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.

New Paper: 60% of early B stars also have close companion

29 May

eso1230aEarly B-type stars, are the little brothers of the brighter, hotter and more massive O type stars, which are  nearly always found in close binary systems.  How often are the little brothers found in binary systems?  Are the binary systems different than in the case of O type stars?  Those are the questions addressed in this paper.

Using the Very Large Telescope in Chili nearly a thousand stars were observed as part of the Tarantula Survey of Massive stars. Dunstall et al.  investigated 408 B stars in the sample, the larges homogeneous survey to date. They find that a quarter of these have companions. Taking into account the companions missed they derive that 60% of the B type stars have a companion, slightly less but similar to what was found for the O stars. The B type stars appear to have a less strong preference for extremely close systems.

The VLT-FLAMES Tarantula SurveyX XII. Multiplicity properties of the B-type stars” by
P. R. Dunstall, P. L. Dufton, H. Sana, C. J. Evans, I. D. Howarth, S. Simon-Dıaz
S. E. de Mink, N. Langer, J. Maız Apellaniz, W. D. Taylor, accepted for publiacation in A&A.

Poster prizes Ylva Gotberg, Abel Schootemeijer and Nathan Grin

22 May

Abel_Ylva_NathanRain of poster prizes for BinCosmos group members at the 70th Dutch Astronomy conference 2015.

Ylva Gotberg, first year PhD student received the 1st prize for her very clear poster “Did massive Binaries contribute to the Epoch of Reionization?” Abel Schootemeijer, MSc student, received a shared 3rd prize for his poster discussing the binary system “Phi Persei: a clue to missing type Ibc progenitors”.  Nathan Grin, MSc student also received the 3rd prize for his poster “Rotational Mixing in Massive  Stars”.

Photo was taken by Manos Zapartas who gave his first conference talk: “The Deaths of Massive Stars in Binaires: the delay time distribution”.

Screen Shot 2015-05-29 at 8.20.45 PMScreen Shot 2015-05-29 at 8.21.30 PMScreen Shot 2015-05-29 at 8.22.07 PM

Discovery of a Light Echo Around Supernova 2012aw in LEGUS

21 May

Screen Shot 2015-05-29 at 8.46.59 PMIf you were too late to see the light flash of a supernovae, you may still be able to see the reflected light many years later as it scatters on nearby layers of dust. One of such light echoes was found serendipitously in the LEGUS data set. LEGUS is a large treasury survey proving Ultra violet images with the Hubble Space Telescope of nearby galaxies. Schuyler van Dijk et al present the discovery of the light echo around the location where a supernova was found in 2012.

LEGUS Discovery of a Light Echo Around Supernova 2012aw, to appear in the Astrophysical Journal: Schuyler D. Van Dyk, Janice C. Lee, Jay Anderson, Jennifer E. Andrews, Daniela Calzetti, Stacey N. Bright, Leonardo Ubeda, Linda J. Smith, Elena Sabbi, Eva K. Grebel, Artemio Herrero, Selma E. de Mink

Westerlund 2 images selected as Hubbles 25th Birthday Image

30 Apr


Data of Westerlund 2 were obtained from the HST proposal 13038: A. Nota (ESA/STScI), E. Sabbi and C. Christian (STScI), E. Grebel and P. Zeidler (Astronomisches Rechen-Institut Heidelberg), M. Tosi (INAF, Osservatorio Astronomico di Bologna), A. Bonanos (National Observatory of Athens, Astronomical Institute), and S.E. de Mink (University of Amsterdam)