136 research outputs found
Synthetic properties of bright metal-poor variables. I. "Anomalous" Cepheids
We present new grids of evolutionary models for the so-colled ``Anomalous''
Cepheids (ACs), adopting Z=0.0001 and various assumptions on the progenitor
mass and mass-loss efficiency. These computations are combined with the results
of our previous set of pulsation models and used to build synthetic populations
of the predicted pulsators as well as to provide a Mass-Luminosity relation in
the absence of mass-loss. We investigate the effect of mass-loss on the
predicted boundaries of the instability strip and we find that the only
significant dependence occurs in the Period-Magnitude plane, where the
synthetic distribution of the pulsators is, on average, brighter by about 0.1
mag than the one in absence of mass-loss. Tight Period-Magnitude relations are
derived in the K band for both fundamental and first overtone pulsators,
providing a useful tool for distance evaluations with an intrinsic uncertainty
of about 0.15 mag, which decreases to about 0.04 mag if the mass term is taken
into account. The constraints provided by the evolutionary models are used to
derive evolutionary (i.e, mass-independent) Period-Magnitude-Color relations
which provide distance determinations with a formal uncertainty of the order of
about 0.1 mag, once the intrinsic colors are well known. We also use model
computations from the literature to investigate the effect of metal content
both on the instability strip and on the evolutionary Period-Magnitude-Color
relations. Finally, we compare our theoretical predictions with observed
variables and we confirm that a secure identification of actual ACs requires
the simultaneous information on period, magnitude and color, that also provide
constraints on the pulsation mode.Comment: accepte
Variable stars in the globular cluster M28 (NGC 6626)
We present a new search for variable stars in the Galactic globular cluster
M28 (NGC 6626). The search is based on a series of BVI images obtained with the
SMARTS Consortium's 1.3m telescope at Cerro Tololo Inter-American Observatory,
Chile. The search was carried out using the ISIS v2.2 image subtraction
package. We find a total of 25 variable stars in the field of the cluster, 9
being new discoveries. Of the newly found variables, 1 is an ab-type RR Lyrae
star, 6 are c-type RR Lyrae, and 2 are long-period/semi-regular variables. V22,
previously classified as a type II Cepheid, appears as a bona-fide RRc in our
data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be
properly phased with any reasonable period. The properties of the ab-type RR
Lyrae stars in M28 appear most consistent with an Oosterhoff-intermediate
classification, which is unusual for bona-fide Galactic globulars clusters.
However, the cluster's c-type variables do not clearly support such an
Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another
possibility, thus raising the intriguing possibility of multiple populations
being present in M28. Coordinates, periods, and light curves in differential
fluxes are provided for all the detected variables.Comment: A&A, in pres
Detection of terminal complement components in experimental immune glomerular injury
Detection of terminal complement components in experimental immune glomerular injury. Complement mediates glomerulonephritis by inflammatory cell-dependent and non-inflammatory cell-independent effects on glomerular permeability. The latter may involve terminal components of the complement system. We examined several models of immunologic renal injury in the rat by immunofluorescence (IF) for terminal complement components C5, C6, C7, and C8 in glomeruli using antisera to human C5-8, which cross-react with the analogous rat complement components. Rats with the heterologous and autologous phases of passive Heymann nephritis (PHN) had proteinuria and 1 to 2+ capillary wall deposits of heterologous or rat IgG, rat C3, and C5-8. Complement depletion with cobra venom factor (CVF) significantly decreased proteinuria in both models and prevented deposition of all complement components. Rats with active Heymann nephritis had similar deposits of rat IgG and C5-8. Rats with anti-GBM nephritis and aminonucleoside nephrosis had severe proteinuria which was not affected by CVF treatment and deposits of C5-8 were absent. The presence of terminal complement components in immune deposits in experimental glomerular disease correlates with a functional role for complement in mediating glomerular injury. These data support the hypothesis that the terminal complement pathway may be a major mediator of some types of immune glomerular injury.Détection des constituants terminaux du complément au cours de lésions immunes glomérulaires expérimentales. Le complément est le médiateur d'une glomérulonéphrite par des effets inflammatoires cellule-dépendants, et non inflammatoires cellule-indépendants sur la perméabilité glomérulaire. Ces derniers pourraient mettre en jeu les constituants terminaux du système complémentaire. Nous avons examiné plusieurs modèles de lésions rénales immunologiques chez le rat par la immunofluorescence (IF) en ce qui concerne les constituants complémentaires terminaux C5, C6, C7, et C8 dans les glomérules en utilisant des antisérums contre C5-8 humain qui croisent avec les constituants complémentaires analogues du rat. Des rats dans les phases hétérologue et autologue d'une néphrite passive de Heymann (PHN) avaient une proléinurie et des dépôts sur les parois capillaires 1 à 2 + d'IgG hétérologue ou de rat, de C3 et de C5-8 de rat. Une déplétion complémentaire avec du facteur de venin de cobra (CVF) a diminué significativement la protéinurie dans les deux modèles et a prévenu le dépôt de tous les constituants complémentaires. Des rats atteints d'une nephrite active de Heymann avaient des dépôts identiques d'IgG et de C5-8 de rat. Des rats atteints de néphrite anti-GBM et de néphrose aux aminoglucosides avaient une protéinurie sévère non affectée par le traitement au CVF et les dépôts de C5-8 étaient absents. La présence des constituants terminaux de complément dans les dépôts immuns lors des glomérulopathies expérimentales est corrélée avec un rôle fonctionnel du complément dans la médiation des lésions glomérulaires. Ces données sont en faveur de l'hypothèse que la voie terminale du complément peut être un médiateur majeur de certains types de lésions glomérulaires immunes
Anomalous Cepheids in the Large Magellanic Cloud: Insight into their origin and connection with the star formation history
Context. The properties of variable stars can give independent constraints on
the star formation history of the host galaxy, by determining the age and
metallicity of the parent population. Aims. We investigate the pulsation
properties of 84 Anomalous Cepheids (ACs) detected by the OGLE-III survey in
the Large Magellanic Cloud (LMC), in order to understand the formation
mechanism and the characteristics of the parent population they came from.
