arXiv subjects

M. W. Feast

Publications and source records attributed to M. W. Feast.

15 recordsLinked to original sources

arXiv2018

Luminous AGB variables in the dwarf Irregular Galaxy, NGC 3109

In a shallow near-infrared survey of the dwarf Irregular galaxy, NGC 3109, near the periphery of the Local Group, we have found eight Mira variables, seven of which appear to be oxygen-rich (O-Miras). The periods range from about 430 days to almost 1500 days. Because of our relatively bright limiting magnitude, only 45 of the more than 400 known carbon stars were measured, but none was found to be a large amplitude variable. One of the Miras may be an unrecognised C star. Five of the O-Miras are probably hot-bottom burning stars considering that they are brighter than expected from the period--luminosity relation of Miras and that, by comparison with theoretical evolutionary tracks, they appear to have masses >~4 Msun. A census of very long period (P>1000 days) Miras in the Galaxy and Magellanic Clouds is presented and discussed together with the newly discovered long period, but relatively blue, variables in NGC 3109. New $JHKL$ photometry is presented for three O-rich long period Miras i n the SMC (including a candidate super-AGB star).

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arXiv2018

R Coronae Borealis: Radial-velocity and other observations, 1950-2007

Radial-velocity observations made on more than a thousand nights are presented for the type star of the R Coronae Borealis (RCB) class. There are four principal sources: the Lick Observatory (1950-1953), the original Cambridge radial-velocity spectrometer (1968-1991), and the Haute-Provence and Cambridge Coravels (1986-1998 and 1997- 2007, respectively). In the case of the last set the size (equivalent width) and width (expressed as if Vsin(i)) of the Coravel cross-correlation ('dip') profiles are also given, and the variation and complexity of those profiles are discussed. Although there is often evidence of cyclical behaviour in radial velocity, no coherent periodicity is found in any of the series. From time to time, and especially over 100 days before the great decline of 2007, the atmosphere was highly disturbed, with evidence of high-velocity components. We suggest that those are associated with large turbulent elements and result in mass ejection to sufficient distances for the formation of soot and other solids and thus the initiation of RCB-type declines. We associate the changes in light and radial velocity near maximum light primarily with the combined effect of such turbulent elements, and not with coherent pulsation. There is some evidence for a variation in the mean radial velocity on a time scale of about ten thousand days.

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arXiv2016

The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud

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: Attempt to, for the first time, spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud. Methods: ALMA was used to observe 2 OH/IR stars and 4 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. Gas-to-dust ratios are derived that make the predicted velocities agree with the observed ones. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with "extreme" C-stars that have properties more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than derived from the 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 it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear similar. This is in agreement with dynamical dust-driven wind models.

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arXiv2014

The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase

We initiated long-term optical interferometry monitoring of the diameters of unstable yellow hypergiants (YHG) with the goal of detecting both the long-term evolution of their radius and shorter term formation related to large mass-loss events. We observed HR5171 A with AMBER/VLTI. We also examined archival photometric data in the visual and near-IR spanning more than 60 years, as well as sparse spectroscopic data. HR5171A exhibits a complex appearance. Our AMBER data reveal a surprisingly large star for a YHG R*=1315+/-260Rsun\ (~6.1AU) at the distance of 3.6+/-0.5kpc. The source is surrounded by an extended nebulosity, and these data also show a large level of asymmetry in the brightness distribution of the system, which we attribute to a newly discovered companion star located in front of the primary star. The companion's signature is also detected in the visual photometry, which indicates an orbital period of Porb=1304+/-6d. Modeling the light curve with the NIGHTFALL program provides clear evidence that the system is a contact or possibly over-contact eclipsing binary. A total current system mass of 39^{+40}_{-22} solar mass and a high mass ratio q>10 is inferred for the system. The low-mass companion of HR5171 A is very close to the primary star that is embedded within its dense wind. Tight constraints on the inclination and vsini of the primary are lacking, which prevents us from determining its influence precisely on the mass-loss phenomenon, but the system is probably experiencing a wind Roche-Lobe overflow. Depending on the amount of angular momentum that can be transferred to the stellar envelope, HR5171 A may become a fast-rotating B[e]/Luminous Blue Variable (LBV)/Wolf-Rayet star. In any case, HR5171 A highlights the possible importance of binaries for interpreting the unstable YHGs and for massive star evolution in general.

