6,241 research outputs found
Galactic Calibration of the Tip of the Red Giant Branch
Indications from Gaia data release 2 (DR2) are that the tip of the red giant
branch (TRGB, a population II standard candle related to the helium flash in
low mass stars) is close to -4 in absolute I magnitude in the Cousins
photometric system. Our sample is high latitude southern stars from the thick
disk and inner halo, and our result is consistent with longstanding findings
from globular clusters, whose distances were calibrated with RR Lyrae stars. As
the Gaia mission proceeds, there is every reason to think an accurate Galactic
geometric calibration of TRGB will be a significant outcome.Comment: to appear in PAS
The Ca II triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]
Measurements are presented and analysed of the strength of the Ca II triplet lines in red giants in Galactic globular and open clusters, and in a sample of red giants in the LMC disc that have significantly different [Ca/Fe] abundance ratios to the Galactic objects. The Galactic objects are used to generate a calibration between Ca II triplet line strength and [Fe/H], which is then used to estimate [Fe/H]CaT for the LMC stars. The values are then compared with the [Fe/H]spec determinations from high-dispersion spectroscopy. After allowance for a small systematic offset, the two abundance determinations are in excellent agreement. Further, as found in earlier studies, the difference is only a very weak function of the [Ca/Fe] ratio. For example, changing [Ca/Fe] from +0.3 to −0.2 causes the Ca II-based abundance to underestimate [Fe/H]spec by only ∼0.15 dex, assuming a Galactic calibration. Consequently, the Ca II triplet approach to metallicity determinations can be used without significant bias to study stellar systems that have substantially different chemical evolution histories
Clarification of the Nature of the Galaxy CFC97 Cen 05
The galaxy CFC97 Cen 05 has in the past been considered an HI-rich dwarf
galaxy in the nearby Centaurus A group. We have used Australia Telescope
Compact Array observations to show that the HI associated with CFC97 Cen 05 by
Cote et al. (1997) is most likely a Galactic High Velocity Cloud that is
centered ~17' from the optical image of the galaxy. At the optical location of
the galaxy, which is not that tabulated by Banks et al. (1999), there is no
indication of the presence of HI for velocities less than the upper limit of
the HIPASS survey at ~12,500 km/s. In addition, WFPC2 images of CFC97 Cen 05
obtained from the HST Archive reveal that this galaxy is in fact not a dwarf at
all, but rather is a distant background spiral not associated with the
Centaurus A group.Comment: accepted for publication in the PASP (November 2004
A new red giant-based distance modulus of 13.3 Mpc to the Antennae galaxies and its consequences
The Antennae galaxies are the closest example of an ongoing major galaxy
merger, and thereby represent a unique laboratory for furthering the
understanding of the formation of exotic objects (e.g., tidal dwarf galaxies,
ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper
HST/WFPC2 observations were used to demonstrate that the Antennae system might
be at a distance considerably less than that conventionally assumed in the
literature. Here we report new, much deeper HST/ACS imaging that resolves the
composite stellar populations, and most importantly, reveals a well-defined red
giant branch. The tip of this red giant branch (TRGB) is unambiguously detected
at Io(TRGB)=26.65 +/- 0.09 mag. Adopting the most recent calibration of the
luminosity of the TRGB then yields a distance modulus for the Antennae of
(m-M)o= 30.62 +/- 0.17 corresponding to a distance of 13.3 +/- 1.0 Mpc. This is
consistent with our earlier result, once the different calibrations for the
standard candle are considered. We briefly discuss the implications of this now
well determined shorter distance.Comment: 11 pages, 3 figures, accepted for publication in the Ap
Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud
We present Gemini South GMOS g,i photometry of 14 intermediate-age Large
Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203,
2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a
continuing project to investigate the extended Main Sequence Turnoff (EMSTO)
phenomenon. Extensive artificial star tests were made over the observed field
of view. These tests reveal the observed behaviour of photometric errors with
magnitude and crowding. The cluster stellar density radial profiles were traced
from star counts over the extent of the observed field. We adopt clus- ter
radii and build colour-magnitude diagrams (CMDs) with cluster features clearly
identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching
of theoretical isochrones. The studied LMC clusters are confirmed to be
intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162
and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the
basis of having dual red clumps.Comment: MNRAS, accepte
Detection of neutral hydrogen in early-type dwarf galaxies of the Sculptor Group
We present our results of deep 21 cm line (HI) observations of five early and
mixed-type dwarf galaxies in the nearby Sculptor group using the ATNF 64m
Parkes Radio Telescope. Four of these objects, ESO294-G010, ESO410-G005,
ESO540-G030, and ESO540-G032, were detected in HI with neutral hydrogen masses
in the range of 2-9x10^5 M_{\odot} ( = 0.08, 0.13, 0.16, and
0.18, respectively). These HI masses are consistent with the gas mass expected
from stellar outflows over a large period of time. Higher resolution radio data
from the Australia Telescope Compact Array were further analysed to measure
more accurate positions and the distribution of the HI gas. In the cases of
dwarfs ESO294-G010 and ESO540-G030, we find significant offsets of 290 pc and
460 pc, respectively, between the position of the HI peak flux and the center
of the stellar component. These offsets are likely to have internal cause such
as the winds from star-forming regions. The fifth object, the spatially
isolated dwarf elliptical Scl-dE1, remains undetected at our 3\sigma limit of
22.5 mJy km/s and thus must contain less than 10^5 M_{\odot} of neutral
hydrogen. This leaves Scl-dE1 as the only Sculptor group galaxy known where no
interstellar medium has been found to date. The object joins a list of similar
systems including the Local Group dwarfs Tucana and Cetus that do not fit into
the global picture of the morphology-density relation where gas-rich dwarf
irregulars are in relative isolation and gas-deficient dwarf ellipticals are
satellites of more luminous galaxies.Comment: 21 pages, 4 figures, to be published in AJ (accepted
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