We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species towards the archetypical circumstellar envelope of IRC+10216. The observations ...have been carried out with the HIFI instrument on board
and with the IRAM 30-m telescope. They cover several observing periods spreading over 3 years. The line intensity variations for molecules produced in the external layers of the envelope most probably result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations have to take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The sub-mm and FIR lines of AGB stars cannot anymore be considered as safe intensity calibrators.
ABSTRACT
We recover the spatially resolved star formation history across the entire main body and Wing of the Small Magellanic Cloud (SMC), using 14 deep tile images from the VISTA survey of the ...Magellanic Clouds (VMC) in the YJKs filters. The analysis is performed on 168 subregions of size 0.143 deg2 covering a total contiguous area of 23.57 deg2. We apply a colour–magnitude diagram (CMD) reconstruction method that returns the best-fitting star formation rate SFR(t), age–metallicity relation, distance and mean reddening, together with their confidence intervals, for each subregion. With respect to previous analyses, we use a far larger set of the VMC data, updated stellar models, and fit the two available CMDs (Y − Ks versus Ks and J − Ks versus Ks) independently. The results allow us to derive a more complete and more reliable picture of how the mean distances, extinction values, star formation rate, and metallicities vary across the SMC, and provide a better description of the populations that form its Bar and Wing. We conclude that the SMC has formed a total mass of (5.31 ± 0.05) × 108 M⊙ in stars over its lifetime. About two-thirds of this mass is expected to be still locked in stars and stellar remnants. 50 per cent of the mass was formed prior to an age of 6.3 Gyr, and 80 per cent was formed between 8 and 3.5 Gyr ago. We also illustrate the likely distribution of stellar ages and metallicities in different parts of the CMD, to aid the interpretation of data from future astrometric and spectroscopic surveys of the SMC.
Abstract
We present the results of targeted observations and a survey of 1612-, 1665- and 1667-MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) ...in the Small Magellanic Cloud (SMC), using the Parkes and Australia Telescope Compact Array (ATCA) radio telescopes. No clear OH maser emission has been detected in any of our observations targeting luminous, long-period, large-amplitude variable stars, which have been confirmed spectroscopically and photometrically to be mid- to late-M spectral type. These observations have probed 3–4 times deeper than any OH maser survey in the SMC. Using a bootstrapping method with Large Magellanic Cloud (LMC) and Galactic OH/IR star samples and our SMC observation upper limits, we have calculated the likelihood of not detecting maser emission in any of the two sources considered to be the top maser candidates to be less than 0.05 per cent, assuming a similar pumping mechanism as the LMC and Galactic OH/IR sources. We have performed a population comparison of the Magellanic Clouds and used Spitzer IRAC and MIPS photometry to confirm that we have observed all high luminosity SMC sources that are expected to exhibit maser emission. We suspect that, compared to the OH/IR stars in the Galaxy and LMC, the reduction in metallicity may curtail the dusty wind phase at the end of the evolution of the most massive cool stars. We also suspect that the conditions in the circumstellar envelope change beyond a simple scaling of abundances and wind speed with metallicity.
ABSTRACT The Infrared Spectrograph on the Spitzer Space Telescope observed 184 carbon stars in the Magellanic Clouds. This sample reveals that the dust-production rate (DPR) from carbon stars ...generally increases with the pulsation period of the star. The composition of the dust grains follows two condensation sequences, with more SiC condensing before amorphous carbon in metal-rich stars, and the order reversed in metal-poor stars. MgS dust condenses in optically thicker dust shells, and its condensation is delayed in more metal-poor stars. Metal-poor carbon stars also tend to have stronger absorption from C2H2 at 7.5 m. The relation between DPR and pulsation period shows significant apparent scatter, which results from the initial mass of the star, with more massive stars occupying a sequence parallel to lower-mass stars, but shifted to longer periods. Accounting for differences in the mass distribution between the carbon stars observed in the Small and Large Magellanic Clouds reveals a hint of a subtle decrease in the DPR at lower metallicities, but it is not statistically significant. The most deeply embedded carbon stars have lower variability amplitudes and show SiC in absorption. In some cases they have bluer colors at shorter wavelengths, suggesting that the central star is becoming visible. These deeply embedded stars may be evolving off of the asymptotic giant branch and/or they may have non-spherical dust geometries.
Context. The Cepheid period-luminosity (PL) relation is unquestionably one of the most powerful tools at our disposal for determining the extragalactic distance scale. While significant progress has ...been made in the past few years towards its understanding and characterization both on the observational and theoretical sides, the debate on the influence that chemical composition may have on the PL relation is still unsettled. Aims. With the aim to assess the influence of the stellar iron content on the PL relation in the V and K bands, we have related the V-band and the K-band residuals from the standard PL relations of Freedman et al. (2001, ApJ, 553, 47) and Persson et al. (2004, AJ, 128, 2239), respectively, to Fe/H. Methods. We used direct measurements of the iron abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra. Results. We find a mean iron abundance (Fe/H) about solar (\sigma = 0.10) for our Galactic sample (32 stars), similar to -0.33 dex (\sigma = 0.13) for the Large Magellanic Cloud (LMC) sample (22 stars) and similar to -0.75 dex (\sigma = 0.08) for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundance measurements of the Magellanic Cepheids double the number of stars studied up to now at high resolution. The metallicity affects the V- band Cepheid PL relation and metal-rich Cepheids appear to be systematically fainter than metal-poor ones. These findings depend neither on the adopted distance scale for Galactic Cepheids nor on the adopted LMC distance modulus. Current data do not allow us to reach a firm conclusion concerning the metallicity dependence of the K-band PL relation. The new Galactic distances indicate a small effect, whereas the old ones support a marginal effect. Conclusions. Recent robust estimates of the LMC distance and current results indicate that the Cepheid PL relation is not Universal.
