709 research outputs found
Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry
The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars
constitutes an important index of evolutionary and environment/metallicity
factor. We develop a method for mass C/O classification of AGBs in photometric
surveys without using periods. For this purpose we rely on the slopes in the
tracks of individual stars in the colour-magnitude diagram. We demonstrate that
our method enables the separation of C-rich and O-rich AGB stars with little
confusion. For the Magellanic Clouds we demonstrate that this method works for
several photometric surveys and filter combinations. As we rely on no period
identification, our results are relatively insensitive to the phase coverage,
aliasing, and time-sampling problems that plague period analyses. For a
subsample of our stars, we verify our C/O classification against published C/O
catalogues. With our method we are able to produce C/O maps of the entire
Magellanic Clouds. Our purely photometric method for classification of C- and
O-rich AGBs constitutes a method of choice for large, near-infrared photometric
surveys. Because our method depends on the slope of colour-magnitude variation
but not on magnitude zero point, it remains applicable to objects with unknown
distances.Comment: 14 pages, 16 figures, 1 table, accepted for publication in Astronomy
& Astrophysic
Possible Detection of OVI from the LMC Superbubble N70
We present FUSE observations toward four stars in the LMC superbubble N70 and
compare these spectra to those of four comparison targets located in nearby
field and diffuse regions. The N70 sight lines show OVI 1032 absorption that is
consistently stronger than the comparison sight lines by ~60%. We attribute the
excess column density (logN_OVI=14.03 cm^-2) to hot gas within N70, potentially
the first detection of OVI associated with a superbubble. In a survey of 12 LMC
sight lines, Howk et al. (2002a) concluded that there was no correlation
between ISM morphology and N_OVI. We present a reanalysis of their measurements
combined with our own and find a clear difference between the superbubble and
field samples. The five superbubbles probed to date with FUSE show a
consistently higher mean N_OVI than the 12 non-superbubble sight lines, though
both samples show equivalent scatter from halo variability. Possible ionization
mechanisms for N70 are discussed, and we conclude that the observed OVI could
be the product of thermal conduction at the interface between the hot, X-ray
emitting gas inside the superbubble and the cooler, photoionized material
making up the shell seen prominently in Halpha. We calculate the total hydrogen
density n_H implied by our OVI measurements and find a value consistent with
expectations. Finally, we discuss emission-line observations of OVI from N70.Comment: 9 pages in emulateapj style. Accepted to Ap
Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope
We present adaptive optics (AO) images of the central starburst region of the
dwarf irregular galaxy IC10. The Keck 2 telescope laser guide star was used to
achieve near diffraction-limited performance at H and K' (Strehls of 18% and
32%, respectively). The images are centered on the putative Wolf-Rayet (W-R)
object [MAC92]24. We combine our AO images with F814W data from HST. By
comparing the K' vs. [F814W]-K' color-magnitude diagram (CMD) with theoretical
isochrones, we find that the stellar population is best represented by at least
two bursts of star formation, one ~ 10 Myr ago and one much older (150-500
Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This
population represents an OB association with a half-light radius of about 3 pc.
We resolve the W-R object [MAC92]24 into at least six blue stars. Four of these
components have near-IR colors and luminosities that make them robust WN star
candidates. By matching the location of C-stars in the CMD with those in the
SMC we derive a distance modulus for IC10 of about 24.5 mag. and a foreground
reddening of E(B-V) = 0.95. We find a more precise distance by locating the tip
of the giant branch in the F814W, H, and K' luminosity functions. We find a
weighted mean distance modulus of 24.48 +/- 0.08. The systematic error in this
measurement, due to a possible difference in the properties of the RGB
populations in IC10 and the SMC, is +/- 0.16 mag.Comment: 13 pages, 13 figures, ApJ in pres
Variability and spectral classification of LMC giants: results from DENIS and EROS
We present the first cross-identifications of sources in the near-infrared
DENIS survey and in the micro-lensing EROS survey in a field of about 0.5
square degrees around the optical center (OC) of the Large Magellanic Cloud. We
analyze the photometric history of these stars in the EROS data base and obtain
light-curves for about 800 variables. Most of the stars are long period
variables (Miras and Semi-Regulars), a few Cepheids are also present. We also
present new spectroscopic data on 126 asymptotic giant branch stars in the OC
field, 30 previously known and 96 newly discovered by the DENIS survey. The
visible spectra are used to assign a carbon- (C-) or oxygen-rich (O-rich)
nature to the observed stars on the basis of the presence of molecular bands of
TiO, VO, CN, C2. For the remaining of the stars we used the (J-Ks) color to
determine whether they are O-rich or C-rich. Plotting Log(period) versus Ks we
find three very distinct period-luminosity relations, mainly populated by
