Lezing: De wonderlijke levens van de helderste sterren in ons universum

24 Oct

Tijdens deze voordracht laten sterrenkundigen Selma de Mink (UvA) en Samayra Straal (ASTRON/UvA) je kennismaken met de exotische levens van zware sterren. Ze vertellen over het dramatische einde van het leven van dit soort sterren en hoe ze hun leven na de explosie voortzetten. In het bijzonder gaan ze in op hoe ze radiostraling en de pas waargenomen zwaartekrachtsgolven gebruiken om meer te weten te komen over dit soort sterren.

Datum en tijd: woensdag 26 oktober 2016, 20:00 – 21:30
Plaats: SPUI25, Spui 25-27, 1012 XM Amsterdam
Entree: Gratis (na aanmelding). Aanmelden is niet vrijblijvend. Bent u verhinderd, dan graag doorgeven via spui25@uva.nl | T: 020 525 8142.
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ALMA time: “The Extreme UV of massive stars through ALMA’s eyes”

20 Oct

Extreme Ultraviolet (UV) light is blocked by our atmosphere (fortunately for us, because they are dangerous for our skin). Even from space it is very hard to measure. At the moment there is no single satellite telescope that can give information about this part of the spectrum.  Bad news for astronomers, because they really want to know how many of such ionizing photons are emitted by massive stars. Such photons play a very important role  in heating their surrounding clouds where low mass stars and their planets are still forming. These photons are also believed to have played a very big role early in the history of our Universe. They ionized the intergalactic H during the Cosmic Epoch of reionization. This made our universe transparent and ended the so-called Cosmic Dark Ages.

For this study we use a trick proposed by Scoville & Murchikova (2013) that allows to indirectly measure the extreme UV using ALMA. ALMA is an array of antennas in the dry desert of Chile (image), which can observe the sky in (sub)millimeter wavelengths, i.e. in wavelengths that are too red to see with the naked eye.   Instead of measuring the ionizing photons directly, we will measure the effect they have on the surrounding H and He gas by  measuring the “high-order recombination lines” that ALMA is sensitive too.

This proposal ALMA 2016.1.01015.S (PI De Mink) has been awarded 6.6 hours. The project design and analysis is lead by E. Muchicova (Caltech) under supervision of N. Scoville (Caltech). Image credit:  Credit ALMA/NAOJ/NRAO J. Hills

A hundred massive binaries in the Tarantula nebula: a universal phenomenon

15 Oct

screen-shot-2016-10-15-at-2-39-09-pmMassive stars are often found in close binary system, two stars that orbit each other.  So far the data samples analyzed were still relatively small and limited to stars in our own galaxy.

New results from a large extensive survey of the TMBM (Tarantula Massive Binary Monitoring) survey (PI H. Sana) contain data for over a hundred massive hot binary stars, the largest homogeneous sample to date and the first of this size outside our own galaxy.   In the first paper, Leonardo Almeida, presents the survey and first results.  The  They show that the properties are very similar as found in earlier studies. These results point at binarity being a universal phenomenon for massive stars.

The paper “The Tarantula Massive Binary Monitoring: I. Observational campaign and OB-type spectroscopic binaries” by  L.A. Almeida, H. Sana, W. Taylor, R. Barbá, A. Bonanos, P. Crowther, A. Damineli, A. de Koter, S.E. de Mink, C.J. Evans, M. Gieles, N.J. Grin, V. Hénault-Brunet, N. Langer, D. Lennon, S. Lockwood, J. Maíz Apellániz, A.F.J. Moffat, C. Neijssel, C. Norman, O.H. Ramírez-Agudelo, N.D. Richardson, A. Schootemeijer, T. Shenar, I. Soszyński, F. Tramper & J.S. Vink is available on the arxiv.

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.