arXiv subjects

Chris Flynn

Publications and source records attributed to Chris Flynn.

102 records · Page 6Linked to original sources

arXiv1999

Metallicities of nearby K-dwarfs

The so-called ``G-dwarf problem'' emerged in the 1960's when it was noticed that the observed metal distribution of nearby stars differs from what is expected in simple models of the evolution of the Galaxy. We have determined metallicities for a sample of over 100 K dwarfs drawn from the Hipparcos catalog, using R-I colours to estimate effective temperatures and Geneva photometry to estimate [Fe/H]. We show there is a K-dwarf problem. K-dwarfs have life-times older than the Galactic disc, so these are an ideal star with which to investigate the disc's chemical evolution. Further photometric observations of K dwarfs are under way.

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arXiv1998

Star Counts and the Warped and Flaring Milky Way Disk

This paper reports the star count predictions of warped and flaring models of the outer Milky Way disk. These have been suggested as possible locations of the lenses responsible for the microlensing events towards the Large Magellanic Cloud (LMC). Three such models are investigated in detail and the theoretical predictions are confronted with {\it Hubble Space Telescope} (HST) star count data in 7 low latitude fields ($30^\circ < | b| < 40^\circ$). If the warped and flaring disk population has the same characteristics as disk stars in the solar neighbourhood, then the models can be unambiguously ruled out. Metallicity gradients are well-known in disk galaxies and may cause the outer disk population to differ in colors and luminosity from that locally. This effect is studied using a simple ansatz for the variation in the color-magnitude relation with position and while it is shown to lead to better agreement with the star counts, upper limits on the contribution of the warped and flaring disk to the optical depth are still below the value measured towards the LMC. Only if the warp is very asymmetric or if the luminosity function changes strongly with Galactocentric radius can the models be made consistent with the starcounts.

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arXiv1998

Faint star counts with HST

We describe a program of star counts in the range 19 < I < 26 made with the WFPC cameras aboard the Hubble Space Telescope. Red (V-I > 1.0) stars at these magnitudes are primarily disk and spheroid M dwarfs. The stars are found both on dedicated images as part of the parallel program and by using appropriate archive data. We measure the faint end of the luminosity functions of the disk and spheroid (i.e. stellar halo). We measure the low mass end of the mass function and show that M dwarfs do not dominate the total disk or spheroid mass. We place strong I-band constraints on the amount of halo dark matter in the form of low mass stars (such as M dwarfs or cool white dwarfs). The disk and spheroid contribute only a minor amount of optical depth toward the Magellanic clouds.

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arXiv1998

The Spheroid Luminosity and Mass Functions From HST Star Counts

We analyze 166 spheroid subdwarfs $(6.5 M >0.09 Msun. The spheroid therefore does not contribute significantly to microlensing unless the mass function changes slope dramatically in the substellar range. The total local mass density of spheroid stars (including remnants and unseen binary companions) is ρ~ 6.4E-5 Msun/pc^3, with an uncertainty of about 50%. The power-law indices α=0.25 for the spheroid and α=0.44 for the disk (both uncorrected for binaries) are similar to those of globular clusters of moderate to high metallicity.

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arXiv1997

Hipparcos and the Age of the Galactic Disc

We have used the Hipparcos satellite colour magnitude diagram to determine the age of the Galactic disc. We first measure the metallicities of Hipparcos giants using DDO photometry (Høg & Flynn 1997). With accurate metallicities and distances we compute the minimum age of the Galactic disc as $11\pm1$ Gyr using the colour of the reddest clump stars as a function of metallicity. In conjunction with the new ages derived for Globular Clusters using the same method (Jimenez et al 1996) we show that any delay between the formation of the halo and the disc was only 2-3 Gyr. Since disc galaxies may have been detected to redshifts as high as $z=3.15$ (Lu, Sargent and Barlow 1997) an age for our own Galactic disc of 11 Gyr favours open or non-zero $Λ$ cosmological models. We show that a Reimers mass-loss law is sufficient to explain the morphology of the red clump.

