Coryn A. L. Bailer-Jones

The Milky Way Tomography with SDSS. II. Stellar Metallicity

In addition to optical photometry of unprecedented quality, the Sloan Digital Sky Survey (SDSS) is producing a massive spectroscopic database which already contains over 280,000 stellar spectra. Using eectiv e temperature and metallicity derived from SDSS spectra for 60,000 F and G type main sequence stars (0:2 < g r < 0:6), we develop polynomial models, reminiscent of traditional methods based on the UBV photometry, for estimating these parameters from the SDSS u g and g r colors. These estimators reproduce SDSS spectroscopic parameters with a root-mean-square scatter of 100 K for eectiv e temperature, and 0.2 dex for metallicity (limited by photometric errors), which are similar to random and systematic uncertainties in spectroscopic determinations. We apply this method to a photometric catalog of coadded SDSS observations and study the photometric metallicity distribution of 200,000 F and G type stars observed in 300 deg 2 of high Galactic latitude sky. These deeper (g < 20:5) and photometrically precise ( 0.01 mag) coadded data enable an accurate measurement of the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The best-t number ratio of the two components is consistent with that implied by the decomposition of stellar counts proles into exponential disk and power-law halo components by Juri c et al. (2008). The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component can be modeled as a spatially invariant Gaussian distribution with a mean of [F e=H] = 1:46 and a standard deviation of 0.3 dex. The disk metallicity distribution is non-Gaussian, with a remarkably small scatter (rms 0.16 dex) and the median smoothly decreasing with distance from the plane from 0:6 at 500 pc to 0:8 beyond several kpc. Similarly, we nd using proper motion measurements that a nonGaussian rotational velocity distribution of disk stars shifts by 50 km/s as the distance from the plane increases from 500 pc to several kpc. Despite this similarity, the metallicity and rotational velocity distributions of disk stars are not correlated (Kendall’s = 0:017 0:018). This absence of a correlation between metallicity and kinematics for disk stars is in a conict with the traditional decomposition in terms of thin and thick disks, which predicts a strong correlation ( = 0:30 0:04) at 1 kpc from the mid-plane. Instead, the variation of the metallicity and rotational velocity distributions can be modeled using non-Gaussian functions that retain their shapes and only shift as the distance from the mid-plane increases. We also study the metallicity distribution using a shallower (g < 19:5) but much larger sample of close to three million stars in 8500 sq. deg. of sky included in SDSS Data Release 6. The large sky coverage enables the detection of coherent substructures in the kinematics{ metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of [F e=H] = 0:95, with an rms scatter of only 0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper motion measurements accurate to 0.2-0.5 mas/yr, for about 200 million F/G dwarf stars within a distance limit of 100 kpc (g < 23:5). Subject headings: methods: data analysis | stars: statistics | Galaxy: halo, kinematics and dynamics, stellar content, structure

Two stellar components in the halo of the Milky Way

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components—an inner and an outer halo—that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.

THE SEGUE STELLAR PARAMETER PIPELINE. I. DESCRIPTION AND COMPARISON OF INDIVIDUAL METHODS

We describe the development and implementation of the Sloan Extension for Galactic Exploration and Understanding (SEGUE) Stellar Parameter Pipeline (SSPP). The SSPP is derived, using multiple techniques, radial velocities, and the fundamental stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) for AFGK-type stars, based on medium-resolution spectroscopy and ugriz photometry obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE). The SSPP also provides spectral classification for a much wider range of stars, including stars with temperatures outside the window where atmospheric parameters can be estimated with the current approaches. This is Paper I in a series of papers on the SSPP; it provides an overview of the SSPP, and tests of its performance using several external data sets. Random and systematic errors are critically examined for the current version of the SSPP, which has been used for the sixth public data release of the SDSS (DR-6).

The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters

We validate the accuracy and precision of the current SEGUE (Sloan Extension for Galactic Understanding and Exploration) Stellar Parameter Pipeline (SSPP), which determines stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) and radial velocities (RVs), by comparing these estimates for selected members of three globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M 67) to the literature values. Spectroscopic and photometric data obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its first extension (SDSS-II/SEGUE) are used to determine atmospheric parameter and RV estimates for stars in these clusters. Based on the scatter in the metallicities derived for the members of each cluster, we quantify the typical uncertainty of the SSPP values, σ ([Fe/H]) = 0.13 dex for stars in the range of –0.3 ≤ g – r ≤ 1.3 and 2.0 ≤ log g ≤ 5.0, at least over the metallicity interval spanned by the clusters studied (–2.3 ≤ [Fe/H] ≤ 0). The surface gravities and effective temperatures derived by the SSPP are also compared with those estimated from the comparison of color-magnitude diagrams with stellar evolution models; we find satisfactory agreement (σ(T eff)< 200 K and σ(log g) ≤ 0.4 dex).

