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
De 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.
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.
We 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 40−60M⊙. 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.
The 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,
The 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.
Dorottya 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.
The 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.
Early 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.
Rain 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”.
If 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