Semi-Regular of type a (SRa), b (SRb) and Mira variables. Carbon-rich stars
occupy mostly the upper part of these relations. We find that 65% of the
asymptotic giant branch population are long period variables (LPVs).Comment: 9 pages, 7 figures, 4 tables (2 via CDS), accepted by A&A journa
The VMC survey - XVII : The proper motions of the Small Magellanic Cloud and the Milky Way globular cluster 47 Tucanae
In this study we use multi-epoch near-infrared observations from the VISTA survey of the Magellanic Cloud system (VMC) to measure the proper motion of different stellar populations in a tile of 1.5 deg sq. in size in the direction of the Galactic globular cluster 47 Tuc. We obtain the proper motion of the cluster itself, of the Small Magellanic Cloud (SMC), and of the field Milky Way stars. Stars of the three main stellar components are selected from their spatial distribution and their distribution in colour-magnitude diagrams. Their average coordinate displacement is computed from the difference between multiple Ks-band observations for stars as faint as Ks=19 mag. Proper motions are derived from the slope of the best-fitting line among 10 VMC epochs over a time baseline of ~1 yr. Background galaxies are used to calibrate the absolute astrometric reference frame. The resulting absolute proper motion of 47 Tuc is (mu_alpha cos(delta), mu_delta)=(+7.26+/-0.03, -1.25+/-0.03) mas/yr. This measurement refers to about 35000 sources distributed between 10 and 60 arcmin from the cluster centre. For the SMC we obtain (mu_alpha cos(delta), mu_delta)=(+1.16+/-0.07, -0.81+/-0.07) mas/yr from about 5250 red clump and red giant branch stars. The absolute proper motion of the Milky Way population in the line-of-sight (l =305.9, b =-44.9) of this VISTA tile is (mu_alpha cos(delta), mu_delta)=(+10.22+/-0.14, -1.27+/-0.12) mas/yr and results from about 4000 sources. Systematic uncertainties associated to the astrometric reference system are 0.18 mas/yr. Thanks to the proper motion we detect 47 Tuc stars beyond its tidal radius.Peer reviewe
Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC
We present a detailed study of the viewing angles of the LMC disk plane. We
find that our viewing direction differs considerably from the commonly accepted
values, which has important implications for the structure of the LMC. The
discussion is based on an analysis of spatial variations in the apparent
magnitude of features in the near-IR color-magnitude diagrams extracted from
the DENIS and 2MASS surveys. Sinusoidal brightness variations with a
peak-to-peak amplitude of approximately 0.25 mag are detected as function of
position angle, for both AGB and RGB stars. This is naturally interpreted as
the result of distance variations, due to one side of the LMC plane being
closer to us than the opposite side. The best fitting geometric model of an
inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and
line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative
evidence that the LMC disk plane may be warped. Traditional methods to estimate
the position angle of the line of nodes have used either the major axis
position angle Theta_maj of the spatial distribution of tracers on the sky, or
the position angle Theta_max of the line of maximum gradient in the velocity
field, given that for a circular disk Theta_maj = Theta_max = Theta. The
present study does not rely on the assumption of circular symmetry, and is
considerably more accurate than previous studies of its kind. We find that the
actual position angle of the line of nodes differs considerably from both
Theta_maj and Theta_max, for which measurements have fallen in the range
140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not
circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm
Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
I construct a near-IR star count map of the LMC and demonstrate, using the
viewing angles derived in Paper I, that the LMC is intrinsically elongated. I
argue that this is due to the tidal force from the Milky Way. The near-IR data
from the 2MASS and DENIS surveys are used to create a star count map of RGB and
AGB stars, which is interpreted through ellipse fitting. The radial number
density profile is approximately exponential with a scale-length 1.3-1.5 kpc.
However, there is an excess density at large radii that may be due to the tidal
effect of the Milky Way. The position angle and ellipticity profile converge to
PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At
large radii there is a drift of the center of the star count contours towards
the near side of the plane, which can be undrestood as due to viewing
perspective. The fact that PA_maj differes from the line of nodes position
angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not
circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the
general direction of the Galactic center, and is elongated perpendicular to the
Magellanic Stream and the velocity vector of the LMC center of mass. This
suggests that the elongation of the LMC has been induced by the tidal force of
the Milky Way. The position angle of the line of nodes differs from the
position angle Theta_max of the line of maximum line of sight velocity
gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a)
streaming along non-circular orbits in the elongated disk; (b) uncertainties in
the transverse motion of the LMC center of mass; (c) precession and nutation of
the LMC disk as it orbits the Milky Way (expected on theoretical grounds).
