Sample Of Dwarfs Research Articles

A new visual – near-infrared diagnostic to estimate the metallicity of cluster and field dwarf stars

We present a theoretical calibration of a new metallicity diagnostic based on the Stroemgren index m1 and on visual -- near-infrared (NIR) colors to estimate the global metal abundance of cluster and field dwarf stars. To perform the metallicity calibration we adopt alpha-enhanced evolutionary models transformed into the observational plane by using atmosphere models computed adopting the same chemical mixture. We apply the new visual - NIR Metallicity-Index-Color (MIC) relations to two different samples of field dwarfs and we find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex, with a dispersion smaller than sigma = 0.3 dex. We apply the same MIC relations to a metal-poor (M 92) and a metal-rich (47 Tuc) globular cluster. We find a peak of -2.01+/-0.08 (sigma = 0.30 dex) and -0.47+/-0.01 (sigma = 0.42 dex), respectively.

LOW CO LUMINOSITIES IN DWARF GALAXIES

[Abridged] We present maps of CO 2-1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13 arcsec angular resolution, ~250 pc at our average distance of 4 Mpc, and sample the galaxies by 10-1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarfs outside the Local Group ranging from single lines-of-sight, stacked over IR-bright regions of embedded star formation, and stacked over the entire galaxy. We detect 5 dwarfs in CO with total luminosities of L_CO = 3-28 1e6 Kkmspc2. The other 11 dwarfs remain undetected in CO even in the stacked data and have L_CO < 0.4-8 1e6 Kkmspc2. We combine our sample of dwarfs with a large literature sample of spirals to study scaling relations of L_CO with M_B and metallicity. We find that dwarfs with metallicities of Z ~ 1/2-1/10 Z_sun have L_CO about 1e2-1e4x smaller than spirals and that their L_CO per unit L_B is 10-100x smaller. A comparison with tracers of star formation (FUV and 24 micron) shows that L_CO per unit SFR is 10-100x smaller in dwarfs. One possible interpretation is that dwarfs form stars much more efficiently, however we argue that the low L_CO/SFR ratio is due to significant changes of the CO-to-H2 conversion factor, alpha_CO, in low metallicity environments. Assuming a constant H2 depletion time of 1.8 Gyr (as found for nearby spirals) implies alpha_CO values for dwarfs with Z ~ 1/2-1/10 Z_sun that are more than 10x higher than those found in solar metallicity spirals. This significant increase of alpha_CO at low metallicity is consistent with previous studies, in particular those which model dust emission to constrain H2 masses. Even though it is difficult to parameterize the metallicity dependence of alpha_CO, our results suggest that CO is increasingly difficult to detect at lower metallicities.

Bayesian fitting of Taurus brown dwarf spectral energy distributions

We present derived stellar and disc parameters for a sample of Taurus brown dwarfs both with and without evidence of an associated disc. These parameters have been derived using an online fitting tool (http://bd-server.astro.ex.ac.uk/), which includes a statistically robust derivation of uncertainties, an indication of pa- rameter degeneracies, and a complete treatment of the input photometric and spectroscopic observations. The observations of the Taurus members with indications of disc presence have been fitted using a grid of theoretical models including detailed treatments of physical processes accepted for higher mass stars, such as dust sublimation, and a simple treatment of the accretion flux. This grid of models has been designed to test the validity of the adopted physical mechanisms, but we have also constructed models using parameterisation, for example semi-empirical dust sublimation radii, for users solely interested in parameter derivation and the quality of the fit. The parameters derived for the naked and disc brown dwarf systems are largely consistent with literature observations. However, our inner disc edge locations are consistently closer to the star than previous results and we also derive elevated accretion rates over non-SED based accretion rate derivations. For inner edge locations we attribute these differences to the detailed modelling we have performed of the disc structure, particularly at the crucial inner edge where departures in geometry from the often adopted vertical wall due to dust sublimation (and therefore accretion flux) can compensate for temperature (and therefore distance) changes to the inner edge of the dust disc. In the case of the elevated derived accretion rates, in some cases, this may be caused by the intrinsic stellar luminosities of the targets exceeding that predicted by the isochrones we have adopted.

