Investigating mixing in massive hot giant stars

5 Sep

screen-shot-2016-10-15-at-2-22-17-pmTheory predicts that rotating massive stars “get stirred”, bringing elements from the deep center to the surface. Nathan Grin, former MSc student in Amsterdam, now PhD student in Bonn tried to test the model predictions.  He analyzed the spectra of more than 72 massive hot giant stars in the Tarantula nebula to determine their surface composition.  In particular he looked at the nitrogen abundance, which is known to be a good tracer of mixing processes.

Many of the slowly rotating stars in the sample show surprisingly high levels of nitrogen, much higher than the theory predict. This is worrying, since this group comprises about a third of the sample.  Nathan also tried to measure nitrogen for the rapidly rotating stars, but with the current data only upper limits could be determined.  Additional spectra will be needed for the full picture.

His paper “The VLT-FLAMES Tarantula Survey XXV. Surface nitrogen abundances of O-type giants and supergiants” has been accepted for publication in Astronomy & Astrophysics.  The team consists of N.J. Grin, O.H. Ramirez-Agudelo, A. de Koter, H. Sana, J. Puls, I. Brott, P.A. Crowther, P.L. Dufton, C.J. Evans, G. Graefener, A. Herrero, N. Langer, D.J. Lennon, J.Th. van Loon, N. Markova, S.E. de Mink, F. Najarro, F.R.N. Schneider, W.D. Taylor, F. Tramper, J.S. Vink, W.R. Walborn

VENI prize fellowship for Silvia Toonen to work on triple stars

15 Jul

Dr. Silvia Toonen has been awarded the prestigious postdoctoral VENI fellowship, awarded by the Dutch Science Foundation (NWO) to work on the evolution of triple star systems at the Anton Pannekoek Institute at the University of Amsterdam, where she plans to interact with the BinCosmos group as well as others.  The fellowship provides funding for three years.

New channel for the formation of binary black holes: Predictions for LIGO

7 Mar

De Mink & Mandel (2016), subm. to MNRAS, March 7, 2016

De Mink & Mandel, MNRAS 2016 in press 

We explore the predictions for detectable gravitational-wave signals from merging binary black holes formed through chemically homogeneous evolution in massive short-period stellar binaries. We find that ∼ 500 events per year could be detected with advanced ground-based detectors operating at full sensitivity. We analyze the distribution of detectable events, and conclude that there is a very strong preference for detecting events with nearly equal components (mass ratio > 0.66 at 90% confidence in our default model) and high masses (total source-frame mass between 57 and 103M⊙ at 90% confidence). We consider multiple alternative variations to analyze the sensitivity to uncertainties in the evolutionary physics and cosmological parameters, and conclude that while the rates are sensitive to assumed variations, the mass distributions are robust predictions. Finally, we consider the recently reported results of the analysis of the first 16 double-coincident days of the O1 LIGO observing run, and find that this formation channel is fully consistent with the inferred parameters of the GW150914 binary black hole detection and the inferred merger rate.

Nine Monster Stars at the heart of the Tarantula Nebula

1 Mar

The left side of this collage shows the central part of the young star cluster R136 as it can be seen in the ultraviolet. Due to the high-resolution of Hubble in the ultraviolet the individual stars in this dense cluster can be resolved and studied. The right side shows a pseudo-image, created from the UV spectra collected with the Space Telescope Imaging Spectrograph (STIS). These spectra have been used by scientists to determine the properties of the stars in R136. The boundary of the 17 slit locations is outlined in white in the left image. The long-slit data from the spectrograph have been compressed to the width of the slits and stacked to create a pseudo-image. This allows the slit locations to be matched to stars in the left image.

The star cluster R136 at the heart of the Tarantula nebula was known to harbor 4 very massive stars. New data taken with the STIS spectrograph on board Hubble reveals a total of 9 monsters stars that appear to have masses around or well in excess of a hundred solar masses.

This is the first paper in a series discussing the UV spectra of the brightest objects.  The “monster stars” fully dominate the strong helium emission lines that have also been seen in other very young massive star clusters, indicating that such extreme stars are more common than we thought. This argues in favor of extending the initial mass function well beyond the canonical limit of 100 solar masses.

A new scenario to form double black-hole binaries observable with LIGO

23 Jan

Screen Shot 2016-02-08 at 6.32.52 PMWhen to black holes in a binary merge, the ripples can in principle be observed with  the gravitational wave detectors aLIGO/VIRGO.  In this paper, Ilya Mandel and I investigate the black hole – black hole binaries produced through “homogeneous evolution”, a formation channel originally hypothesized in De Mink et al. (2009).  In this scenario two stars in a close binary are strongly perturbed and internally mixed. The stars remain compact as they evolve, eventually forming two black holes, which, we estimate, typically merge 4 to 11 Gyr after formation.  We estimate the cosmic merger rate.   Events from this channel may be distinguished by the preference for nearly equal mass components and high masses, with typical total masses between 50 and 110 solar masses.  Mandel & de Mink (2016, subm.),  available on the Arxiv

A failed search for a surviving companion of Supernova 1994I

1 Jan

Screen Shot 2016-01-01 at 8.16.55 PMCore-collapse supernovae (SNe) are among the most powerful explosions in the Universe. They mark the deaths of massive stars, most of which are thought to spend their life with in close binary systems. Twenty years after the explosion SN 1994I, long considered to be the result of a binary system, we use Hubble to search for a companion. No companion was detected. We derive deep upper limits in the ultra violet, which allow us to exclude the presence of a main sequence companion with a mass > 10M

We compare with extensive simulations conducted by Manos Zapartas and show that the new data excludes interacting binaries with semi-conservative (late Case A or early Case B) mass transfer. The limits tend to favor systems with non-conservative, late Case B mass transfer with intermediate initial orbital periods and mass ratios. The most likely mass range for a putative main sequence companion would be ∼5–12M, the upper end of which corresponds to the inferred upper detection limit.

Results will be published in the Astrophysical Journal, by Van Dyk, de Mink and Zapartas, 2016.