[Abridged]Comment: Astronomical Journal, in press. 34 pages, LaTeX, with 7 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution color figures from
http://www.stsci.edu/~marel/abstracts/abs_R32.htm
The VMC survey III : Mass-loss rates and luminosities of LMC AGB stars
Context. Asymptotic giant branch (AGB) stars are major contributors to both the chemical enrichment of the interstellar medium and the integrated light of galaxies. Despite its importance, the AGB is one of the least understood phases of stellar evolution. The main difficulties associated with detailed modelling of the AGB are related to the mass-loss process and the 3rd dredge-up efficiency Aims. We provide direct measures of mass-loss rates and luminosities for a complete sample of AGB stars in the Large Magellanic Cloud, disentangling the C- and O-rich stellar populations. Methods. Dust radiative transfer models are presented for all 374 AGB stars candidates in one of the fields observed by the new VISTA survey of the Magellanic Clouds (VMC). Mass-loss rates, luminosities and a classification of C-and O-rich stars are derived by fitting the models to the spectral energy distribution (SED) obtained by combining VMC data with existing optical, near-, and mid-infrared photometry. Results. The classification technique is reliable at a level of - at worst -75% and significantly better for the reddest dusty stars. We classified none of the stars with a relevant mass-loss rate as O-rich, and we can exclude the presence of more than one dusty O-rich star at a similar to 94% level. The bolometric luminosity function we obtained is fully consistent with most of the literature data on the LMC and with the prediction of theoretical models, with a peak of the C-star distribution at M-bol similar or equal to -4.8 mag and no stars brighter than the classical AGB tip, at M-bol = -7.1 mag. Conclusions. This exploratory study shows that our method provides reliable mass-loss rates, luminosities and chemical classifications for all AGB stars. These results offer already important constraints to AGB evolutionary models. Most of our conclusions, especially for the rarer dust-enshrouded extreme AGB stars, are however strongly limited by the relatively small area covered by our study. Forthcoming VMC observations will easily remove this limitation.Peer reviewe
Preliminary results for RR Lyrae stars and Classical Cepheids from the Vista Magellanic Cloud (VMC) Survey
The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting
-band time series photometry of the system formed by the two Magellanic
Clouds (MC) and the "bridge" that connects them. These data are used to build
-band light curves of the MC RR Lyrae stars and Classical Cepheids and
determine absolute distances and the 3D geometry of the whole system using the
-band period luminosity (), the period - luminosity - color ()
and the Wesenhiet relations applicable to these types of variables. As an
example of the survey potential we present results from the VMC observations of
two fields centered respectively on the South Ecliptic Pole and the 30 Doradus
star forming region of the Large Magellanic Cloud. The VMC -band light
curves of the RR Lyrae stars in these two regions have very good photometric
quality with typical errors for the individual data points in the range of
0.02 to 0.05 mag. The Cepheids have excellent light curves (typical
errors of 0.01 mag). The average magnitudes derived for both types
of variables were used to derive relations that are in general good
agreement within the errors with the literature data, and show a smaller
scatter than previous studies.Comment: 7 pages, 6 figure. Accepted for publication in Astrophysics and Space
Science. Following a presentation at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and the Gaia Perspective", Naples, May 201
QoSatAr: a cross-layer architecture for E2E QoS provisioning over DVB-S2 broadband satellite systems
This article presents QoSatAr, a cross-layer architecture developed to provide end-to-end quality of service (QoS) guarantees for Internet protocol (IP) traffic over the Digital Video Broadcasting-Second generation (DVB-S2) satellite systems. The architecture design is based on a cross-layer optimization between the physical layer and the network layer to provide QoS provisioning based on the bandwidth availability present in the DVB-S2 satellite channel. Our design is developed at the satellite-independent layers, being in compliance with the ETSI-BSM-QoS standards. The architecture is set up inside the gateway, it includes a Re-Queuing Mechanism (RQM) to enhance the goodput of the EF and AF traffic classes and an adaptive IP scheduler to guarantee the high-priority traffic classes taking into account the channel conditions affected by rain events. One of the most important aspect of the architecture design is that QoSatAr is able to guarantee the QoS requirements for specific traffic flows considering a single parameter: the bandwidth availability which is set at the physical layer (considering adaptive code and modulation adaptation) and sent to the network layer by means of a cross-layer optimization. The architecture has been evaluated using the NS-2 simulator. In this article, we present evaluation metrics, extensive simulations results and conclusions about the performance of the proposed QoSatAr when it is evaluated over a DVB-S2 satellite scenario. The key results show that the implementation of this architecture enables to keep control of the satellite system load while guaranteeing the QoS levels for the high-priority traffic classes even when bandwidth variations due to rain events are experienced. Moreover, using the RQM mechanism the user’s quality of experience is improved while keeping lower delay and jitter values for the high-priority traffic classes. In particular, the AF goodput is enhanced around 33% over the drop tail scheme (on average)
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