4,612 research outputs found
Period-Color and Amplitude-Color Relations in Classical Cepheid Variables - VI. New Challenges for Pulsation Models
We present multiphase Period-Color/Amplitude-Color/Period-Luminosity
relations using OGLE III and Galactic Cepheid data and compare with state of
the art theoretical pulsation models. Using this new way to compare models and
observations, we find convincing evidence that both Period-Color and
Period-Luminosity Relations as a function of phase are dynamic and highly
nonlinear at certain pulsation phases. We extend this to a multiphase Wesenheit
function and find the same result. Hence our results cannot be due to reddening
errors. We present statistical tests and the urls of movies depicting the
Period-Color/Period Luminosity and Wesenheit relations as a function of phase
for the LMC OGLE III Cepheid data: these tests and movies clearly demonstrate
nonlinearity as a function of phase and offer a new window toward a deeper
understanding of stellar pulsation. When comparing with models, we find that
the models also predict this nonlinearity in both Period-Color and
Period-Luminosity planes. The models with (Z=0.004, Y=0.25) fare better in
mimicking the LMC Cepheid relations, particularly at longer periods, though the
models predict systematically higher amplitudes than the observations
On the Impact of Helium Content on the RR Lyrae Distance Scale
Indexación: Scopus.We constructed new sets of He-enhanced (Y = 0.30, Y = 0.40) nonlinear, time-dependent convective hydrodynamical models of RR Lyrae (RRL) stars covering a broad range in metal abundances (Z = 0.0001-0.02). The increase in He content from the canonical value (Y = 0.245) to Y = 0.30-0.40 causes a simultaneous increase in stellar luminosity and in pulsation period. To investigate the dependence of the RRL distance scale on the He abundance, we computed new optical (RI) and near-infrared (JHK) Period-luminosity-metallicity-helium relations. Interestingly enough, the increase in He content causes a minimal change in the coefficients of both period and metallicity terms, since canonical and He-enhanced models obey similar PLZ relations. On the contrary, the classical B-And V-band mean magnitude metallicity relations and the R-band PLZ relation display a significant dependence on the He content. The He-enhanced models are, at fixed metal content, 0.2-0.5 mag brighter than canonical ones. This variation is only marginally affected by evolutionary effects. The quoted distance diagnostics once calibrated with trigonometric parallaxes (Gaia) will provide the opportunity to estimate the He content of field and cluster RRLs. Moreover, the use of either spectroscopic or photometric metal abundances will pave the way to new empirical constraints on the universality of the helium-To-metal enrichment ratio in old (t10 Gyr) stellar tracers. © 2018. The American Astronomical Society. All rights reserved.https://iopscience.iop.org/article/10.3847/2041-8213/aada1
Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822
On the basis of a new photometric analysis of the Local Group dwarf irregular
galaxy NCG 6822 based on observations obtained with the Advanced Camera for
Surveys onboard the Hubble Space Telescope, we have obtained a new estimate of
the extinction of two fields located in the southeast region of the galaxy.
Because of significant differences in the distance estimates to NGC 6822
available in literature, we decided to provide an independent determination of
the distance to this galaxy based on an updated and self-consistent theoretical
calibration of the tip of the red giant branch brightness. As a result we newly
determined the distance to NGC 6822 to be equal to , and compared our measurement with the most recent determinations of this
distance.Comment: 5 pages, 5 figures, Astronomy & Astrophysics (Research Note), in
press (proof correction included
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV observations of some massive globular clusters uncovered a significant
population of very hot stars below the hot end of the horizontal branch (HB),
the so-called blue hook stars. This feature might be explained either as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich
stars show strong carbon enrichment as predicted by the late hot flasher
scenario. We conclude that the helium-rich HB stars in omega Cen cannot be
explained solely by the helium-enrichment scenario invoked to explain the blue
main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy
We present a study of short period, central helium-burning variable stars in
the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and
51 Cepheids. So far, this is the largest sample of Cepheids and the largest
Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with
other Local Group dwarf spheroidals, Carina and Fornax, shows that the period
distribution of RR Lyrae stars is quite similar, suggesting similar properties
of the parent populations, whereas the Cepheid period distribution in LeoI
peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a
broader range, from 0.5 to 1.78d.
Evolutionary and pulsation predictions indicate, assuming a mean metallicity
peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids
traces a unique mix of Anomalous Cepheids (blue extent of the red--clump,
partially electron degenerate central helium-burning stars) and short-period
classical Cepheids (blue-loop, quiescent central helium-burning stars). Current
evolutionary prescriptions also indicate that the transition mass between the
two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars
metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different
implications of the current findings on the star formation history of LeoI.Comment: 5 Pages, 4 Figures, ApJ letter, accepte
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
A state-of-the-art analysis of the dwarf irregular galaxy NGC 6822
We present a detailed photometric study of the dwarf irregular galaxy NGC
6822 aimed at investigating the properties of its stellar populations and, in
particular, the presence of stellar radial gradients. Our goal is to analyse
the stellar populations in six fields, which cover the whole bar of this dwarf
galaxy. We derived the quantitative star formation history (SFH) of the six
fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we
derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the
past 500 Myr. The SFRs of the other fields are almost extinguished at very
recent epochs and. We study the radial gradients of the SFR and consider the
total mass converted into stars in two time intervals (between 0 and 0.5 Gyr
ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the
young and intermediate-to-old populations are perfectly compatible, with the
exception of the young populations in Fields 1 and 3. The recent SF in these
two fields is greater than in the other ones. This might be an indication that
in these two fields we are sampling incipient spiral arms. Further evidence and
new observations are required to prove this hypothesis. In addition, we derived
the age-metallicity relations. As expected, the metallicity increases with time
for all of the fields. We do not observe any radial gradient in the
metallicity.Comment: 9 pages, 11 figures, Accepted to A&
Improvement of the CORS method for Cepheids radii determination based on Stromgren photometry
In this paper we present a modified version of the CORS method based on a new
calibration of the Surface Brightness function in the Stromgren photometric
system. The method has been tested by means of synthetic light and radial
velocity curves derived from nonlinear pulsation models. Detailed simulations
have been performed to take into account the quality of real observed curves as
well as possible shifts between photometric and radial velocity data. The
method has been then applied to a sample of Galactic Cepheids with Stromgren
photometry and radial velocity data to derive the radii and a new PR relation.
As a result we find log R = (1.19 +- 0.09) + (0.74 +- 0.11) log P (r.m.s=0.07).
The comparison between our result and previous estimates in the literature is
satisfactory. Better results are expected from the adoption of improved model
atmosphere grids.Comment: 13 pages including 12 postscript figures and 3 jpeg figures; accepted
for publication on A&
Predicted properties of RR Lyrae stars in the SDSS photometric system
The luminosities and effective temperatures, as well as the whole bolometric
lightcurves of nonlinear convective RR Lyrae models with 0.0001<Z<0.006 are
transformed into the SDSS photometric system. The obtained ugriz lightcurves,
mean magnitudes and colors, pulsation amplitudes and color-color loops are
shown and analytical relations connecting pulsational to intrinsic stellar
parameters, similarly to the ones currently used in the Johnson-Cousins
filters, are derived. Finally the behaviour in the color-color planes is
compared with available observations in the literature and possible systematic
uncertainties affecting this comparison are discussed.Comment: accepted for publication in MNRA
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