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arXiv2012

The Cepheid distance to the Local Group Galaxy NGC 6822

Recent estimates of the Cepheid distance modulus of NGC 6822 differ by 0.18 mag. To investigate this we present new multi-epoch JHKs photometry of classical Cepheids in the central region of NGC 6822 and show that there is a zero-point difference from earlier work. These data together with optical and mid-infrared observations from the literature are used to derive estimates of the distance modulus of NGC 6822. A best value of 23.40 mag is adopted, based on an LMC distance modulus of 18.50 mag. The standard error of this quantity is ~0.05 mag. We show that to derive consistent moduli from Cepheid observations at different wavelengths, it is necessary that the fiducial LMC period-luminosity relations at these wavelengths should refer to the same subsample of stars. Such a set is provided. A distance modulus based on RR Lyrae variables agrees with the Cepheid result.

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arXiv2011

Three classical Cepheid variable stars in the nuclear bulge of the Milky Way

The nuclear bulge is a region with a radius of about 200 parsecs around the centre of the Milky Way. It contains stars with ages ranging from a few million years to over a billion years, yet its star-formation history and the triggering process for star formation remain to be resolved. Recently, episodic star formation, powered by changes in the gas content, has been suggested. Classical Cepheid variable stars have pulsation periods that decrease with increasing age, so it is possible to probe the star-formation history on the basis of the distribution of their periods. Here we report the presence of three classical Cepheids in the nuclear bulge with pulsation periods of approximately 20 days, within 40 parsecs (projected distance) of the central black hole. No Cepheids with longer or shorter periods were found. We infer that there was a period about 25 million years ago, and possibly lasting until recently, in which star formation increased relative to the period of 30-70 million years ago.

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arXiv2009

Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy

We present spectroscopic observations from the {\it Spitzer Space Telescope} of six carbon-rich AGB stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C$_2$H$_2$ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the LMC, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the DUSTY radiative transfer model and determine their dust mass-loss rates to be in the range 1.0--3.3$\times 10^{-8} $M$_{\odot}$yr$^{-1}$. The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars show the strongest SiC feature in our present Local Group sample.

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arXiv2009

Discovery of the first symbiotic star in NGC6822

We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in the Local Group dwarf irregular galaxy NGC6822. This star was identified during a spectral survey of Ha emission-line objects using the Southern African Large Telescope (SALT) during its performance-verification phase. The observed strong emission lines of HI and HeII suggest a high electron density and T* < 130 000 K for the hot companion. The infrared colours allow us to classify this object as an S-type symbiotic star, comprising a red giant losing mass to a compact companion. The red giant is an AGB carbon star, and a semi-regular variable, pulsating in the first overtone with a period of 142 days. Its bolometric magnitude is M_bol=-4.4 mag. We review what is known about the luminosities of extragalactic symbiotic stars, showing that most, possibly all, contain AGB stars. We suggest that a much larger fraction of Galactic symbiotic stars may contain AGB stars than was previously realised.

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arXiv2007

Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy

We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C2H2 bands are stronger at lower metallicity because of the higher C/O ratio. In contrast, the SiC fraction is reduced at low metallicity, due to low silicon abundance. The total mass-loss rate from all known carbon-rich AGB stars into the interstellar medium of this galaxy is of the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf irregular galaxy WLM, which has a similar visual luminosity and metallicity. The difference is attributed to the younger stellar population of WLM. The suppressed gas-return rate to the ISM accentuates the difference between the relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies. Our study will be useful to constrain gas and dust recycling processes in low metallicity galaxies.

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arXiv2006

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space telescope spectroscopic observations of 14 carbon-rich AGB stars in the Small Magellanic Cloud. SiC dust is seen in most of the carbon-rich stars but it is weak compared to LMC stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas the reverse situation occurs in the Galaxy where SiC condenses at higher temperatures and forms first. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-micron band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C$_2$H$_2$ bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour-colour diagram based on Spitzer IRAC and MIPS colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for $\sim 6 \times 10^5$ yr. For the LMC, this lifetime is $\sim 3 \times 10^5$ yr. The superwind phase traced with Spitzer lasts for $\sim 10^4$ yr. Spitzer spectra of a K supergiant and a compact HII region are also given.

