1,040 research outputs found
Baade-Wesselink distances to Galactic and Magellanic Cloud Cepheids and the effect of metallicity
(abridged) The aim of this paper is to investigate the metallicity dependence
of the PL relation in V and K, based on a sample of 128 Galactic, 36 LMC, and 6
SMC Cepheids with individual Baade-Wesselink (BW) distances and individually
determined metallicities from high-resolution spectroscopy. The p-relation
finally adopted is 1.50 -0.24log P. The slope of this relation is based on the
condition that the distance to the LMC does not depend on period or (V-K)
colour and that the slope of the PL relation based on the BW distances agrees
with that based on apparent magnitude. The zero point of the relation is tight
to the Cepheids with HST and revised Hipparcos parallaxes as well as to
Cepheids in clusters. The slope of the Galactic and LMC K-band relation
formally agrees within the errors, and combining all Cepheids (including the
SMC) results in a negligible metallicity dependence. A similar conclusion is
found for the reddening-free Wesenheit relation. In the V-band the situation is
more complex. The slope of the LMC and the Galactic PL relation differ at the
3sigma level. Combining the sample nevertheless results in a metallicity term
significant at the 2sigma level. The details of the comparison of BW-based
distances and Cepheids with HST and revised Hipparcos parallaxes also play a
role. The method used by Storm et al. would lead to larger DM of 18.37 and
18.81 for the LMC and SMC, respectively. The LMC DM is shorter than the
currently accepted value, which is in the range 18.42 to 18.55 (Walker 2012),
and it is speculated that the p-factor may depend on metallicity.Comment: Accepted for A&
The Cepheid period -- luminosity -- metallicity relation based on Gaia DR2 data
We use parallax data from the Gaia second data release (GDR2), combined with
parallax data based on Hipparcos and HST data, to derive the
period-luminosity-metallicity (PLZ) relation for Galactic classical cepheids
(CCs) in the V,K, and Wesenheit WVK bands. An initial sample of 452 CCs are
extracted from the literature with spectroscopically derived iron abundances.
Reddening values, pulsation periods, and mean magnitudes are taken from the
literature.
Based on nine CCs with a goodness-of-fit (GOF) statistic <8 and with an
accurate non-Gaia parallax, a parallax zero-point offset of -0.049 +- 0.018 mas
is derived. Selecting a GOF statistic <8 removes about 40\% of the sample most
likely related due to binarity. Excluding first overtone and multi-mode
cepheids and applying some other criteria reduces the sample to about 200
stars.
The derived PL(Z) relations depend strongly on the parallax zero-point
offset. The slope of the PL relation is found to be different from the
relations in the LMC at the 3 sigma level. Fixing the slope to the value found
in the LMC leads to a distance modulus (DM) to the LMC of order 18.7 mag,
larger than the canonical distance. The canonical DM of around 18.5 mag would
require a parallax zero-point offset of order mas.
Given the strong correlation between zero point, period and metallicity
dependence of the PL relation, and the parallax zero-point offset there is no
evidence for a metallicity term in the PLZ relation.
The GDR2 release does not allow us to improve on the current distance scale
based on CCs. The value of and the uncertainty on the parallax zero-point
offset leads to uncertainties of order 0.15 mag on the distance scale. The
parallax zero-point offset will need to be known at a level of 3 microas or
better to have a 0.01 mag or smaller effect on the zero point of the PL
relation and the DM to the LMC.Comment: Accepted 23 July by A&
First detection of rotational CO line emission in an RGB star
Context: For stars with initial masses below about 1 solar mass, the mass
loss during the first red giant branch (RGB) phase dominates mass loss in the
later asymptotic giant branch (AGB) phase. Nevertheless, mass loss on the RGB
is still often parameterised by a simple Reimers law in stellar evolution
models. Aims: To try to detect CO thermal emission in a small sample of nearby
RGB stars with reliable Hipparcos parallaxes that were shown to have infrared
excess in an earlier paper. Methods: A sample of five stars was observed in the
CO J=2-1 and J=3-2 lines with the IRAM and APEX telescopes. Results: One star,
the one with the largest mass-loss rate based on the previous analysis of the
spectral energy distribution, was detected. The expansion velocity is
unexpectedly large at 12 km/s. The line profile and intensity are compared to
the predictions from a molecular line emission code. The standard model
predicts a double-peaked profile, while the observations indicate a flatter
profile. A model that does fit the data has a much smaller CO envelope (by a
factor of 3), and a CO abundance that is two times larger and/or a larger
mass-loss rate than the standard model. This could indicate that the phase of
large mass loss has only recently started. Conclusions: The detection of CO in
