596 research outputs found
Photometric Variability Among the Brightest Asymptotic Giant Branch Stars Near the Center of M32
Deep K' images with 0.1 arcsec angular resolution, obtained with ALTAIR+NIRI
on Gemini North, are used to investigate photometric variablity among the
brightest asymptotic giant branch (AGB) stars in the central regions of M32.
Based on a comparison with brightnesses obtained from the K-band data discussed
by Davidge et al. (2000, ApJ, 545, L89), it is concluded that (1) at least 60%
of bright AGB stars near the center of M32 are photometrically variable, and
(2) the amplitudes of the light variations are similar to those of long period
variables in the Galactic bulge. We do not find evidence for a population of
large amplitude variables, like those detected by IRAS in the Galactic bulge.
The technique discussed here may prove useful for conducting an initial
reconnaisance of photometric variability among AGB stars in spheroids in the
Virgo cluster and beyond, where the required long exposure times may restrict
observations to only a few epochs.Comment: 8 pages of text, 3 postscript figures. ApJ (letters) in pres
Vascular Aging and Hemodynamic Stability in the Intraoperative Period
The proportion of elderly people in the population is steadily increasing, and the inevitable consequence is that this subpopulation is more frequently represented in common medical procedures and surgeries. Understanding the circulatory changes that accompany the aging process is therefore becoming increasingly timely and relevant. In this short review, we discuss aspects of vascular control in aging that are particularly relevant in the maintenance of intraoperative hemodynamic stability. We subsequently review the effects of certain notable anesthetic agents with respect to the aging vasculature
High Angular Resolution JHK Imaging of the Centers of the Metal-Poor Globular Clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341 (M92)
The Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) has
been used to obtain high angular resolution JHK images of the centers of the
metal-poor globular clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341
(M92). The color-magnitude diagrams (CMDs) derived from these data include the
upper main sequence and most of the red giant branch (RGB), and the cluster
sequences agree with published photometric measurements of bright stars in
these clusters. The photometric accuracy is limited by PSF variations, which
introduce systematic errors of a few hundredths of a magnitude near the AO
reference star.
The clusters are paired according to metallicity, and the near-infrared CMDs
and luminosity functions are used to investigate the relative ages within each
pair. The near-infrared CMDs provide the tightest constraints on the relative
ages of the classical second parameter pair NGC5272 and NGC6205, and indicate
that these clusters have ages that differ by no more than +/- 1 Gyr. These
results thus support the notion that age is not the second parameter. We
tentatively conclude that NGC6287 and NGC6341 have ages that differ by no more
than +/- 2 Gyr. However, the near-infrared spectral energy distributions of
stars in NGC6287 appear to differ from those of stars in outer halo clusters,
bringing into question the validity of this age estimate.Comment: 22 pages, 17 figures. To be published in the Astronomical Journa
The Recent Stellar Archeology of M31 - The Nearest Red Disk Galaxy
We examine the star-forming history (SFH) of the M31 disk during the past few
hundred Myr. The luminosity functions (LFs) of main sequence stars at distances
R_GC > 21 kpc (i.e. > 4 disk scale lengths) are matched by models that assume a
constant star formation rate (SFR). However, at smaller R_GC the LFs suggest
that during the past ~10 Myr the SFR was 2 - 3 times higher than during the
preceding ~100 Myr. The rings of cool gas that harbor a significant fraction of
the current star-forming activity are traced by stars with ages ~100 Myr,
indicating that (1) these structures have ages of at least 100 Myr, and (2)
stars in these structures do not follow the same relation between age and
random velocity as their counterparts throughout the disks of other spiral
galaxies, probably due to the inherently narrow orbital angular momentum
distribution of the giant molecular clouds in these structures. The
distribution of evolved red stars is not azimuthally symmetric, in the sense
that their projected density along the north east segment of the major axis is
roughly twice that on the opposite side of the galaxy. The north east arm of
the major axis thus appears to be a fossil star-forming area that dates to
intermediate epochs. Such a structure may be the consequence of interactions
with a companion galaxy.Comment: To appear in The Astrophysical Journa
<i>AKARI</i>/IRC source catalogues and source counts for the IRAC Dark Field, ELAIS North and the <i>AKARI</i> Deep Field South
We present the first detailed analysis of three extragalactic fields (IRAC Dark Field, ELAIS-N1, ADF-S) observed by the infrared satellite, AKARI, using an optimized data analysis toolkit specifically for the processing of extragalactic point sources. The InfaRed Camera (IRC) on AKARI complements the SpitzerSpace Telescope via its comprehensive coverage between 8–24 μm filling the gap between the Spitzer/IRAC and MIPS instruments. Source counts in the AKARI bands at 3.2, 4.1, 7, 11, 15 and 18 μm are presented. At near-infrared wavelengths, our source counts are consistent with counts made in other AKARI fields and in general with SpitzerIRAC (except at 3.2 μm where our counts lie above). In the mid-infrared (11 – 18 μm), we find our counts are consistent with both previous surveys by AKARI and the Spitzer peak-up imaging survey with the InfraRed Spectrograph (IRS). Using our counts to constrain contemporary evolutionary models, we find that although the models and counts are in agreement at mid-infrared wavelengths there are inconsistencies at wavelengths shortward of 7 μm, suggesting either a problem with stellar subtraction or indicating the need for refinement of the stellar population models. We have also investigated the AKARI/IRC filters, and find an active galactic nucleus selection criteria out to z AKARI 4.1, 11, 15 and 18 μm colours
The Upper Asymptotic Giant Branch of the Elliptical Galaxy Maffei 1, and Comparisons with M32 and NGC 5128
Deep near-infrared images obtained with adaptive optics systems on the Gemini
North and Canada-France-Hawaii telescopes are used to investigate the bright
stellar content and central regions of the nearby elliptical galaxy Maffei 1.
Stars evolving on the upper asymptotic giant branch (AGB) are resolved in a
field 3 arcmin from the center of the galaxy. The locus of bright giants on the
(K, H-K) color-magnitude diagram is consistent with a population of stars like
those in Baade's Window reddened by E(H-K) = 0.28 +/- 0.05 mag. This
corresponds to A_V = 4.5 +/- 0.8 mag, and is consistent with previous estimates
of the line of sight extinction computed from the integrated properties of
Maffei 1. The AGB-tip occurs at K = 20.0, which correponds to M_K = -8.7;
hence, the AGB-tip brightness in Maffei 1 is comparable to that in M32, NGC
5128, and the bulges of M31 and the Milky-Way. The near-infrared luminosity
functions (LFs) of bright AGB stars in Maffei 1, M32, and NGC 5128 are also in
excellent agreement, both in terms of overall shape and the relative density of
infrared-bright stars with respect to the fainter stars that dominate the light
at visible and red wavelengths. It is concluded that the brightest AGB stars in
Maffei 1, NGC 5128, M32, and the bulge of M31 trace an old, metal-rich
population, rather than an intermediate age population. It is also demonstrated
that Maffei 1 contains a distinct red nucleus, and this is likely the optical
signature of low-level nuclear activity and/or a distinct central stellar
population. Finally, there is an absence of globular clusters brighter than the
peak of the globular cluster LF in the central 700 x 700 parsecs of Maffei 1.Comment: 22 pages of text and 9 postscript figures; to appear in the
Astronomical Journa
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