572 research outputs found
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
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
Carbon Star Survey in the Local Group. VI. The Dwarf Spheroidal Galaxy NGC 205
We present a CFH12K survey of the carbon stars in NGC 205 and its surrounding field. We find that the number of C stars in NGC 205 is low (~500) for its luminosity and that very few C stars are seen outside of the 10' isophote, suggesting tidal stripping by M31. Their MI = -4.54, a magnitude nearly identical to what has been found in other galaxies with numerous C stars. Stars with accurate (R-I) photometry, to I â 22.5, are used to determine the outer profile of NGC 205. A King profile with rt = 1977 ± 300 fits the data for distances larger than 7'. This rt implies that NGC 205 reached a minimum distance to M31 of ~42 kpc. The spatial distribution of C stars in the surrounding field reveals that some C stars belong to to the disk of M31, seen in projection, and that a surplus of C stars seen west of NGC 205 could be part of tidal debris left along its orbit. Finally, we report evidence of a transient period of enhanced star formation that occurred 1â2 Gyr ago in the northwest half of NGC 205
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
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
The extent of NGC 6822 revealed by its C stars population
Using the CFH12K camera, we apply the four band photometric technique to
identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C
stars, outside of this area were also discovered with the Las Campanas Swope
Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading
to an average absolute I magnitude of
-4.70 mag, a value essentially identical to the mean magnitude obtained for
the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC
6822, C stars are seen at large radial distances and trace a huge slightly
elliptical halo which do not coincide with the huge HI cloud surrounding
NGC6822. The previously unknown stellar component of NGC 6822 has a exponential
scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M
ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.Comment: accepted, to be published in A
- âŠ