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arXiv2006

Spitzer observations of acetylene bands in carbon-rich AGB stars in the Large Magellanic Cloud

We investigate the molecular bands in carbon-rich AGB stars in the Large Magellanic Cloud (LMC), using the InfraRed Spectrograph (IRS) on board the Spitzer Space Telescope (SST) over the 5--38 micron range. All 26 low-resolution spectra show acetylene (C2H2) bands at 7 and 14 micron. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 micron C2H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 micron band by circumstellar dust emission. This 14 micron band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2H2,column density, derived from the 13.7 micron band, shows a gas mass-loss rate in the range 3x10^-6 to 5x10^{-5} Msun yr-1. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 micron C2H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2H2 abundance among LMC and Galactic stars. This reflects the effect of the 3rd dredge-up bringing self-produced carbon to the surface, leading to high C/O ratios at low metallicity.

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arXiv2006

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescope spectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The spectra cover the wavelength range 5--38 micron. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The relations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-micron C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-micron absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitrite dust. The line-to-continuum ratio of theSiC and MgS bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J-K vs. K-A colours, and show that these colours are relatively insensitive to chemical type. Metal-poor carbon stars form amorphous carbon dust from self-produced carbon. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compare to the Galaxy.

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arXiv2005

Three-micron spectra of AGB stars and supergiants in nearby galaxies

The dependence of stellar molecular bands on the metallicity is studied using infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C2H2 abundance can be explained with a high carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra-galactic carbon stars show the 3.5micron HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity (i.e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra-galactic and Galactic carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30angstrom for LMC metallicity. Several possible explanations are discussed. The observations imply that LMC and SMC carbon stars could reach mass-loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich stars. The effect on the ISM dust enrichment is discussed.

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arXiv1997

The Luminosities and Diameters of Mira Variables from Hipparcos Parallaxes

Hipparcos trigonometrical parallaxes of Mira-type variables have been combined with ground-based angular diameter measurements to derive linear diameters. Of eight stars with ground-based data, six have diameters indicating overtone pulsation whilst two, both with periods over 400 day, are pulsating in the fundamental. Hipparcos parallaxes of 11 Miras have been combined with extensive infrared photometry to determine the zero-point of the Mira period-luminosity relation. Adopting the relation at K (2.2 micron), since this is less likely to be subject to abundance effects than that at Mbol, leads to a distance modulus for the LMC of 18.6 mag with a uncertainty of slightly less than 0.2 mag. A brief discussion is given of the preliminary analysis of the parallaxes of a much larger sample of Miras. Some consideration is given to possible problems in interpreting the Hipparcos data which arise because of the physical characteristics of the Mira variables. Finally the apparent low-luminosity of the carbon Mira, R Lep, implied by the Hipparcos results leads to an interesting problem in AGB evolution.

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arXiv1997

Hipparcos Parallaxes and Proper Motions of Cepheids and their Implications

A summary is given of an analysis of the Hipparcos trigonometrical parallaxes and proper motions of classical Cepheids. It is possible for the first time to derive zero-points for the period-luminosity and period-luminosity-colour relations from parallaxes alone, avoiding the problems of less direct methods. The results imply an increase of 8 to 10 percent in the extragalactic distance scale based on Cepheids. The proper motions are used to derive the constants of galactic rotation. Comparison with radial velocity data leads to a confirmation of the Cepheid distance scale derived from the parallaxes and indicates a kinematic distance to the galactic centre of 8.5 +/- 0.5 kpc. From the new Cepheid distances to the LMC and M31, the absolute magnitude of RR Lyrae variables in metal-poor globular clusters is derived. Applying this to data on metal-poor clusters in our own Galaxy leads to an age of about 11 Gyr for these clusters, considerably less than previously thought. Other evidence from Hipparcos on these matters is briefly reviewed and it is suggested that the Cepheid results currently provide the most reliable scale on which to base distances and ages.

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