278 research outputs found
Near-Infrared Adaptive Optics Imaging of the Central Regions of Nearby Sc Galaxies. II. NGC 247 and NGC 2403
J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive
optics system are used to investigate the star-forming histories of the central
regions of the Sc galaxies NGC 247 and NGC 2403. The brightest resolved red
stars within 15 arcsec of the nucleus of each galaxy are red supergiants,
indicating that the central few hundred parsecs of these galaxies experienced
star formation within the last ~ 0.1 Gyr. However, when averaged over Gyr time
scales, the star-forming histories of the inner disks of these galaxies have
been remarkably similar, as expected if the long-term evolution of disks is
defined by local characteristics such as mass density. It is demonstrated that
NGC 247 and NGC 2403, like M33, harbour nuclear star clusters with stellar
contents that differ from the surrounding central light concentrations. The
nucleus of NGC 2403 is significantly bluer than that of the other two galaxies
and the K-band surface brightnesses near the centers of NGC 247 and NGC 2403
are 1 -- 2 mag per square arcsec lower than in M33. Finally, it is noted that
young or intermediate-age nuclear star clusters are a common occurence in
nearby spirals, indicating that nuclear star formation in these objects is
either continuous or episodic on time scales of 0.1 - 1 Gyr.Comment: 27 pages of text and 14 figures; to appear in the Astronomical
Journa
Investigating Ca II emission in the RS CVn binary ER Vulpeculae using the Broadening Function Formalism
The synchronously rotating G stars in the detached, short-period (0.7 d),
partially eclipsing binary, ER Vul, are the most chromospherically active
solar-type stars known. We have monitored activity in the Ca II H & K reversals
for almost an entire orbit. Rucinski's Broadening Function Formalism allows the
photospheric contribution to be objectively subtracted from the highly blended
spectra. The power of the BF technique is also demonstrated by the good
agreement of radial velocities with those measured by others from less crowded
spectral regions. In addition to strong Ca II emission from the primary and
secondary, there appears to be a high-velocity stream flowing onto the
secondary where it stimulates a large active region on the surface 30 - 40
degrees in advance of the sub-binary longitude. A model light curve with a spot
centered on the same longitude also gives the best fit to the observed light
curve. A flare with approximately 13% more power than at other phases was
detected in one spectrum. We suggest ER Vul may offer a magnified view of the
more subtle chromospheric effects synchronized to planetary revolution seen in
certain `51 Peg'-type systems.Comment: Accepted to AJ; 17 pages and 16 figure
The Disk and Extraplanar Environment of NGC 247
The stellar content of the spiral galaxy NGC 247 is investigated. The main
sequence turn-off (MSTO) in the inner 12 kpc of the disk corresponds to an age
of 6 Myr. A mean star formation rate (SFR) of 0.1 solar masses per year during
the past 16 Myr is computed from star counts. The color of the red supergiant
plume does not change with radius, suggesting that the mean metallicity of
young stars does not vary by more than 0.1 dex. The number of bright main
sequence stars per local stellar mass density climbs towards larger radii out
to a distance of 12 kpc; the scale lengths that characterize the radial
distributions of young and old stars in the disk thus differ. The density of
bright main sequence stars with respect to projected HI mass gradually drops
with increasing radius. The population of very young stars disappears in the
outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds
to 16 Myr, while between 15 and 18 kpc the age is > 40 Myr. Red giant branch
(RGB) stars are resolved at a projected minor axis galactocentric distance of
12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean
[M/H] = -1.2. The RGB-tip occurs at i' = 24.5 +/- 0.1, indicating that the
distance modulus is 27.9 +/- 0.1. Luminous AGB stars with an age 3 Gyr are also
seen in this field.Comment: Includes 16 eps figures; to appear in the Astrophysical Journa
Deep ALTAIR + NIRI Imaging of the Disk and Bulge of M31
Deep J, H, and K' images, recorded with the ALTAIR adaptive optics system and
NIRI imager on Gemini North, are used to probe the stellar content of the disk
and bulge of the Local Group galaxy M31. With FWHM near 0.08 arcsec in K, these
are the highest angular resolution near-infrared images yet obtained of this
galaxy. Four fields that sample M31 at galactocentric radii of 62, 9, 4, and 2
arcmin were observed. The RGB-tip occurs between K = 17.0 and 17.2, and the
color of the RGB in the field closest to the center of M31 is consistent with
that of NGC 6528. After accounting for random photometric errors, the upper RGB
in each field has a width on the (K, J-K) CMD that is consistent with a +/- 0.5
dex dispersion in [Fe/H], in rough agreement with what is seen in other disk
and spheroid fields in M31. A population of very bright red stars, which we
identify as C stars, are seen in the three fields that are closest to the
center of M31. The spatial distribution of these objects suggests that they are
well mixed throughout this part of M31, and so likely did not form in a compact
region near the galactic nucleus, but more probably formed in the inner disk.
