arXiv subjects

Michael W. Feast

Publications and source records attributed to Michael W. Feast.

At least 19 recordsLinked to original sources

arXiv2020

Light and colour variations of Mira variables in the Small Magellanic Cloud

The goal of this paper is to characterise the light variation properties of Mira variables in the Small Magellanic Cloud. We have investigated a combined optical and near infrared multi-epoch dataset of Mira variables based on our monitoring data obtained over 15 years. Bolometric correction relations are formulated for various near-infrared colours. We find that the same bolometric correction equation holds for both the bolometricly brightest and faintest pulsation phases. Period-bolometric magnitude relations and period-colour relations were derived using time-averaged values. Phase lags between bolometric phase and optical and near-infrared phases were detected from the O-rich (the surface C/O number ratio is below unity) Mira variables, while no significant systematic lags were observed in most of the C-rich (the C/O ratio is over unity) ones. Some Miras show colour phase inversions, e.g., $H-K_{\rm s}$ at its bluest while $J-H$ and $J-K_{\rm s}$ are at their reddest values at about the bolometricly brightest phase. Their occurrence conditions were studied but no clear direct or indirect trigger was found. A large NIR colour change unassociated with stellar pulsation was observed in Miras with long secondary periods, and its possible explanation is described.

astro-ph.SR↗

arXiv2018

A near infrared variable star survey in the Magellanic Clouds: The Small Magellanic Cloud data

A very long term near-infrared variable star survey towards the Large and Small Magellanic Clouds was carried out using the 1.4m InfraRed Survey Facility at the South African Astronomical Observatory. This project was initiated in December 2000 in the LMC, and in July 2001 in the SMC. Since then an area of 3 square degrees along the bar in the LMC and an area of 1 square degree in the central part of the SMC have been repeatedly observed. This survey is ongoing, but results obtained with data taken until December 2017 are reported in this paper. Over more than 15 years we have observed the two survey areas more than one hundred times. This is the first survey that provides near-infrared time-series data with such a long time baseline and on such a large scale. This paper describes the observations in the SMC and publishes a point source photometric catalogue, a variable source catalogue, and time-series data.

astro-ph.SR↗

arXiv2017

A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy

We report and discuss JHKs photometry for Sgr dIG, a very metal-deficient galaxy in the Local Group, obtained over 3.5 years with the Infrared Survey Facility in South Africa. Three large amplitude asymptotic giant branch variables are identified. One is an oxygen-rich star that has a pulsation period of 950 days, that was until recently undergoing hot bottom burning, with Mbol~-6.7. It is surprising to find a variable of this sort in Sgr dIG, given their rarity in other dwarf irregulars. Despite its long period the star is relatively blue and is fainter, at all wavelengths shorter than 4.5microns, than anticipated from period-luminosity relations that describe hot bottom burning stars. A comparison with models suggests it had a main sequence mass Mi~5 times solar and that it is now near the end of its AGB evolution. The other two periodic variables are carbon stars with periods of 670 and 503 days (Mbol~-5.7 and -5.3). They are very similar to other such stars found on the AGB of metal deficient Local Group Galaxies and a comparison with models suggests Mi~3 times solar. We compare the number of AGB variables in Sgr dIG to those in NGC6822 and IC1613, and suggest that the differences may be due to the high specific star formation rate and low metallicity of Sgr dIG.

astro-ph.GA↗

arXiv2017

Discovery of carbon-rich Miras in the Galactic bulge

Only one carbon-rich (C-rich, hereinafter) Mira variable has so far been suggested as a member of the Galactic bulge and this is in a symbiotic system. Here we describe a method for selecting C-rich candidates from an infrared colour-colour diagram, (J-Ks) vs ([9]-[18]). Follow-up low-resolution spectroscopy resulted in the detection of 8 C-rich Mira variables from a sample of36 candidates towards the Galactic bulge. Our near-infrared photometry indicates that two of these, including the known symbiotic, are closer than the main body of the bulge while a third is a known foreground object. Of the 5 bulge members, one shows He I and [O II] emission and is possibly another symbiotic star. Our method is useful for identifying rare C-rich stars in the Galactic bulge and elsewhere. The age of these C-rich stars and the evolutionary process which produced them remain uncertain. They could be old and the products of either binary mass transfer or mergers, i.e. the descendants of blue stragglers, but we cannot rule out the possibility that they belong to a small in-situ population of metal-poor intermediate age (less than 5 Gyr) stars in the bulge or that they have been accreted from a dwarf galaxy.