A close look at the Centaurus A group of galaxies

We study a sample of 5 dwarf irregular galaxies in the CenA/M83 group, which are companions to the giant elliptical CenA. We aim at deriving their physical properties over their lifetime and compare them to those of dwarfs located in different environments. We use archival HST/ACS data and apply synthetic color-magnitude diagram fitting in order to reconstruct the past star formation activity of the target galaxies. The average star formation rate for the studied galaxies ranges from 10^{-3} up to \sim 7x10^{-2} M_odot/yr, and their mean metallicities correlate with their luminosities (from [Fe/H]\sim -1.4 up to \sim -1.0). The form of the star formation histories varies across the sample, with quiescent periods alternating with intermittent enhancements in the star formation (from a few up to several times the average lifetime value). The dwarfs in this sample formed ~35% to ~60% of their stellar content prior to ~5 Gyr ago. The resulting star formation histories for the CenA companions are similar to those found for comparable Local Group and M81 group dwarfs. We consider this sample of dwarfs together with 5 previously studied M83 dwarf irregular companions. We find no trend of the average star formation rate with tidal index or distance from the main galaxy of the group. However, dwarfs with higher baryonic masses do show higher average star formation rates, underlining the importance of intrinsic properties in governing the evolution of these galaxies. On the other hand, there is also a clear trend when looking at the recent (~0.5-1 Gyr) level of activity. Namely, dwarfs within a denser region of the group appear to have had their star formation quenched while dwarfs located in the group outskirts show a wide range of possible star formation rates, thus indicating that external processes play a fundamental role, complementary to mass, in shaping the star formation histories of dwarf galaxies.

A HERSCHEL SEARCH FOR COLD DUST IN BROWN DWARF DISKS: FIRST RESULTS

We report initial results from a {\it Herschel} program to search for far-infrared emission from cold dust around a statistically significant sample of young brown dwarfs. The first three objects in our survey are all detected at 70\micron, and we report the first detection of a brown dwarf at 160\micron. The flux densities are consistent with the presence of substantial amounts of cold dust in the outer disks around these objects. We modeled the SED's with two different radiative transfer codes. We find that a broad range of model parameters provides a reasonable fit to the SED's, but that the addition of our 70\micron, and especially the 160\micron\ detection enables strong lower limits to be placed on the disk masses since most of the mass is in the outer disk. We find likely disk masses in the range of a few $\times 10^{-6}$ to $10^{-4}$ \msun. Our models provide a good fit to the SED's and do not require dust settling.

Analysis of old very metal rich stars in the solar neighbourhood

A sample of mostly old metal-rich dwarf and turn-off stars with high eccentricity and low maximum height above the Galactic plane has been identified. From their kinematics, it was suggested that the inner disk is their most probable birthplace. Their chemical imprints may therefore reveal important information about the formation and evolution of the still poorly understood inner disk. To probe the formation history of these stellar populations, a detailed analysis of a sample of very metal-rich stars is carried out. We derive the metallicities, abundances of \alpha\ elements, ages, and Galactic orbits. The analysis of 71 metal-rich stars is based on optical high-resolution \'echelle spectra obtained with the FEROS spectrograph at the ESO 1.52-m Telescope at La Silla, Chile. The metallicities and abundances of C, O, Mg, Si, Ca, and Ti were derived based on LTE detailed analysis, employing the MARCS model atmospheres. We confirm the high metallicity of these stars reaching up to [Fe I/H]~0.58, and the sample of metal-rich dwarfs can be kinematically subclassified in samples of thick disk, thin disk, and intermediate stellar populations. All sample stars show solar \alpha-Fe ratios, and most of them are old and still quite metal rich. The orbits suggest that the thin disk, thick disk and intermediate populations were formed at Galactocentric distances of ~8 kpc, ~6 kpc, and ~7 kpc, respectively. The mean maximum height of the thick disk subsample of Z_max~380 pc, is lower than for typical thick disk stars. A comparison of \alpha-element abundances of the sample stars with bulge stars shows that the oxygen is compatible with a bulge or inner thick disk origin. Our results suggest that models of radial mixing and dynamical effects of the bar and bar/spiral arms might explain the presence of these old metal-rich dwarf stars in the solar neighbourhood.

