so close that they touch each other. The two stars in the extreme system VFTS 352 could be heading for a dramatic end, during which the two stars either coalesce to create a single giant star, or form a binary black hole.

Almeida, Sana, de Mink et al. ApJ in press,  preprint


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Merging Neutron Stars and Black holes

5 Oct

Screen Shot 2015-06-13 at 6.15.35 PMDe Mink & Belcynski, (2015, accepted for publication in ApJ)

With the advanced Gravitational Wave detectors coming online, astrophysicists hope to soon detect the signal from the merger of two neutron stars (NS) or black holes (BH). We investigate the impact of the new constraints on the birth properties of massive stars on the predicted rates for NS and BH mergers. Despite the large changes with previous assumptions (larger binary fraction, stronger preference for very tight systems), we only find an increase of less than a factor 2 (insignificant compared with evolutionary uncertainties of typically a factor 10-100).  The uncertainties do not significantly affect (within a factor of 2) our predictions of double compact object merger rates. An exception is the uncertainty in IMF (variations by a factor of 6 up and down). No significant changes in the distributions of final component masses, mass ratios, chirp masses and delay times are found.  We conclude that the predictions are, for practical purposes, robust against uncertainties in the initial conditions concerning binary parameters with exception of the IMF. This eliminates an important layer of the many uncertain assumptions affecting the predictions of merger detection rates with the gravitational wave detectors aLIGO/aVirgo.

Extremely hot stars on the verge of exploding, just a few thousand years to live

19 Sep

Oxygen Wolf-Rayet (WO) stars represent a very rare late stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. They can teach us about the final evolutionary stages of massive stars as they are on the verge of exploding as supernovae.

Screen Shot 2015-09-19 at 3.13.52 PMWe use the X-Shooter instrument on the Very Large Telescope in Chili investigate several WO stars.  We find that the stars are extremely hot, 150 kK to 210 kK, more than 100,000 times brighter than the sun, with surfaces rich in oxygen and carbon (helium mass fractions less than  44% -14%).  They lose mass at a rate that is 2-3 times higher than predicted.

We conclude that WO stars indeed represent the final evolutionary stage of very massive star that start their live with masses of about 4060M. They are in their final evolutionary stages (post core-helium burning) and predicted to explode as “type Ic supernovae” within just a few thousand years.

Tramper et al. (2015), Accepted for publication in Astronomy & Astrophysics.

Brightest young star clusters in LEGUS: host to Extremely Massive Stars?

19 Sep

Screen Shot 2015-09-19 at 12.59.23 PMThe nearby dwarf starburst galaxy NGC5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission.

To investigate the role of these clusters in the starburst energetics, we use Hubble Space Telescope images of NGC5253 combined with data of the Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the brightest clusters.The clusters have ages ~1-15 Myr and masses ~10,000 – 250,000 solar masses.

The most massive cluster is in the radio nebula; with a mass 250,000 solar masses and an age ~1 Myr,  2-4 times less massive and younger than previously estimated. The second radio nebula cluster is also ~1 Myr old, confirming the extreme youth of the starburst region.  These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars.

Calzetti & the LEGUS collaboration (2015), Accepted for publication in the Astrophysical Journal,

Starcluster Westerlund 2 has two hearts.

12 Sep

Screen Shot 2015-09-12 at 1.35.06 AMThe star cluster Westerlund 2 is one of the most massive young star clusters known in the Milky Way.  New data taken with the Hubble Space Telescope allowed Peter Zeidler to study the very young low mass stars in the dense center.  These low mass stars are so young that they have not even started nuclear fusion in their center and still reside on the pre main sequence. Surprisingly two clumps were found with an age of less than 2 Myr.

This data of this program was used for Hubble’s 25th birthday image.  Zeidler et al. (2015), published in AJ.

Studying very metal pool massive stars in I Zw 18 with new models: “Pop III imposters” ?

12 Sep
Screen Shot 2015-09-12 at 1.29.07 AMDorottya Szecsi’s new paper presents an extensive grid of models of massive stars for the galaxy with the inspiring name “I Zw 18”. With a metal content being 50 times lower than in the neighbourhood of the Sun, this galaxy is a unique opportunity to study massive stars that are more similar to the earliest generations of stars in the Universe.
Szecsi presents discusses the models that evolve “chemically homogeneously”. These models reach extreme temperatures. and become “Transparent Wind Ultraviolet INtense stars (TWUIN stars)”.  These provide an attractive explanation to the large observed He II ionizing photon flux, previously interpreted as a sign for Population III stars.
Low-metallicity massive single stars with rotation. Evolutionary models applicable to I Zwicky 18, D. Szécsi, N. Langer, S.-C. Yoon, D. Sanyal, S. E. de Mink, C.J. Evans, T. Dermine

Rotation rates for >100 binary stars in 30 Doradus

12 Jul

Screen Shot 2015-07-12 at 12.05.55 PMThe initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Oscar Ramirez-Agudelo, PhD student in Amsterdam, measured the rotation rates for 114 O-type binaries observed as part of the VLT FLAMES Tarantula Survey.

We find that the wide binaries have a distribution that  is very similar to that of single stars. This is surprising as it seems to hint that binaries and single stars obtain their birth spin in similar ways.  The big exception is the complete lack of  very rapidly spinning stars among binary systems is consistent with the idea  proposed in De Mink et al (2013) that most stars with v sin i > 300 km/s in the single star sample are spun-up post-binary interaction products.

Ramırez-Agudelo, Sana, de Mink et al. (2015) “VFTS XXI: Stellar spin rates of O-type spectroscopic binaries” accepted for publication in A&A.