We present K
s-band light curves for 299 Cepheids in the Small Magellanic Cloud (SMC) of which 288 are new discoveries that we have identified using multi-epoch near-infrared photometry obtained by ...the VISTA survey of the Magellanic Clouds system (VMC). The new Cepheids have periods in the range from 0.34 to 9.1 d and cover the magnitude interval 12.9 ≤ 〈K
s〉 ≤ 17.6 mag. Our method was developed using variable stars previously identified by the optical microlensing survey OGLE. We focus on searching new Cepheids in external regions of the SMC for which complete VMC K
s-band observations are available and no comprehensive identification of different types of variable stars from other surveys exists yet.
ABSTRACT
We present results from an analysis of ∼29 000 RR Lyrae stars located in the Large Magellanic Cloud (LMC). For these objects, near-infrared time-series photometry from the VISTA survey of ...the Magellanic Clouds system (VMC) and optical data from the Optical Gravitational Lensing Experiment (OGLE) IV survey and the Gaia Data Release 2 catalogue of confirmed RR Lyrae stars were exploited. Using VMC and OGLE IV magnitudes we derived period–luminosity (PL), period–luminosity–metallicity (PLZ), period–Wesenheit (PW), and period–Wesenheit–metallicity (PWZ) relations in all available bands. More that 7000 RR Lyrae were discarded from the analysis because they appear to be overluminous with respect to the PL relations. The $PL_{K_{\mathrm{s}}}$ relation was used to derive individual distance to ${\sim}22\, 000$ RR Lyrae stars, and study the three-dimensional structure of the LMC. The distribution of the LMC RR Lyrae stars is ellipsoidal with the three axis S1 = 6.5 kpc, S2 = 4.6 kpc, and S3 = 3.7 kpc, inclination i = 22 ± 4° relative to the plane of the sky and position angle of the line of nodes θ = 167 ± 7° (measured from north to east). The north-eastern part of the ellipsoid is closer to us and no particular associated substructures are detected and neither any metallicity gradient.
In this study, we constructed spectral energy distributions (SEDs) for a sample of 142 Large Magellanic Cloud (LMC) and 77 Small Magellanic Cloud (SMC) fundamental-mode classical Cepheids (CCs) using ...photometric data from the literature. When possible, the data were taken to be representative of mean light or averaged over the light curve. The sample was built from stars that either have a metallicity determination from high-resolution spectroscopy or have been used in Baade-Wesselink types of analyses, or have a radial velocity curve published in
Gaia
DR3 or have Walraven photometry, or have their light- and radial-velocity curves modelled by pulsation codes. The SEDs were fitted with stellar photosphere models to derive the best-fitting luminosity and effective temperature. Distance and reddening were taken from the literature. Only one star with a significant infrared (IR) excess was found in the LMC and none in the SMC. IR excess in MW CCs is not uncommon suggesting that IR excess may be more prominent in MW cepheids than in the Magellanic Clouds. The stars were plotted in a Hertzsprung-Russell diagram (HRD) and compared to evolutionary tracks for CCs and to theoretical instability strips. For the large majority of stars, the position in the HRD is consistent with the instability strip. Period-luminosity (PL) and period-radius relations were derived and compared to these relations in the MW. For a fixed slope, the zero point of the bolometric PL relation does not depend on metallicity, contrary to recent findings of a significant metallicity term when considering the PL relation in different photometric bands. The mass-luminosity (ML) relation is derived and it points to an over luminosity of about +0.3 dex with respect to a canonical
ML
relation. The most intriguing result concerns the flux-weighted gravity (FWG, a quantity derived from gravity and effective temperature) and its relation to period and luminosity. Both relations agree with theory, with the results for the MW and with the independent estimates from the six known LMC eclipsing binaries that contain CCs. However, the FWG (as determined from dedicated high-resolution spectroscopy for the sample) is too low by about 0.8 dex in 90% of the cases. Recent works on time-series data on 20 CCs in the MW were analysed finding a similar (but less extreme) offset in gravity and the FWG. Most importantly, other time-series data on the same 20 CCs are in full agreement with the FWG-period relation. The observed time-series of spectroscopic data and from a two-dimensional hydrodynamical cepheid model was used to investigate the so-called effective gravity, that is, the gravity corrected for a dynamical term related to the time derivative of the radial velocity. There is a reasonable good correspondence between the predicted effective gravity and the observed gravity as a function of pulsation phase, which would potentially allow for an independent estimate of the projection factor, but the dynamical term is too small to explain the overall difference between the observed (flux-weighted) gravity and the (flux-weighted) gravity derived from the SED modelling and stellar mass estimates.
We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 mu m data and improved imaging ...quality at 100 and 160 mu m compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 mu m OI line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 mu m flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5 + or - 0.1 M sub(middot in circle) of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 M sub(middot in circle) of amorphous carbon and 0.5 M sub(middot in circle) of silicates, totalling 0.8 M sub(middot in circle) of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies.
Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important ...in quantifying the contribution of evolved stars to the enrichment of the interstellar medium. Aims. This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC). Methods. The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2−1 line. Results. We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC. Conclusions. Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models.
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