The cold gas content of bulgeless dwarf galaxies

We present an analysis of the neutral hydrogen (HI) properties of a fully cosmological hydrodynamical dwarf galaxy, run with varying simulation parameters. As reported by Governato et al. (2010), the high resolution, high star formation density threshold version of this galaxy is the first simulation to result in the successful reproduction of a (dwarf) spiral galaxy without any associated stellar bulge. We have set out to compare in detail the HI distribution and kinematics of this simulated bulgeless disk with what is observed in a sample of nearby dwarfs. To do so, we extracted the radial gas density profiles, velocity dispersion (e.g., velocity ellipsoid, turbulence), and the power spectrum of structure within the cold interstellar medium from the simulations. The highest resolution dwarf, when using a high density star formation threshold comparable to densities of giant molecular clouds, possesses bulk characteristics consistent with those observed in nature, though the cold gas is not as radially extended as that observed in nearby dwarfs, resulting in somewhat excessive surface densities. The lines-of-sight velocity dispersion radial profiles have values that are in good agreement with observed dwarf galaxies, but due to the fact that only the streaming velocities of particles are tracked, a correction to include the thermal velocities can lead to profiles that are quite flat. The ISM power spectra of the simulations appear to possess more power on smaller spatial scales than that of the SMC. We conclude that unavoidable limitations remain due to the unresolved physics of star formation and feedback within pc-scale molecular clouds.

FOUR NEW T DWARFS IDENTIFIED IN Pan-STARRS 1 COMMISSIONING DATA

A complete well-defined sample of ultracool dwarfs is one of the key science programs of the Pan-STARRS 1 optical survey telescope (PS1). Here we combine PS1 commissioning data with 2MASS to conduct a proper motion search (0.1--2.0\arcsec/yr) for nearby T dwarfs, using optical+near-IR colors to select objects for spectroscopic followup. The addition of sensitive far-red optical imaging from PS1 enables discovery of nearby ultracool dwarfs that cannot be identified from 2MASS data alone. We have searched 3700 sq. deg. of PS1 y-band (0.95--1.03 um) data to y$\approx$19.5 mag (AB) and J$\approx$16.5 mag (Vega) and discovered four previously unknown bright T dwarfs. Three of the objects (with spectral types T1.5, T2 and T3.5) have photometric distances within 25 pc and were missed by previous 2MASS searches due to more restrictive color selection criteria. The fourth object (spectral type T4.5) is more distant than 25 pc and is only a single-band detection in 2MASS. We also examine the potential for completing the census of nearby ultracool objects with the PS1 3$\pi$ survey.

Transformations between the WISE, 2MASS, SDSS and BVRI photometric systems - I. Transformation equations for dwarfs

We present colour transformations for the conversion of the W1 and W2 magnitudes of WISE photometric system to the Johnson-Cousins' BVRI, SDSS (gri), and 2MASS (JHK_s) photometric systems, for dwarfs. The W3 and W4 magnitudes were not considered due to their insufficient signal to noise ratio (S/N). The coordinates of 825 dwarfs along with their BVRI, gri, and JHK_s data, taken from Bilir et al. (2008) were matched with the coordinates of stars in the preliminary data release of WISE (Wright et al., 2010) and a homogeneous dwarf sample with high S/N ratio have been obtained using the following constraints: 1) the data were dereddened, 2) giants were identified and excluded from the sample, 3) sample stars were selected according to data quality, 4) transformations were derived for sub samples of different metallicity range, and 5) transformations are two colour dependent. These colour transformations, coupled with known absolute magnitudes at shorter wavelenghts, can be used in space density evaluation for the Galactic (thin and thick) discs, at distances larger than the ones evaluated with JHK_s photometry.

