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

17 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.

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