Sloan Fellows

About: Sloan Fellows is a based out in . It is known for research contribution in the topics: Galaxy & Star formation. The organization has 55 authors who have published 253 publications receiving 35008 citations. The organization is also known as: Sloan Fellows.

Supersonic Shear Instabilities in Astrophysical Boundary Layers

Abstract: Disk accretion onto weakly magnetized astrophysical objects often proceeds via a boundary layer (BL) that forms near the object's surface, in which the rotation speed of the accreted gas changes rapidly. Here, we study the initial stages of formation for such a BL around a white dwarf or a young star by examining the hydrodynamical shear instabilities that may initiate mixing and momentum transport between the two fluids of different densities moving supersonically with respect to each other. We find that an initially laminar BL is unstable to two different kinds of instabilities. One is an instability of a supersonic vortex sheet (implying a discontinuous initial profile of the angular speed of the gas) in the presence of gravity, which we find to have a growth rate of order (but less than) the orbital frequency. The other is a sonic instability of a finite width, supersonic shear layer, which is similar to the Papaloizou-Pringle instability. It has a growth rate proportional to the shear inside the transition layer, which is of order the orbital frequency times the ratio of stellar radius to the BL thickness. For a BL that is thin compared to the radius of the star, the shear rate is much larger than the orbital frequency. Thus, we conclude that sonic instabilities play a dominant role in the initial stages of nonmagnetic BL formation and give rise to very fast mixing between disk gas and stellar fluid in the supersonic regime.

Microlensing by Stellar Black Holes around Sagittarius A

Abstract: We show that at any given time, the Galactocentric black hole Sgr A* is expected to be microlensing Nlens ~ 1.7 bulge stars if the threshold of detectability of the fainter image is Kthr = 21, and Nlens ~ 8 sources if Kthr = 23. The lensed images then provide a unique way to detect stellar-mass black holes predicted to cluster around Sgr A*. If a black hole passes close to a microlensed image, it will give rise to a short (weeks long) microlensing event. We show that the mass and projected velocity of the stellar-mass black hole can both be measured unambiguously from such an event, provided that either a caustic crossing is observed or the astrometric displacement is measured. For Kthr = 23 and moderate magnifications by Sgr A*, the microlensing event rate from a cluster of 20000 black holes within a radius of 0.7 pc is only 0.06 yr- 1; however, if highly magnified images of a star were found, the rate of events by the stellar black holes would be much higher. In addition, the Nlens sources lensed by Sgr A* provide a unique probe of extinction behind the Galactic center along 2Nlens lines of sight.

Fossil group origins V. the dependence of the luminosity function on the magnitude gap

Abstract: Context. In nature we observe galaxy aggregations that span a wide range of magnitude gaps between the two first-ranked galaxies of a system (Δm12). Thus, there are systems with gaps close to zero (e.g., the Coma cluster), and at the other extreme of the distribution, the largest gaps are found among the so-called fossil systems. The observed distribution of magnitude gaps is thought to be a consequence of the orbital decay of M ∗ galaxies in massive halos and the associated growth of the central object. As a result, to first order the amplitude of this gap is a good statistical proxy for the dynamical age of a system of galaxies. Fossil and non-fossil systems could therefore have different galaxy populations that should be reflected in their luminosity functions. Aims. In this work we study, for the first time, the dependence of the luminosity function parameters on Δm12 using data obtained by the fossil group origins (FOGO) project. Methods. We constructed a hybrid luminosity function for 102 groups and clusters at z ≤ 0.25 using both photometric data from the SDSS-DR7 and redshifts from the DR7 and the FOGO surveys. The latter consists of ∼1200 new redshifts in 34 fossil system candidates. We stacked all the individual luminosity functions, dividing them into bins of Δm12, and studied their best-fit Schechter parameters. We additionally computed a “relative” luminosity function, expressed as a function of the central galaxy luminosity, which boosts our capacity to detect differences – especially at the bright end. Results. We find trends as a function of Δm12 at both the bright and faint ends of the luminosity function. In particular, at the bright end, the larger the magnitude gap, the fainter the characteristic magnitude M ∗ . The characteristic luminosity in systems with negligible gaps is more than a factor three brighter than in fossil-like ones. Remarkably, we also find differences at the faint end. In this region, the larger the gap, the flatter the faint-end slope α. Conclusions. The differences found at the bright end support a dissipationless, dynamical friction-driven merging model for the growth of the central galaxy in group- and cluster-sized halos. The differences in the faint end cannot be explained by this mechanism. Other processes – such as enhanced tidal disruption due to early infall and/or prevalence of eccentric orbits – may play a role. However, a larger sample of systems with Δm12 > 1.5 is needed to establish the differences at the faint end.

A New Class of Luminous Transients and A First Census of Their Massive Stellar Progenitors

Abstract: The progenitors of SN 2008S and the 2008 transient in NGC300 were dust-enshrouded, with extremely red mid-infrared (MIR) colors and relatively low luminosities. The transients were optically faint (-13 < M_V < -15) compared to normal core-collapse supernovae (SNe), and their spectra exhibited narrow emission lines. These events are unique among transient-progenitor pairs and hence constitute a new class. Whether they are true SNe or bright massive-star eruptions, we argue that their rate is ~20% of the SN rate. This fact is remarkable in light of the observation that a very small fraction of all massive stars have the MIR colors of the SN 2008S and NGC300 progenitors, as we show using MIR and optical luminosity, color, and variability properties of massive stars in M33. We find that the fraction of massive stars with colors consistent with these progenitors is 1/10000. In fact, only < 10 similar objects exist in M33 - all of which lie at the luminous red extremum of the AGB sequence. That these transients are relatively common with respect to SNe, while their progenitors are rare compared to the massive star population, implies that the dust-enshrouded phase is a short-lived phase in the lives of many massive stars. This shrouded epoch can occur only in the last ~10^4 yr before explosion, be it death or merely eruption. We discuss the implications of this finding for the evolution and census of ``low-mass'' massive stars (8-12 Msun), and we connect it with theoretical discussions of electron-capture SNe and the explosive birth of white dwarfs. A systematic census with (warm) Spitzer of galaxies in the local universe for analogous progenitors would significantly improve our knowledge of this channel to massive stellar explosions, and potentially to others with obscured progenitors. (Abridged)

HAT-P-49b: A 1.7 M J PLANET TRANSITING A BRIGHT 1.5 M ☉ F-STAR*

Abstract: We report the discovery of the transiting extrasolar planet HAT-P-49b. The planet transits the bright (V = 10.3) slightly evolved F-star HD 340099 with a mass of 1.54 M {sub ☉} and a radius of 1.83 R {sub ☉}. HAT-P-49b is orbiting one of the 25 brightest stars to host a transiting planet which makes this a favorable candidate for detailed follow-up. This system is an especially strong target for Rossiter-McLaughlin follow-up due to the host star's fast rotation, 16 km s{sup –1}. The planetary companion has a period of 2.6915 days, mass of 1.73 M {sub J}, and radius of 1.41 R {sub J}. The planetary characteristics are consistent with that of a classical hot Jupiter but we note that this is the fourth most massive star to host a transiting planet with both M{sub p} and R{sub p} well determined.