209 research outputs found
Identification and Analysis of Young Star Cluster Candidates in M31
We present a method for finding clusters of young stars in M31 using
broadband WFPC2 data from the HST data archive. Applying our identification
method to 13 WFPC2 fields, covering an area of ~60 arcmin^2, has revealed 79
new candidate young star clusters in these portions of the M31 disk. Most of
these clusters are small (~<5 pc) young (~10-200 Myr) star groups located
within large OB associations. We have estimated the reddening values and the
ages of each candidate individually by fitting isochrones to the stellar
photometry. We provide a catalog of the candidates including rough
approximations of their reddenings and ages. We also look for patterns of
cluster formation with galactocentric distance, but our rough estimates are not
precise enough to reveal any clear patterns.Comment: 32 pages, 9 figures, 5 tables, accepted to Ap
Extension of the C star rotation curve of the Milky Way to 24 kpc
Demers and Battinelli published, in 2007 the rotation curve of the Milky Way
based on the radial velocity of carbon stars outside the Solar circle. Since
then we have established a new list of candidates for spectroscopy. The goal of
this paper is to determine the rotation curve of the galaxy, as far as possible
from the galactic center, using N type carbon stars. The stars were selected
from their dereddened 2MASS colours, then the spectra were obtained with the
Dominion Astrophysical Observatory and Asiago 1.8 meter telescopes. This
publication adds radial velocities and Galactrocentric distances of 36 carbon
stars, from which 20 are new confirmed. The new results for stars up to 25 kpc
from the galactic center, suggest that the rotation curve shows a slight
decline beyond the Solar circle.Comment: 13 pages, 3 figures, 1 table; accepted for publication in
Astrophysic
The assessment of the near infrared identification of Carbon stars. I. The Local Group galaxies WLM, IC 10 and NGC 6822
{The selection of AGB C and M stars from NIR colours has been done in recent
years using adjustable criteria that are in needs of standardization if one
wants to compare, in a coherent manner, properties of various populations.} We
intend to assess the NIR colour technique to identify C and M stars. We compare
the NIR colours of several C stars previously identified from spectroscopy or
narrow band techniques in WLM, IC 10 and NGC 6822. We demonstrate that very few
M stars have but a non negligible number of C stars are bluer
than this limit. Thus, counts of M and C stars based on such limit do not
produce pure samples. C/M ratios determined from NIR colours must be regarded
as underestimates mainly because the M numbers include many warm C stars and
also K stars if no blue limit is considered.Comment: A&A accepted 18.07.200
C stars in the outer spheroid of NGC 6822
From a 2 x 2 degree survey of NGC 6822 we have previously established that
this Local Group dwarf irregular galaxy possesses a huge spheroid having more
than one degree in length. This spheroid is in rotation but its rotation curve
is known only within ~15' from the center. It is therefore critical to identify
bright stars belonging to the spheroid to characterize, as far as possible, its
outer kinematics. We use the new wide field near infrared imager CPAPIR,
operated by the SMARTS consortium, to acquire J, Ks images of two 34.8' x 34.8'
areas in the outer spheroid to search for C stars. The colour diagram of the
fields allows the identification of 192 C stars candidates but a study of the
FWHM of the images permits the rejection of numerous non-stellar objects with
colours similar to C stars. We are left with 75 new C stars, their mean Ks
magnitude and mean colour are similar to the bulk of known NGC 6822 C stars.
This outer spheroid survey confirms that the intermediate-age AGB stars are a
major contributor to the stellar populations of the spheroid. The discovery of
some 50 C stars well beyond the limit of the previously known rotation curve
calls for a promising spectroscopic follow-up to a major axis distance of 40'.Comment: 13 page
Photometric Survey of the Polar ring galaxy NGC 6822
We have previously established, from a carbon star survey, that the Local
Group dwarf irregular galaxy NGC 6822 is much larger than its central bright
core.
Four MegaCam fields are acquired to survey a 2 2 area
centred on NGC 6822 to fully determine its extent and map its stellar
populations. Photometry of over one million stars is obtained in the SDSS g,
r, i to three magnitudes below the TRGB. RGB stars, selected from their
magnitudes and colours, are used to map the NGC 6822 stellar distribution up to
a distance of 60 arcmin. We map the reddening over the whole area. We establish
that the stellar outer structure of NGC 6822 is elliptical in shape, with
and a major-axis PA = 65, contrasting with the
orientation of the HI disk. The density enhancement can be seen up to a
semi-major axis of 36 making NGC 6822 as big as the Small Magellanic Cloud.
We fit two exponentials to the surface density profile of the spheroid, and
identify a bulge with a scale length of 3.85 and an outer spheroid with a
scale length of 10.0. We find intermediate-age C stars up to 40
while demonstrating that the SDSS filters are unsuitable to identify
extragalactic C stars. NGC 6822 is a unique Local Group galaxy with shape and
structure suggesting a polar ring configuration. Radial velocities of carbon
stars have indeed demonstrated that there are two kinematical systems in NGC
6822.Comment: A&A in pres
Carbon star survey in the Local Group. VII. NGC 3109 a galaxy without a stellar halo
We present a CFH12K wide field survey of the carbon star population in and
around NGC 3109. Carbon stars, the brightest members of the intermediate-age
population, were found nearly exclusively in and near the disk of NGC 3109,
ruling out the existence of an extensive intermediate-age halo like the one
found in NGC 6822. Over 400 carbon stars identified have = -4.71,
confirming the nearly universality of mean magnitude of C star populations in
Local Group galaxies. Star counts over the field reveal that NGC 3109 is a
truncated disk shaped galaxy without an extensive stellar halo. The minor axis
star counts reach the foreground density between 4' and 5', a distance that can
be explained by an inclined disk rather than a spheroidal halo. We calculate a
global C/M ratio of 1.75 +/- 0.20, a value expected for such a metal poor
galaxy.Comment: Accepted in Astronomy and Astrophysic
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