Space Telescope Science Institute

About: Space Telescope Science Institute is a facility organization based out in Baltimore, Maryland, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 2448 authors who have published 14154 publications receiving 947296 citations. The organization is also known as: STScI.

The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core

Abstract: Doppler measurements from Subaru and Keck have revealed radial velocity variations in the V = 8.15, G0 IV star HD 149026 consistent with a Saturn-mass planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory have detected three complete transit events with depths of 0.003 mag at the predicted times of conjunction. HD 149026 is now the second-brightest star with a transiting extrasolar planet. The mass of the star, based on interpolation of stellar evolutionary models, is 1.3 ± 0.1 M_☉; together with the Doppler amplitude K_1 = 43.3 m s^(-1), we derive a planet mass M sin i = 0.36M_J and orbital radius 0.042 AU. HD 149026 is chromospherically inactive and metal-rich with spectroscopically derived [Fe/H] = +0.36, T_(eff) = 6147 K, log g = 4.26, and v sin i = 6.0 km s^(-1). Based on T_(eff) and the stellar luminosity of 2.72 L_☉, we derive a stellar radius of 1.45 R_☉. Modeling of the three photometric transits provides an orbital inclination of 85o.3 ± 1o.0 and (including the uncertainty in the stellar radius) a planet radius of (0.725 ± 0.05)R_J. Models for this planet mass and radius suggest the presence of a ~67 M_⊕ core composed of elements heavier than hydrogen and helium. This substantial planet core would be difficult to construct by gravitational instability.

The ACS Nearby Galaxy Survey Treasury. IX. Constraining Asymptotic Giant Branch Evolution with Old Metal-poor Galaxies

Abstract: In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metal-poor stars (with M < 1.5 M_⊙, [Fe/H] ≾ -1.0). Indeed, those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses between 0.51 and 0.55 M_⊙. This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.

The Major and Minor Galaxy Merger Rates at z < 1.5

Abstract: Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates, and a cosmologically-averaged‘observability’ timescale hTobs(z)ifor identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, hTobs(z)i for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate hTobs(z)i for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G−M20, and (3) the morphology indicator asymmetry A. Then we apply these timescales to the observed merger fractions at z < 1.5 from the recent literature. When our physically-motivated timescales are adopted, the observed galaxy merger rates become largely consistent. The remaining differences between the galaxy merger rates are explained by the differences in the range of mass-ratio measured by different techniques and differing parent galaxy selection. The major merger rate per unit comoving volume for samples selected with constant number density evolves much more strongly with redshift (∝ (1 + z) +3.0±1.1 ) than samples selected with constant stellar mass or passively evolving luminosity (∝ (1 + z) +0.1±0.4 ). We calculate the minor merger rate (1:4 < Msat/Mprimary . 1:10) by subtracting the major merger rate from close pairs from the ‘total’ merger rate determined by G − M20. The implied minor merger rate is ∼ 3 times the major merger rate at z ∼ 0.7, and shows little evolution with redshift. Subject headings: galaxies:evolution – galaxies:high-redshift – galaxies:interacting – galaxies:structure

Evidence for TP-AGB Stars in High-Redshift Galaxies, and Their Effect on Deriving Stellar Population Parameters

Abstract: We explore the effects of using different stellar population models on estimates of star formation histories, ages, and masses of high-redshift galaxies by fitting the SEDs with models by Maraston (hereafter M05) and by Bruzual & Charlot (hereafter BC03). We focus on the thermally pulsing asymptotic giant branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy. In this respect, BC03 models are representative of other models whose treatment of the TP-AGB phase is similar. Moreover, M05 and BC03 models adopt stellar tracks with different assumptions on convective overshooting. For our experiment we use a sample of high-z (1.4 z 2.7) galaxies, for which rest-frame UV spectroscopy and spectroscopic redshifts are available, along with Spitzer IRAC and MIPS photometry from GOODS. The mid-UV spectra of these galaxies exhibit features typical of A- or F-type stars, indicative of ages in the range ~0.2-2 Gyr, when the contribution of TP-AGB stars is expected to be maximum. We find that the TP-AGB phase plays a key role in the interpretation of the Spitzer data, where the rest-frame near-IR is sampled. Generally, M05 models give better fits than BC03 models and indicate systematically lower ages and lower masses (by ~60%, on average). Photometric redshifts derived using M05 models are also in better agreement with the spectroscopic ones, especially when the rest-frame near-IR fluxes from Spitzer IRAC are included in the fit. We argue that the different results are primarily a consequence of the different treatment of the TP-AGB phase, although other differences in the input stellar evolution also contribute. This work provides a first direct evidence for a strong contribution by TP-AGB stars to the SED of galaxies in the high-redshift universe (z ~ 2).

Deep galaxy counts, extragalactic background light and the stellar baryon budget

Abstract: We assess the constraints imposed by the observed extragalactic background light (EBL) on the cosmic history of star formation and the stellar-mass density today. The logarithmic slope of the galaxy number–magnitude relation from the Southern Hubble Deep Field imaging survey is flatter than 0.4 in all seven UBVIJHK optical bandpasses, i.e. the light from resolved galaxies has converged from the UV to the near-IR. We find a lower limit to the surface brightness of the optical extragalactic sky of about 15 nW m−2 sr−1, comparable to the intensity of the far-IR background from COBE data. Assuming a Salpeter initial mass function with a lower cut-off consistent with observations of M subdwarf disc stars, we set a lower limit of Ωg+sh2>0.0013 I50 to the visible (processed gas + stars) mass density required to generate an EBL at a level of 50 I50 nW m−2 sr−1; our ‘best-guess’ value is Ωg+sh2≈0.0031 I50. Motivated by the recent microlensing results of the MACHO collaboration, we consider the possibility that massive dark haloes around spiral galaxies are composed of faint white dwarfs, and show that only a small fraction (≲5 per cent) of the nucleosynthetic baryons can be locked in the remnants of intermediate-mass stars forming at zF≲5, as the bright early phases of such haloes would otherwise overproduce the observed EBL.