USING M DWARF SPECTRA TO MAP EXTINCTION IN THE LOCAL GALAXY

We use spectra of more than 56,000 M dwarfs from the Sloan Digital Sky Survey (SDSS) to create a high-latitude extinction map of the local Galaxy. Our technique compares spectra from the stars in the SDSS Data Release 7 M dwarf sample in low-extinction lines of sight, as determined by Schlegel, Finkbeiner, & Davis, to other SDSS M dwarf spectra in order to derive improved distance estimates and accurate line-of-sight extinctions. Unlike most previous studies, which have used a two-color method to determine extinction, we fit extinction curves to fluxes across the entire spectral range from 5700 to 9200 {\AA} for every star in our sample. Our result is an extinction map that extends from a few tens of pc to approximately 2 kpc away from the sun. We also use a similar technique to create a map of R_V values within approximately 1 kpc of the sun, and find they are consistent with the widely accepted diffuse interstellar medium value of 3.1. Using our extinction data, we derive a dust scale height for the local galaxy of 119\pm15 parsecs and find evidence for a local dust cavity.

The initial period function of late-type binary stars and its variation

The variation of the period distribution function of late-type binaries is studied. It is shown that the Taurus--Auriga pre-main sequence population and the main sequence G dwarf sample do not stem from the same parent period distribution with better than 95 per cent confidence probability. The Lupus, Upper Scorpius A and Taurus--Auriga populations are shown to be compatible with being drawn from the same initial period function (IPF), which is inconsistent with the main sequence data. Two possible IPF forms are used to find parent distributions to various permutations of the available data which include Upper Scorpius B (UScB), Chameleon and Orion Nebula Cluster pre-main sequence samples. All the pre-main sequence samples studied here are consistent with the hypothesis that there exists a universal IPF which is modified through binary-star disruption if it forms in an embedded star cluster leading to a general decline of the observed period function with increasing period. The pre-main sequence data admit a log-normal IPF similar to that arrived at by Duquennoy & Mayor (1991) for main sequence stars, provided the binary fraction among pre-main sequence stars is significantly higher. But, for consistency with proto-stellar data, the possibly universal IPF ought to be flat in log-P or log-semi-major axis and must be similar to the K1 IPF form derived through inverse dynamical population synthesis, which has been shown to lead to the main sequence period function if most stars form in typical embedded clusters.

FIRST DIRECT EVIDENCE THAT BARIUM DWARFS HAVE WHITE DWARF COMPANIONS

Barium ii (Ba) stars are chemically peculiar F-, G-, and K-type objects that show enhanced abundances of s-process elements. Since s-process nucleosynthesis is unlikely to take place in stars prior to the advanced asymptotic giant branch (AGB) stage, the prevailing hypothesis is that each present Ba star was contaminated by an AGB companion which is now a white dwarf (WD). Unless the initial mass ratio of such a binary was fairly close to unity, the receiving star is thus at least as likely to be a dwarf as a giant. So although most known Ba stars appear to be giants, the hypothesis requires that Ba dwarfs be comparably plentiful and moreover that they should all have WD companions. However, despite dedicated searches with the IUE satellite, no WD companions have been directly detected to date among the classical Ba dwarfs, even though some 90% of those stars are spectroscopic binaries, so the contamination hypothesis is therefore presently in some jeopardy. In this paper, we analyze recent deep, near-UV and far-UV Galaxy Evolution Explorer (GALEX) exposures of four of the brightest of the class (HD 2454, 15360, 26367, and 221531), together with archived GALEX data for two newly recognized Ba dwarfs: HD 34654 and HD 114520 (which also prove to be spectroscopic binaries). The GALEX observations of the Ba dwarfs as a group show a significant far-UV excess compared to a control sample of normal F-type dwarfs. We suggest that this ensemble far-UV excess constitutes the first direct evidence that Ba dwarfs have WD companions.

Studying the diverse nature of faint galaxies in nearby clusters of the WINGS sample

AbstractWe present the first results of our X‐shooter observations for a sample of dwarf (–17 &lt; MB &lt; –15) galaxies in nearby (0.04 &lt; z &lt; 0.07) galaxy clusters. This luminosity range is fundamental to trace the evolution of higher‐z star‐forming cluster galaxies down to the present day, and to explore the galaxy scaling relations of early‐type galaxies over a broad mass range. Thanks to high resolution and availability of several lines we can derive the velocity dispersion of the galaxies in this range of luminosities and we begin the construction of the fundamental plane of faint early‐type galaxies (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A search for southern ultracool dwarfs in young moving groups