Methods. We used an updated theoretical pulsation scenario to derive the mass
and the pulsation mode of each AC in the sample. We also used a
Kolmogorov-Smirnov test to analyze the spatial distribution of the ACs, in
comparison with that of other groups of variable stars, and connect their
properties with the star formation history of the LMC. Results. We find that
the mean mass of ACs is 1.2 \pm 0.2Mo. We show that ACs do not follow the same
spatial distribution of classical Cepheids. This and the difference in their
period-luminosity relations further support the hypothesis that ACs are not the
extension to low luminosity of classical Cepheids. The spatial distribution of
ACs is also different from that of bona-fide tracers of the old population,
such as RR Lyrae stars and population II Cepheids. We therefore suggest that
the majority of ACs in the LMC are made of intermediate-age (1-6Gyr),
metal-poor single stars. Finally, we investigate the relation between the
frequency of ACs and the luminosity of the host galaxy, disclosing that purely
old systems follow a very tight relation and that galaxies with strong
intermediate-age and young star formation tend to have an excess of ACs, in
agreement with their hosting ACs formed via both single and binary star
channels.Comment: 10 pages, 7 figures, accepted for publication on A&
M22: A [Fe/H] Abundance Range Revealed
Intermediate resolution spectra at the Ca II triplet have been obtained for
55 candidate red giants in the field of the globular cluster M22 with the
VLT/FORS instrument. Spectra were also obtained for a number of red giants in
standard globular clusters to provide a calibration of the observed line
strengths with overall abundance [Fe/H]. For the 41 M22 member stars that lie
within the V-V_HB bounds of the calibration, we find an abundance distribution
that is substantially broader than that expected from the observed errors
alone. We argue that this broad distribution cannot be the result of
differential reddening. Instead we conclude that, as has long been suspected,
M22 is similar to omega Cen in having an intrinsic dispersion in heavy element
abundance. The observed M22 abundance distribution rises sharply to a peak at
[Fe/H] = -1.9 with a broad tail to higher abundances: the highest abundance
star in our sample has [Fe/H] = -1.45 dex. If the unusual properties of omega
Cen have their origin in a scenario in which the cluster is the remnant nucleus
of a disrupted dwarf galaxy, then such a scenario likely applies also to M22.Comment: 29 pages, 9 figures, accepted for publication in the Astrophysical
Journa
The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI
We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion to M31,
for variable stars using F450W and F555W observations obtained with the Hubble
Space Telescope. A total of 118 variables were found, with 111 being RR Lyrae,
6 anomalous Cepheids, and 1 variable we were unable to classify. We find that
the Andromeda VI anomalous Cepheids have properties consistent with those of
anomalous Cepheids in other dwarf spheroidal galaxies. We revise the existing
period-luminosity relations for these variables. Further, using these and other
available data, we show that there is no clear difference between fundamental
and first-overtone anomalous Cepheids in a period-amplitude diagram at shorter
periods, unlike the RR Lyrae. For the Andromeda VI RR Lyrae, we find that they
lie close to the Oosterhoff type I Galactic globular clusters in the
period-amplitude diagram, although the mean period of the RRab stars, =
0.588 d, is slightly longer than the typical Oosterhoff type I cluster. The
mean V magnitude of the RR Lyrae in Andromeda VI is 25.29+/-0.03, resulting in
a distance 815+/-25 kpc on the Lee, Demarque, & Zinn distance scale. This is
consistent with the distance derived from the I magnitude of the tip of the red
giant branch. Similarly, the properties of the RR Lyrae indicate a mean
abundance for Andromeda VI which is consistent with that derived from the mean
red giant branch color.Comment: 23 pages, including 13 figures and 6 tables, emulateapj5/apjfonts
style. Accepted by the Astronomical Journal. We recommend the interested
reader to download the preprint with full-resolution figures, which can be
found at http://www.noao.edu/noao/staff/pritzl/M31dwarfs.htm
Stellar Archaeology in the Galactic halo with the Ultra-Faint Dwarfs: VI. Ursa Major II
We present a B, V color-magnitude diagram (CMD) of the Milky Way dwarf