We speculate that these C stars may be the most luminous members of the
intermediate age population that has been detected previously in studies of the
integrated spectrum of the central regions of M31.Comment: 36 pages of text + 16 eps figures; Astronomical Journal in pres
HD 51106 and HD 50747: an ellipsoidal binary and a triple system observed with CoRoT
We present an analysis of the observations of HD 51106 and HD 50747 by the
satellite CoRoT, obtained during its initial run, and of the spectroscopic
preparatory observations.
AIMS: We complete an analysis of the light curve, extract the main
frequencies observed, and discuss some preliminary interpretations about the
stars.
Methods: We used standard Fourier transform and pre-whitening methods to
extract information about the periodicities of the stars.
Results: HD 51106 is an ellipsoidal binary, the light curve of which can be
completely explained by the tidal deformation of the star and smaller secondary
effects. HD 50747 is a triple system containing a variable star, which exhibits
many modes of oscillation with periods in the range of a few hours. On the
basis of this period range and the analysis of the physical parameters of the
star, we conclude that HD 50747 is a Gamma-Doradus star.Comment: 7 pages, 8 figures, use (Astronomy-Astrophysics format/macro LAtex
New Light on the Formation and Evolution of M31 and its Globular Cluster System
We present spectroscopic ages, metallicities, and [alpha/Fe] ratios for 70
globular clusters in M31 that were derived from Lick line-index measurements.
In addition to a population of old (>10 Gyr) globular clusters with a wide
range of metallicities, from about -2.0 dex to solar values, we find evidence
for a population of intermediate-age globular clusters with ages between ~5 and
8 Gyr and a mean metallicity around [Z/H]=-0.6. We also confirm the presence of
young M31 globular clusters that were recently identified by Beasley et al.
(2004), which have ages <1 Gyr and relatively high metallicities around -0.4
dex. The M31 globular cluster system has a clearly super-solar mean
[alpha/Fe]=0.14\pm0.04 dex. Intermediate-age and young objects show roughly
solar abundance ratios. We find evidence for an age-[alpha/Fe] relation in the
sense that younger clusters have smaller mean [alpha/Fe] ratios. From a
comparison of indices, mostly sensitive to carbon and/or nitrogen abundance,
with SSP model predictions for nitrogen-enhanced stellar populations, we find a
dichotomy in nitrogen enhancement between young and old M31 globular clusters.
The indices of objects older than 5 Gyr are consistent with a factor of three
or higher in nitrogen enhancement compared to their younger counterparts. Using
kinematical data from Morrison et al. (2004) we find that the globular cluster
sub-population with halo kinematics is old (>9 Gyr), has a bimodal metallicity
distribution, and super-solar [alpha/Fe]. Disk globular clusters have a wider
range of ages, are on average more metal-rich, and have a slightly smaller mean
[alpha/Fe] ratio. (abridged)Comment: 32 pages, 12 figures, accepted for publication in A&
Local Group Dwarf Elliptical Galaxies: II. Stellar Kinematics to Large Radii in NGC 147 and NGC 185
We present kinematic and metallicity profiles for the M31 dwarf elliptical
(dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most
extensive spectroscopic radial coverage for any dE galaxy, extending to a
projected distance of eight half-light radii (8 r_eff = 14'). We achieve this
coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442
member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast
to previous studies, we find that both dEs have significant internal rotation.