We have constructed an 800-strong red object catalogue by cross-referencing optical and infrared catalogues with an extensive proper motion catalogue compiled for red objects in the southern sky to obtain proper motions. We have applied astrometric and photometric constraints to the catalogue in order to select ultracool dwarf moving group candidates. 132 objects were found to be candidates of a moving group. From this candidate list we present initial results. Using spectroscopy we have obtained reliable spectral types and space motions, and by association with moving groups we can infer an age and composition. the further study of the remainder of our candidates will provide a large sample of young brown dwarfs and confirmed members will provide benchmark ultracool dwarfs. These will make suitable targets of AO planet searches.

CLASSIFICATION OF FIELD DWARFS AND GIANTS IN RAVE AND ITS USE IN STELLAR STREAM DETECTION

An efficient separation between dwarfs and giants in surveys of bright stars is important, especially for studies in which distances are estimated through photometric parallax relations. We use the available spectroscopic log g estimates from the second RAVE data release (DR2) to assign each star a probability for being a dwarf or subgiant/giant based on mixture model fits to the log g distribution in different color bins. We further attempt to use these stars as a labeled training set in order to classify stars which lack log g estimates into dwarfs and giants with a SVM algorithm. We assess the performance of this classification against different choices of the input feature vector. In particular, we use different combinations of reduced proper motions, 2MASS JHK, DENIS IJK and USNO-B B2R2 apparent magnitudes. Our study shows that -- for our color ranges -- the infrared bands alone provide no relevant information to separate dwarfs and giants. Even when optical bands and reduced proper motions are added, the fraction of true giants classified as dwarfs (the contamination) remains above 20%. Using only the dwarfs with available spectroscopic log g and distance estimates (the latter from Breddels et al. 2010), we then repeat the stream search by Klement, Fuchs & Rix (2008, KFR08), which assumed all stars were dwarfs and claimed the discovery of a new stellar stream at V = -160 km/s in a sample of 7015 stars from RAVE DR1. Our re-analysis of the pure DR2 dwarf sample exhibits an overdensity of 5 stars at the phase-space position of the KFR08 stream, with a metallicity distribution that appears inconsistent with that of stars at comparably low rotational velocities. Compared to several smooth Milky Way models, the mean standardized deviation of the KFR08 stream is only marginal at 1.6$\pm$0.4... (abbreviated)

Search forp-mode oscillations in DA white dwarfs with VLT-ULTRACAM

Aims. The main goal of this project is to search for p-mode oscillations in a selected sample of DA white dwarfs near the blue edge of the DAV (g-mode) instability strip, where the p-modes should be excited following theoretical models.

Detection limits for close eclipsing and transiting substellar and planetary companions to white dwarfs in the WASP survey

We have performed extensive simulations to explore the possibility of detecting eclipses and transits of close, sub-stellar and planetary companions to white dwarfs in WASP light-curves. Our simulations cover companions $\sim0.3\Re<{\rm R}_{pl}<12\Re$ and orbital periods $2{\rm h}<P<15{\rm d}$, equivalent to orbital radii $0.003{\rm AU} < a < 0.1{\rm AU}$. For Gaussian random noise WASP is sensitive to transits by companions as small as the Moon orbiting a $\textrm{V}\simeq$12 white dwarf. For fainter white dwarfs WASP is sensitive to increasingly larger radius bodies. However, in the presence of correlated noise structure in the light-curves the sensitivity drops, although Earth-sized companions remain detectable in principle even in low S/N data. Mars-sized, and even Mercury-sized bodies yield reasonable detection rates in high-quality light-curves with little residual noise. We searched for eclipses and transit signals in long-term light-curves of a sample of 194 white dwarfs resulting from a cross-correlation of the McCook $&$ Sion catalogue and the WASP archive. No evidence for eclipsing or transiting sub-stellar and planetary companions was found. We used this non-detection and results from our simulations to place tentative upper limits to the frequency of such objects in close orbits at white dwarfs. While only weak limits can be placed on the likely frequency of Earth-sized or smaller companions, brown dwarfs and gas giants (radius $\approx \Rjup$) with periods $<0.1-0.2$ days must certainly be rare ($<10%$). More stringent constraints likely requires significantly larger white dwarf samples, higher observing cadence and continuous coverage. The short duration of eclipses and transits of white dwarfs compared to the cadence of WASP observations appears to be one of the main factors limiting the detection rate in a survey optimised for planetary transits of main sequence stars.