satellite Ursa Major II (UMa II), spanning the magnitude range from V ~ 15 to V
~ 23.5 mag and extending over a 18 {\times} 18 arcmin2 area centered on the
galaxy. Our photometry goes down to about 2 magnitudes below the galaxy's main
sequence turn-off, that we detected at V ~ 21.5 mag. We have discovered a
bona-fide RR Lyrae variable star in UMa II, which we use to estimate a
conservative dereddened distance modulus for the galaxy of (m-M)0 =
17.70{\pm}0.04{\pm}0.12 mag, where the first error accounts for the
uncertainties of the calibrated photometry, and the second reflects our lack of
information on the metallicity of the star. The corresponding distance to UMa
II is 34.7 {\pm} 0.6 ({\pm} 2.0) kpc. Our photometry shows evidence of a spread
in the galaxy subgiant branch, compatible with a spread in metal abundance in
the range between Z=0.0001 and Z=0.001. Based on our estimate of the distance,
a comparison of the fiducial lines of the Galactic globular clusters (GCs) M68
and M5 ([Fe/H]=-2.27 {\pm} 0.04 dex and -1.33 {\pm} 0.02 dex, respectively),
with the position on the CMD of spectroscopically confirmed galaxy members, may
suggest the existence of stellar populations of different metal abundance/age
in the central region of UMa II.Comment: To appear in Ap
The distance to the Galactic Centre based on Population-II Cepheids and RR Lyrae stars
Context: The distance to the Galactic Centre (GC) is of importance for the
distance scale in the Universe. The value derived by Eisenhauer et al. (2005)
of 7.62 +- 0.32 kpc based on the orbit of one star around the central black
hole is shorter than most other distance estimates based on a variety of
different methods. Aim: To establish an independent distance to the GC with
high accuracy. To this end Population-II Cepheids are used that have been
discovered in the OGLE-II and III surveys. Method: Thirty-nine Pop-II Cepheids
have been monitored on 4 nights spanning 14 days. Light curves have been fitted
using the known periods from the OGLE data to determine the mean K-band
magnitude. It so happens that 37 RR Lyrae stars are in the field-of-views and
mean K-band magnitudes are derived for this sample as well. Results: The
period-luminosity relation of Pop-II Cepheids in the K-band is determined, and
the derived slope of -2.24 +- 0.14 is consistent with the value derived by
Matsunaga et al. (2006). Fixing the slope to their more accurate value results
in a zero point, and implies a distance modulus to the GC of 14.51 +- 0.12,
with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR
Lyrae K-band PL-relation we derive a value of 14.48 +- 0.17 (random) +- 0.07
(syst.). The two independent determinations are averaged to find 14.50 +- 0.10
(random) +- 0.07 (syst.), or 7.94 +- 0.37 +- 0.26 kpc.Comment: A&A accepte
RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy
(Abridged) We have surveyed four globular clusters in the Fornax dwarf galaxy
for RR Lyrae stars, using archival HST observations. We identify 197 new RR
Lyrae stars in these four clusters. Despite the short observational baseline,
we derive periods, light-curves, and photometric parameters for each. The
Fornax clusters have exceptionally large RR Lyrae specific frequencies compared
with the Galactic globular clusters. Furthermore, the Fornax cluster RR Lyrae
stars are unusual in that their characteristics are intermediate between the
two Galactic Oosterhoff groups. In this respect the Fornax clusters are similar
to the field populations in several dwarf galaxies. We revise previous
measurements of the HB morphology in each cluster. The Fornax clusters closely
resemble the ``young'' Galactic halo population defined by Zinn. The existence
of the second parameter effect among the Fornax clusters is also confirmed.
Finally, we determine foreground reddening and distance estimates for each
cluster. We find a mean distance modulus to Fornax of (m-M)_0 = 20.66 +/- 0.03
(random) +/- 0.15 (systematic). Our measurements are consistent with a line of
sight depth of 8-10 kpc for this galaxy, matching its projected dimensions, and
incompatible with tidal model explanations for the observed high velocity
dispersions in many dSph galaxies. Dark matter dominance is suggested.Comment: 26 pages, 6 figures. Accepted for publication in MNRAS. Table 2 and
Figure 2 will only be available in the electronic version. On-line data will
soon be available at http://www.ast.cam.ac.uk/STELLARPOPS/Fornax_RRlyr
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