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arXiv1997

Hipparcos absolute magnitudes for metal rich K giants and the calibration of DDO photometry

Parallaxes for 581 bright K giants have been determined using the Hipparcos satellite. We combine the trigonometric parallaxes with ground based photometric data to determine the K giant absolute magnitudes. For all these giants, absolute magnitude estimates can also be made using the intermediate band photometric DDO system (Janes 1975, 1979). We compare the DDO absolute magnitudes with the very accurate Hipparcos absolute magnitudes, finding various systematic offsets in the DDO system. These systematic effects can be corrected, and we provide a new calibration of the DDO system allowing absolute magnitude to be determined with an accuracy of 0.35 mag in the range 2 > M_V > -1. The new calibration performs well when tested on K giants with DDO photometry in a selection of low reddening open-clusters with well-measured distance moduli.

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arXiv1996

M Dwarfs From Hubble Space Telescope Star Counts III: The Groth Strip

We analyze the disk M dwarfs found in 31 new fields observed with the Wide Field Camera (WFC2) on the Hubble Space Telescope (HST) together with the sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary Camera (PC1) fields. The new observations, which include the 28 high-latitude fields comprising the Large Area Multi-Color Survey (``Groth Strip''), increase the total sample to 337 stars, and more than double the number of late M dwarfs (M_V>13.5) from 23 to 47. The mass function changes slope at M~0.6 Msun, from a near-Salpeter power-law index of α=-1.21 to α=0.44. In both regimes the mass function at the Galactic plane is given by ${d^3 N / d\log M d M_V d V} = 8.1\times 10^{-2}\pc^{-3} ({M / 0.59 M_\odot})^α$. The correction for secondaries in binaries changes the low-mass index from α=0.44 to α\sim 0.1. If the Salpeter slope continued to the hydrogen-burning limit, we would expect 500 stars in the last four bins (14.5<M_V<18.5), instead of the 25 actually detected. The explanation of the observed microlensing rate towards the Galactic bulge requires either a substantial population of bulge brown dwarfs or that the disk and bulge mass functions are very different for stars with M~< 0.5 Msun.

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arXiv1996

Metallicities and kinematics of G and K dwarfs

We have developed a photometric abundance indicator for G and K dwarfs, based on accurate, spectroscopically determined abundances for G and K dwarfs of Morell, Källander and Butcher (1992) and Morell (1994). Broadband Cousins R-I photometry is used to estimate effective temperature and the Geneva b_1 colour to estimate line blanketing in the blue and hence abundance. Abundances can be derived in the range -2.0<[Fe/H]<0.5 for G0 to K3 dwarfs, with a scatter in [Fe/H] of 0.2 dex. We apply the method to a sample of Gliese catalog G and K dwarfs, and examine the metallicity and kinematic properties of the stars. The stars show the well established observational features of the disk, thick disk and halo in the solar neighbourhood. We find that the distribution of local K dwarf metallicity is quite similar to local G dwarfs, indicating that there is a ``K-dwarf'' as well as a G-dwarf problem.

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arXiv1996

Hubble Deep Field Constraint on Baryonic Dark Matter

We use a new technique to search for faint red stars in the Hubble Deep Field (HDF) imaged by the Wide Field Camera (WFC2) on the {\it Hubble Space Telescope}. We construct a densely sampled stellar light profile from a set of undersampled images of bright stars in a low-latitude field. Comparison of this stellar light profile to densely sampled profiles of individual objects in the HDF constructed from multiple undersampled dithers allows us to distinguish unambiguously between stars and galaxies to $I=26.3$. We find no stars with $V-I>1.8$ in the outer 90\% of the volume probed. This result places strong and general constraints on the $I$ band luminosity of the constituents of the Galactic dark halo: $$M_I > 15.9 + {5\over 3}\log\biggl(f{0.5 M_\odot\over M}\biggr)\qquad (V-I>1.8),$$ where $M$ is the mass of the objects and $f$ is their density as a fraction of the local halo density, taken to be $ρ_0=9\times 10^{-3}\,M_\odot\,\pc^{-3}$. If the halo is made of white dwarfs, this limit implies that these objects have $M_V\gsim 18.4$ and $V-I\gsim 2.5$. That is, they are $\gsim 2$ magnitudes fainter than the end of the disk white dwarf sequence. Faint red dwarfs account for $<1\%$ of the Galactic dark halo for $M_I<14$, and $<6\%$ for $M_I<15$ at the 95\% confidence level. The density of inter-galactic Local Group stars is at least a factor 3000 smaller than the density of local Population II stars.

