16163950
ACS imaging of star clusters in M 51 : I. Identification and radius distribution
Context. Size measurements of young star clusters are valuable tools to put constraints on the formation and early dynamical evolution of star clusters. Aims. We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and F814W to select a large sample of star clusters with accurate effective radius measurements in an area covering the complete disc ofM51.We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age. Methods. We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii (Reff) by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail. Results. We find cluster radii between 0.5 and ∼10 pc, and one exceptionally large cluster candidate with Reff = 21.6 pc. The median Reff is 2.1 pc. We find 70 clusters in our sample which have colours consistent with being old GC candidates and we find 6 new “faint fuzzy” clusters in, or projected onto, the disc of M51. The radius distribution can not be fitted with a power law similar to the one for star-forming clouds. We find an increase in Reff with colour as well as a higher fraction of clusters with B−V >∼ 0.05 in the interarm regions. We find a correlation between Reff and galactocentric distance (RG) of the formReff ∝ R0.12±0.02 G , which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius: Reff ∝ L0.15±0.02 for the interarm regions and Reff ∝ L−0.11±0.01 for the spiral arm regions, but we do not observe a correlation between cluster mass and radius. Conclusions. The observed radius distribution indicates that shortly after the formation of the clusters from a fractal gas, the radii of the clusters have changed in a non-uniform way. We find tentative evidence suggesting that clusters in spiral arms are more compact.
16540380
Observational evidence for a truncation of the star cluster initial mass function at the high mass end
We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around MV = −8.9. On the bright side the index of the power-law distribution is steeper (α = 2.75) than on the faint-side (α = 1.93), similar to what was found earlier for the “Antennae” galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al. (2006, A&A, accepted), which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cut-off of the form N dM ∝ M−β exp(−M/MC) dM, we find that MC = 105 M . A direct comparison with the LF of the “Antennae” suggests that there MC = 4 × 105 M .
4422653
On the Measurement of Elemental Abundance Ratios in Inner Galaxy H II Regions
Although variations in elemental abundance ratios in the Milky Way certainly exist, details remain uncertain, particularly in the inner Galaxy, where stars and H II regions in the Galactic plane are obscured optically. In this paper we revisit two previously studied, inner Galaxy H II regions: G333.6-0.2 and W43. We observed three new positions in G333.6-0.2 with the Kuiper Airborne Observatory and reobserved the central position with the Infrared Space Observatory's Long Wavelength Spectrometer in far-infrared lines of S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 μm to the suite of lines measured in W43 by Simpson and coworkers. The measured electron densities range from ~40 to over 4000 cm-3 in a single H II region, indicating that abundance analyses must consider density variations, since the critical densities of the observed lines range from 40 to 9000 cm-3. We propose a method to handle density variations and make new estimates of the S/H and N/H abundance ratios. We find that our sulfur abundance estimates for G333.6-0.2 and W43 agree with the S/H abundance ratios expected for the S/H abundance gradient previously reported by Simpson and coworkers, with the S/H values revised to be smaller as a result of changes in collisional excitation cross sections. The estimated N/H, S/H, and N/S ratios are the most reliable because of their small corrections for unseen ionization states (10%). The estimated N/S ratios for the two sources are smaller than what would be calculated from the N/H and S/H ratios in our previous paper. We compute models of the two H II regions to estimate corrections for the other unseen ionization states. We find, with large uncertainties, that oxygen does not have a high abundance, with the result that the N/O ratio is as high (~0.35) as previously reported. The reasons for the uncertainty in the ionization corrections for oxygen are both the nonuniqueness of the H II region models and the sensitivity of these models to different input atomic data and stellar atmosphere models. We discuss these predictions and conclude that only a few of the latest models adequately reproduce H II region observations, including the well-known, relatively large observed Ne++/O++ ratios in low- and moderate-excitation H II regions.
118300339
Observational Constraints, Stellar Models, and Kepler Data for theta Cyg, the Brightest Star Observable by Kepler
The V=4.48 F4 main-sequence star theta Cyg is the brightest star observable in the Kepler spacecraft field-of-view. Short-cadence (58.8 s) photometric data were obtained by Kepler during 2010 June-September. Preliminary analysis shows solar-like oscillations in the frequency range 1200- 2500 microHz. To interpret these data and to motivate further observations, we use observational constraints from the literature to construct stellar evolution and pulsation models for this star. We compare the observed large frequency separation of the solar-like oscillations with the model predictions, and discuss the prospects for gamma Doradus-like g-mode pulsations, given the observational constraints. We discuss the value of angular diameter measurements from optical interferometry for constraining stellar properties and the implications for asteroseismology.
204537563
The Transiting Circumbinary Planets Kepler-34 and Kepler-35
51829745
OVERVIEW OF THE KEPLER SCIENCE PROCESSING PIPELINE
The Kepler Mission Science Operations Center (SOC) performs several critical functions including managing the ~156,000 target stars, associated target tables, science data compression tables and parameters, as well as processing the raw photometric data downlinked from the spacecraft each month. The raw data are first calibrated at the pixel level to correct for bias, smear induced by a shutterless readout, and other detector and electronic effects. A background sky flux is estimated from ~4500 pixels on each of the 84 CCD readout channels, and simple aperture photometry is performed on an optimal aperture for each star. Ancillary engineering data and diagnostic information extracted from the science data are used to remove systematic errors in the flux time series that are correlated with these data prior to searching for signatures of transiting planets with a wavelet-based, adaptive matched filter. Stars with signatures exceeding 7.1? are subjected to a suite of statistical tests including an examination of each star's centroid motion to reject false positives caused by background eclipsing binaries. Physical parameters for each planetary candidate are fitted to the transit signature, and signatures of additional transiting planets are sought in the residual light curve. The pipeline is operational, finding planetary signatures and providing robust eliminations of false positives.
119213717
Initial Characteristics of Kepler Long Cadence Data for Detecting Transiting Planets
The Kepler Mission seeks to detect Earth-size planets transiting solar-like stars in its ~115?deg2 field of view over the course of its 3.5 year primary mission by monitoring the brightness of each of ~156,000 Long Cadence stellar targets with a time resolution of 29.4 minutes. We discuss the photometric precision achieved on timescales relevant to transit detection for data obtained in the 33.5 day long Quarter 1 (Q1) observations that ended 2009 June 15. The lower envelope of the photometric precision obtained at various timescales is consistent with expected random noise sources, indicating that Kepler has the capability to fulfill its mission. The Kepler light curves exhibit high precision over a large dynamic range, which will surely permit their use for a large variety of investigations in addition to finding and characterizing planets. We discuss the temporal characteristics of both the raw flux time series and the systematic error-corrected flux time series produced by the Kepler Science Pipeline, and give examples illustrating Kepler's large dynamic range and the variety of light curves obtained from the Q1 observations.