1,105 research outputs found
On the usefulness of finding charts Or the runaway carbon stars of the Blanco & McCarthy field 37
We have been recently faced with the problem of cross--identifying stars
recorded in historical catalogues with those extracted from recent fully
digitized surveys (such as DENIS and 2MASS). Positions mentioned in the old
catalogues are frequently of poor precision, but are generally accompanied by
finding charts where the interesting objects are flagged. Those finding charts
are sometimes our only link with the accumulated knowledge of past literature.
While checking the identification of some of these objects in several
catalogues, we had the surprise to discover a number of discrepancies in recent
works.The main reason for these discrepancies was generally the blind
application of the smallest difference in position as the criterion to identify
sources from one historical catalogue to those in more recent surveys. In this
paper we give examples of such misidentifications, and show how we were able to
find and correct them.We present modern procedures to discover and solve
cross--identification problems, such as loading digitized images of the sky
through the Aladin service at CDS, and overlaying entries from historical
catalogues and modern surveys. We conclude that the use of good finding charts
still remains the ultimate (though time--consuming) tool to ascertain
cross--identifications in difficult cases.Comment: 4 pages, 1 figure, accepted by A&
Three aspects of red giant studies in the Magellanic Clouds
There are three important aspects concerning the study of the red giant and
in particular of the asymptotic giant branch (AGB) stars in the Magellanic
Clouds. These are: the surface distribution, the luminosity function and the
variability. The spatial distribution of AGB stars is an efficient tool to
study the structure of the galaxies and their metalicity by analysing the ratio
between carbon- and oxygen-rich AGB stars. The shape of the luminosity function
carries informations about the star formation rate in the Clouds and it can be
mathematically related to their history. Most AGB stars vary their magnitude in
a few to several hundred years time; the one epoch DENIS magnitudes for both
Large and Small Magellanic Cloud AGB stars outline the same relations as a
function of period.Comment: 8 pages, 6 figures, invited talk, to be published in: Mass-Losing
Pulsating Stars and their Circumstellar Matter, Y. Nakada & M. Honma (eds)
Kluwer ASSL serie
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
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
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
Evolution and appearance of Be stars in SMC clusters
Star clusters are privileged laboratories for studying the evolution of
massive stars (OB stars). One particularly interesting question concerns the
phases, during which the classical Be stars occur, which unlike HAe/Be stars,
are not pre-main sequence objects, nor supergiants. Rather, they are extremely
rapidly rotating B-type stars with a circumstellar decretion disk formed by
episodic ejections of matter from the central star. To study the impact of
mass, metallicity, and age on the Be phase, we observed SMC open clusters with
two different techniques: 1) with the ESO-WFI in its slitless mode, which
allowed us to find the brighter Be and other emission-line stars in 84 SMC open
clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine
accurately the evolutionary phases of Be stars in the Be-star rich SMC open
cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar
evolution / appearance as a function of metallicity and mass / spectral type is
developed, involving the fractional critical rotation rate as a key parameter.Comment: Proceedings of the IAUS266 of the GA200
New quasars behind the Magellanic Clouds. Spectroscopic confirmation of near-infrared selected candidates
Context. Quasi-stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are difficult to identify because of fore- and background contamination. Deep wide-field, high angular resolution surveys spanning the entire area of nearby galaxies are needed to obtain a complete census of such quasars. Aims. We embarked on a program to expand the quasar reference system behind the Large and the Small Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream that connects the Clouds with the Milky Way. Methods. Hundreds of quasar candidates were selected based on their near-infrared colors and variability properties from the ongoing public ESO VISTA Magellanic Clouds survey. A subset of 49 objects was followed up with optical spectroscopy. Results. We confirmed the quasar nature of 37 objects (34 new identifications): four are low redshift objects, three are probably stars, and the remaining three lack prominent spectral features for a secure classification. The bona fide quasars, identified from their broad emisison lines, are located as follows: 10 behind the LMC, 13 behind the SMC, and 14 behind the Bridge. The quasars span a redshift range from z ~ 0.5 to z ~ 4.1. Conclusions. Upon completion the VMC survey is expected to yield a total of ~1500 quasars with Y< 19.32 mag, J< 19.09 mag, and Ks< 18.04 mag
The 2nd to 4th century explosive activity of Vesuvius: new data on the timing of the upward migration of the post-A.D. 79 magma chamber
ber
(SMM), the eruption cycle occurred at Vesuvius (Italy) in the period
between the A.D. 79 plinian and the A.D. 472 subplinan eruptions. Historical
accounts report only sporadic, poorly reliable descriptions of the
volcanic activity in this period, during which a stratified sequence of ash
and lapilli beds, up to 150 cm thick, with a total volume estimated around
0.15 km3, was widely dispersed on the outer slopes of the volcano. Stratigraphic
studies and component analyses suggest that activity was characterized
by mixed hydromagmatic and magmatic processes. The
eruption style has been interpreted as repeated alternations of continuous
and prolonged ash emission activity intercalated with short-lived, violent
strombolian phases. Analyses of the bulk rock composition reveal that
during the entire eruption cycle, magma maintained an homogeneous
phonotephritic composition. In addition, the general trends of major and
trace elements depicted by the products of the A.D. 79 and A.D. 472 eruptions
converge to the SMM composition, suggesting a common mafic endmember
for these eruptions. The volatile content measured in
pyroxene-hosted melt inclusions indicates two main values of crystallization
pressures, around 220 and 70 MPa, roughly corresponding to the
previously estimated depth of the magma reservoirs of the A.D. 79 and
A.D. 472 eruptions, respectively. The study of SMM eruption cycle may
thus contribute to understand the processes governing the volcano reawakening
immediately after a plinian event, and the timing and modalities
which govern the migration of the magma reservoir
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