We measure a maximum rotational velocity of 17+/-2 km/s for NGC 147 and 15+/-5
km/s for NGC 185. The velocity dispersions decrease gently with radius with an
average dispersion of 16+/-1 km/s for NGC 147 and 24+/-1 km/s for NGC 185. Both
dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for
a radial metallicity gradient. We construct two-integral axisymmetric dynamical
models and find that the observed kinematical profiles cannot be explained
without modest amounts of non-baryonic dark matter. We measure central
mass-to-light ratios of ML_V = 4.2+/-0.6 and ML_V = 4.6+/-0.6 for NGC 147 and
NGC 185, respectively. Both dE galaxies are consistent with being primarily
flattened by their rotational motions, although some anisotropic velocity
dispersion is needed to fully explain their observed shapes. The velocity
profiles of all three Local Group dEs (NGC 147, NGC 185 and NGC 205) suggest
that rotation is more prevalent in the dE galaxy class than previously assumed,
but is often manifest only at several times the effective radius. Since all dEs
outside the Local Group have been probed to only inside the effective radius,
this opens the door for formation mechanisms in which dEs are transformed or
stripped versions of gas-rich rotating progenitor galaxies.Comment: 16 pages, 7 figures. accepted to A
Inner Polar Rings and Disks: Observed Properties
A list of galaxies with inner regions revealing polar (or strongly inclined
to the main galactic plane) disks and rings is compiled from the literature
data. The list contains 47 galaxies of all morphological types, from E to Irr.
We consider the statistics of the parameters of polar structures known from
observations. The radii of the majority of them do not exceed 1.5 kpc. The
polar structures are equally common in barred and unbarred galaxies. At the
same time, if a galaxy has a bar (or a triaxial bulge), this leads to the polar
disk stabilization - its axis of rotation usually coincides with the major axis
of the bar. More than two thirds of all considered galaxies reveal one or
another sign of recent interaction or merging. This fact indicates a direct
relation between the external environment and the presence of an inner polar
structure.Comment: 12 pages, 3 figures, accepted to Astrophysical Bulletin. Minor
changes and corrections are still possibl
Resolving the Stellar Outskirts of M81: Evidence for a Faint, Extended Structural Component
We present a wide field census of resolved stellar populations in the
northern half of M81, conducted with Suprime-Cam on the 8-m Subaru telescope
and covering an area ~ 0.3 square degrees. The resulting color-magnitude
diagram reaches over one magnitude below the red giant branch (RGB) tip,
allowing a detailed comparison between the young and old stellar spatial
distributions. The surface density of stars with ages <~ 100 Myr is correlated
with that of neutral hydrogen in a manner similar to the disk-averaged
Kennicutt-Schmidt relation. We trace this correlation down to gas densities of
~ 2 x 10^20 cm^{-2}, lower than typically probed with H-alpha flux. Both
diffuse light and resolved RGB star counts show compelling evidence for a
faint, extended structural component beyond the bright optical disk, with a
much flatter surface brightness profile. The star counts allow us to probe this
component to significantly fainter levels than is possible with the diffuse
light alone. From the colors of its RGB stars, we estimate this component has a
peak global metallicity [M/H] ~ -1.1 +/- 0.3 at deprojected radii 32 - 44 kpc
assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of
its RGB stars follows a power-law surface density profile, I(r) ~ r^{-gamma},
with gamma ~ 2. [Abridged]Comment: 17 pages including 2 tables and 20 figures, accepted to AJ, version
with high resolution figures available at
http://www.roe.ac.uk/~mkb/astroph/m81hires.pd
Conformal Mapping on Rough Boundaries I: Applications to harmonic problems
The aim of this study is to analyze the properties of harmonic fields in the
vicinity of rough boundaries where either a constant potential or a zero flux
is imposed, while a constant field is prescribed at an infinite distance from
this boundary. We introduce a conformal mapping technique that is tailored to
this problem in two dimensions. An efficient algorithm is introduced to compute
the conformal map for arbitrarily chosen boundaries. Harmonic fields can then
simply be read from the conformal map. We discuss applications to "equivalent"
smooth interfaces. We study the correlations between the topography and the
field at the surface. Finally we apply the conformal map to the computation of
inhomogeneous harmonic fields such as the derivation of Green function for
localized flux on the surface of a rough boundary
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