The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the σ Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin2. We identify 64 very low mass cluster member candidates in this region. We have available three-color (I, Z, and J) photometry for all of them, spectra for 24 objects, and K photometry for 27% of our sample. These data provide a well-defined sequence in the I versus I-J and I versus I-K color-magnitude diagrams and indicate that the cluster exhibits little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 M☉) across the complete brown dwarf domain (0.075 to 0.013 M☉) and into the realm of free-floating planetary-mass objects (≤0.013 M☉). We find that the mass spectrum (dN/dm) m-α increases toward lower masses, with an exponent α = 0.8 ± 0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 M☉ should be abundant in the solar vicinity, with a density similar to M-type stars.

Stellar rotation and variability in the Orion Nebula Cluster

A wide field imager attached to the MPG/ESO 2.2 m telescope on La Silla has been used to monitor the Orion Nebula Cluster on 45 nights between 25 Dec. 1998 and 28 Feb. 1999. Ninety-two images were obtained during this period through an intermediate band filter centered at 815.9 nm. More than 1500 sources with I magnitudes between 12.5 and 20 were monitored. We find that essentially every star brighter than 16th mag (where the precision is 0:1 yielded periods. Our work confirms the existence of a bimodal period distribution, with peaks near 2 and 8 days, for stars with M> 0:25 M and a unimodal distribution peaked near 2 days, for lower mass stars. We show that a statistically significant correlation exists between infrared excess emission and rotation in the sense that slower rotators are more likely to show evidence of circumstellar disks. Slowly rotating stars, with angular velocities,! 2r adians/d, corresponding to rotation periods shorter than 3.14 d, have a much smaller mean of 0:17 0:05. This supports the hypothesis that disks are involved in regulating stellar rotation during the pre-main sequence phase. We explore a simple and commonly adopted model of rotational evolution in which stars conserve angular velocity while locked to a disk and conserve angular momentum once released. If these assumptions are valid, and if the locking period is 8 days, we find that more than half of the stars in the ONC are no longer locked to disks and that an exponential decay model with a disk-locking half-life of about 0.5-1 My fits the observations well. Assuming that the mean ages of the higher and lower mass stars are the same, the faster rotation of the lower mass stars can be understood as either a consequence of a shorter disk-locking time or a shorter initial disk-locking period, or both.

THE SEGUE STELLAR PARAMETER PIPELINE. III. COMPARISON WITH HIGH-RESOLUTION SPECTROSCOPY OF SDSS/SEGUE FIELD STARS*

The authors report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which they compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R {approx} 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), they empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s{sup -1}, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. They estimate random errors for lower S/N spectra based on numerical simulations.

Erratum Variability in ultra cool dwarfs: Evidence for the evolution of surface features

We present photometric light curves for a sample of 21 ultra cool M and L dwarfs in the field and in the young open clusters σ Orionis and the Pleiades. The list of targets includes both low mass hydrogen burning stars and brown dwarfs. Evidence for variability with rms amplitudes (in the I band) of 0.01 to 0.055 magnitudes on timescales of 0.4 to 100 hours is discovered in half of these objects. Power spectral analysis using the CLEAN algorithm was performed to search for evidence of periodic variability. Some objects show strong periodicities at around a few hours, which could be due to rotational modulation of the light curve by surface features. However, several objects do not have any significant periodicities to explain their variability. The

A rotational and variability study of a large sample of PMS stars in NGC 2264

v \sin i

The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z = 2:04 ?;??;???

values of a similar population of objects makes it very likely that our time sampling was sensitive to the expected range of rotation periods, and simulations show that we would have detected these if they were caused by long-lived surface features. We argue that this absence of periodicity is due to the evolution of the brightness, and presumably also the physical size, of surface features on timescales of a few to a few tens of hours. This is supported in the case of 2M1145 for which two light curves have been obtained one year apart and show no common periodicity. The surface features could plausibly be photospheric dust clouds or magnetically-induced spots. The recently observed decline in chromospheric activity for late type M and L dwarfs hints towards the former explanation for at least our later-type objects. Furthermore, our sample suggests that variability is more common in objects later than M9, indicating that the variability may be related to dust formation. One light curve shows a brief, but significant, dip, which could be a short-lived feature or possibly an eclipse by a companion.