an RGB star with a luminosity of only about 1300 solar luminosities and a
mass-loss rate as low as a few 10(-9) solarmasses/yr is important and the
results also raise new questions. However, ALMA observations are required in
order to study the mass-loss process of RGB stars in more detail, both for
reasons of sensitivity (6 hours of integration in superior weather at IRAM were
needed to get a 4sigma detection in the object with the largest detection
probability), and spatial resolution (to determine the size of the CO
envelope).Comment: A&A accepte
Eclipsing binaries in the Galactic Bulge: candidates for distance estimates
The 222~000 I-band light curves of variable stars detected by the OGLE-II
survey in the direction of the Galactic Bulge have been searched for eclipsing
binaries (EBs). A previously developed code to analyze lightcurve shapes and
identify long period variables (LPVs) has been adapted to identify EBs. The
parameters in the modified code have been optimised to recover a list of about
140 detached EBs in the Small Magellanic Cloud previously identified in the
literature as particularly well suited for distance estimates (and wich have
periods \more 0.85 days). The power of the code is demonstrated by identifying
16 and 178 previously uncatalogued EBs in the SMC and LMC, respectively. Among
the 222~000 variable stars in the direction of the Galactic Bulge 3053 EBs have
been identified. Periods and phased lightcurves are presented.Comment: accepted Astronomy and Astrophysic
The photodissociation of CO in circumstellar envelopes
Carbon monoxide is the most abundant molecule after H and is important
for chemistry in circumstellar envelopes around late-type stars. The size of
the envelope is important when modelling low-J transition lines and deriving
mass-loss rates from such lines. Now that ALMA is coming to full power the
extent of the CO emitting region can be measured directly for nearby asymptotic
giant branch (AGB) stars. In parallel, it has become obvious in the past few
years that the strength of the interstellar radiation field (ISRF) can have a
significant impact on the interpretation of the emission lines.
In this paper an update and extension of the classical Mamon et al. (1988;
ApJ 328, 797) paper is presented; these authors provided the CO abundance
profile, described by two parameters, as a function of mass-loss rate and
expansion velocity. Following recent work an improved numerical method and
updated H and CO shielding functions are used and a larger grid is
calculated that covers more parameter space, including the strength of the
ISRF.
The effect of changing the photodissociation radius on the low-J CO line
intensities is illustrated in two cases.Comment: A&A in pres
Infrared excess around nearby RGB stars and Reimers law
(Abridged) The spectral energy distributions of a well-defined sample of 54
RGB stars are constructed, and fitted with the dust radiative transfer model
DUSTY. The central stars are modeled by MARCS model atmospheres. In a first
step, the best-fit MARCS model is derived, determining the effective
temperature. In a second step, models with a finite dust optical depth are
fitted and it is determined whether the reduction in chi2 in such models with
one additional free parameter is statistically significant.
23 stars are found to have a significant infrared excess, which is
interpreted as mass loss. The dust optical depths are translated into mass-loss
rates assuming a typical expansion velocity of 10 km/s and a dust-to-gas ratio
of 0.005.
The mass-loss rates are compared to those derived for luminous stars in
globular clusters, by fitting both the infrared excess, as in the present
paper, and the chromospheric lines. There is excellent agreement between these
values and the mass-loss rates derived from the chromospheric activity. There
is a systematic difference with the literature mass-loss rates derived from
modeling the infrared excess, and this has been traced to technical details on
how the DUSTY radiative transfer model is run.
If the present results are combined with those from modeling the
chromospheric emission lines, we obtain the fits Log Mdot = (1.0 +- 0.3) Log L
+ (-12.0 +- 0.9) and Log Mdot = (0.6 +- 0.2) Log (LR/M) + (-11.9 +- 0.9).
The predictions of these mass-loss rate formula are tested against the RGB
mass loss determination in NGC 6791. Using a scaling factor of (8 +- 5), both
relations can fit this value. That the scaling factor is larger than unity
suggests that the expansion velocity and/or dust-to-gas ratio, or even the dust
opacities, are different from the values adopted.Comment: It was pointed out that the mass used for NGC 6719 is incorrect (its
1.2 and not 1.6 Msol). The numbers in table 6 are correct, but the inference
drawn from it not. The result is that the scaling factors eta_1 and eta_2
become slightly smaller. The conclusions of the paper remain unchanged. This
version has the updated Table 6 and eta's. These changes will appear as an
erratum to the A&A pape
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