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arXiv1996

Kinematics of the Outer Stellar Halo

We have tested whether the simple model for the kinematics of the Galactic stellar halo (in particular the outer halo) proposed by Sommer-Larsen, Flynn and Christensen (SLFC) is physically realizable, by directly integrating particles in a 3-D model of the Galactic potential. We are able to show that the SLFC solution can be realized in terms of a distribution of particles with stationary statistical properties in phase-space. Hence, the SLFC model, which shows a notable change in the anisotropy from markedly radial at the sun to markedly tangential beyond about Galactocentric radius r=20 kpc, seems a tenable description of outer halo kinematics.

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arXiv1995

Disk M Dwarf Luminosity Function From HST Star Counts

We study a sample of 257 Galactic disk M dwarfs (8 12 (M<0.25 \ms), decreasing by a factor \gsim 3 by M_V~14 (M~0.14\ms). This decrease in the LF is in good agreement with the ground-based photometric study of nearby stars by Stobie et al. (1989), and in mild conflict with the most recent LF measurements based on local parallax stars by Reid et al. (1995). The local LF of the faint Galactic disk stars can be transformed into a local mass function using an empirical mass-M_V relation. The mass function can be represented analytically over the mass range 0.1\ms<M<1.6\ms by \log(ϕ)=-1.35-1.34\log(M/\ms)-1.85 [\log(M/\ms)]^2 where ϕis the number density per logarithmic unit of mass. The total column density of M stars is only Σ_M=11.8\pm 1.8\ms\pc^{-2}, implying a total `observed' disk column density of Σ_\obs~=39\ms\pc^{-2}, lower than previously believed, and also lower than all estimates with which we are familiar of the dynamically inferred mass of the disk. The measured scale length for the M-star disk is 3.0\pm 0.4 kpc. The optical depth to microlensing toward the LMC by the observed stars in the Milky Way disk is τ~1x10^{-8}, compared to the observed optical depth found in ongoing experiments τ_\obs~ 10^{-7}. The M-stars show evidence for a population with characteristics intermediate between thin disk and spheroid populations. Approximating what may be a continuum of populations by two separate component, we find characteristic exponential scale heights of ~210 pc and ~740 pc.

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arXiv1994

M Dwarfs, Microlensing, and the Mass Budget of the Galaxy

We show that faint red stars do not contribute significantly to the mass budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field taken with the Wide Field Camera on the newly repaired {\it Hubble Space Telescope}. Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with $2.0 3.0$. Therefore, main-sequence stars with $M_I>10$ that are above the hydrogen-burning limit in the dark halo or the spheroid contribute $<6%$ of the unseen matter. Faint red disk stars, M-dwarfs, contribute at most $ 15%$ to the mass of the disk. We parameterize the faint end of the cumulative distribution of stars, $Φ$, as a function of luminosity $L_V$, $dΦ/d \ln L_V \propto L_V^{-γ}$. For spheroid stars, $γ<0.32$ over the range $6<M_V<17$, with 98% confidence. The disk luminosity function falls, $γ<0$, for $15\lsim M_V\lsim 19$. Faint red stars in the disk or thick disk, and stars with $M_V<16$ in the spheroid contribute $τ< 10^{-8}$ to the optical depth to microlensing toward the Large Magellanic Cloud.

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