THE NIRSPEC ULTRACOOL DWARF RADIAL VELOCITY SURVEY

We report the results of an infrared Doppler survey designed to detect brown dwarf and giant planetary companions to a magnitude-limited sample of ultracool dwarfs. Using the NIRSPEC spectrograph on the Keck II telescope, we obtained approximately 600 radial velocity measurements over a period of six years for a sample of 59 late-M and L dwarfs spanning spectral types M8/L0 to L6. A subsample of 46 of our targets have been observed on three or more epochs. We rely on telluric CH4 absorption features in the Earth's atmosphere as a simultaneous wavelength reference and exploit the rich set of CO absorption features found in the K-band spectra of cool stars and brown dwarfs to measure radial velocities and projected rotational velocities. For a bright, slowly rotating M dwarf standard we demonstrate a radial velocity precision of 50 m/s, and for slowly rotating L dwarfs we achieve a typical radial velocity precision of approximately 200 m/s. This precision is sufficient for the detection of close-in giant planetary companions to mid-L dwarfs as well as more equal mass spectroscopic binary systems with small separations (a<2 AU). We present an orbital solution for the subdwarf binary LSR1610-0040 as well as an improved solution for the M/T binary 2M0320-04. We also combine our radial velocity measurements with distance estimates and proper motions from the literature to estimate the dispersion of the space velocities of the objects in our sample. Using a kinematic age estimate we conclude that our UCDs have an age of 5.0+0.7-0.6 Gyr, similar to that of nearby sun-like stars. We simulate the efficiency with which we detect spectroscopic binaries and find that the rate of tight (a<1 AU) binaries in our sample is 2.5+8.6-1.6%, consistent with recent estimates in the literature of a tight binary fraction of 3-4%. (abridged)

A microvariability study of nearby M dwarfs from the Western Italian Alps: Status update

AbstractSmall ground-based telescopes can effectively be used to look for transiting rocky planets around nearby low-mass M stars, as recently demonstrated for example by the MEarth project. Since December 2009 at the Astronomical Observatory of the Autonomous Region of Aosta Valley (OAVdA) we are monitoring photometrically a sample of red dwarfs with accurate parallax measurements. The primary goal of this ‘pilot study’ is the characterization of the photometric microvariability of each target over a typical period of approximately 2 months. This is the preparatory step to long-term survey with an array of identical small telescopes, with kick-off in early 2011. Here we discuss the present status of the study, describing the stellar sample, and presenting the most interesting results obtained so far, including the aggressive data analysis devoted to the characterization of the variability properties of the sample and the search for transit-like signals.

A Dependence of GALEX FUV Magnitudes of F, G, and K Dwarfs Upon Stellar Activity

ABSTRACTThe magnitudes of F, G, and K dwarf stars measured in the FUV bandpass of the GALEX satellite are found to show correlations with chromospheric activity. Values of a color (mFUV - B) were obtained by combining GALEX FUV and Johnson B magnitudes for a large sample of dwarfs for which published measurements of flux in the chromospheric Ca ii H and K emission lines are available. Among dwarfs with (B - V) < 0.8 the photospheric flux produces a two-color relation between mFUV - B and B - V. A color excess Δ(mFUV - B), obtained by making a first-order correction for this photospheric effect, shows a correlation with chromospheric activity for stars with , where is the ratio between the luminosity in the H and K emission lines and the bolometric luminosity. Among dwarfs with (B - V) > 0.8 the photospheric contribution to the flux in the GALEX FUV bandpass is sufficiently low that the uncorrected mFUV - B color shows a clear variation with . The approximately linear nature of this relation indicates that the combined flux in the FUV band, which for such late-type dwarfs arises from transition-region and chromospheric emission lines, is related in a power-law manner to the flux in the Ca ii H and K emission lines.