astro-ph.SR↗

arXiv2017

San Pedro Meeting on Wide Field Variability Surveys: Some Concluding Comments

This is a written version of the closing talk at the 22nd Los Alamos Stellar pulsation conference on wide field variability surveys. It comments on some of the issues which arise from the meeting. These include the need for attention to photometric standardization (especially in the infrared) and the somewhat controversial problem of statistical bias in the use of parallaxes (and other methods of distance determination). Some major advances in the use of pulsating variables to study Galactic structure are mentioned. The paper includes a clarification of apparently conflicting results from classical Cepheids and RR Lyrae stars in the inner Galaxy and bulge. The importance of understanding non-periodic phenomena in variable stars,particularly AGB variables and RCB stars is stressed, especially for its relevance to mass-loss, in which pulsation may only play a minor role.

astro-ph.IM↗

arXiv2016

A lack of classical Cepheids in the inner part of the Galactic disk

Recent large-scale infrared surveys have been revealing stellar populations in the inner Galaxy seen through strong interstellar extinction in the disk. In particular, classical Cepheids with their period-luminosity and period-age relations are useful tracers of Galactic structure and evolution. Interesting groups of Cepheids reported recently include four Cepheids in the Nuclear Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga et al. and those spread across the inner part of the disk reported by Dekany and collaborators. We here report our discovery of nearly thirty classical Cepheids towards the bulge region, some of which are common with Dekany et al., and discuss the large impact of the reddening correction on distance estimates for these objects. Assuming that the four Cepheids in the NSD are located at the distance of the Galactic Centre and that the near-infrared extinction law, i.e. wavelength dependency of the interstellar extinction, is not systematically different between the NSD and other bulge lines-of-sight, most of the other Cepheids presented here are located significantly further than the Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region of the Galactic disk except the NSD. Recent radio observations show a similar distribution of star-forming regions.

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arXiv2015

The Age and Structure of the Galactic Bulge from Mira Variables

We report periods and JHKL observations for 648 oxygen-rich Mira variables found in two outer bulge fields at b=-7 degrees and l=+/-8 degrees and combine these with data on 8057 inner bulge Miras from the OGLE, Macho and 2MASS surveys, which are concentrated closer to the Galactic centre. Distance moduli are estimated for all these stars. Evidence is given showing that the bulge structure is a function of age. The longer period Miras (log P > 2.6, age about 5 Gyr and younger) show clear evidence of a bar structure inclined to the line of sight in both the inner and outer regions. The distribution of the shorter period (metal-rich globular cluster age) Miras, appears spheroidal in the outer bulge. In the inner region these old stars are also distributed differently from the younger ones and possibly suggest a more complex structure. These data suggest a distance to the Galactic centre, R0, of 8.9 kpc with an estimated uncertainty of 0.4 kpc. The possible effect of helium enrichment on our conclusions is discussed.

astro-ph.SR↗

arXiv2015

The Local Group Galaxy IC1613 and its Asymptotic Giant Branch Variables

JHKs photometry is presented from a three-year survey of the central regions of the Local Group dwarf irregular galaxy IC1613. The morphologies of the colour-magnitude and colour-colour diagrams are discussed with particular reference to the supergiants and M- and C-type asymptotic giant branch (AGB) stars. Mean JHKs magnitudes, amplitudes and periods are given for five O-rich and nine C-rich Mira variables for which bolometric magnitudes are also estimated. A distance of 750 kpc ($(m-M)_0=24.37\pm 0.08$ mag) is derived for IC1613 by fitting a period-luminosity relation to the C-rich Miras. This is in agreement with values from the literature. The AGB stars exhibit a range of ages. A comparison with theoretical isochrones suggests that four luminous O-rich Miras are as young as $2\times 10^8$ yrs. One of these has a lithium absorption line in its spectrum, demonstrating that it is undergoing hot bottom burning (HBB). This supports the idea that HBB is the cause of the high luminosity of these AGB stars, which puts them above the fundamental period-luminosity (PL) relation. Further studies of similar stars, selected from their positions in the PL diagram, could provide insight into HBB. A much fainter, presumed O-rich, Mira is similar to those found in Galactic globular clusters. The C Miras are of intermediate age. The O-rich variables are not all recognized as O-rich, or even as AGB stars, on the basis of their J-Ks colour. It is important to appreciate this when using near-infrared surveys to classify AGB stars in more distant galaxies.

astro-ph.GA↗

arXiv2014

Cepheid Variables in the Flared Outer Disk of our Galaxy

Flaring and warping of the disk of the Milky Way have been inferred from observations of atomic hydrogen, but stars associated with flaring have not hitherto been reported. In the area beyond the Galactic centre the stars are largely hidden from view by dust, and the kinematic distances of the gas cannot be estimated. Thirty-two possible Cepheid stars (young pulsating variable stars) in the direction of the Galactic bulge were recently identified. With their well-calibrated period-luminosity relationships, Cepheid stars are useful distance indicators. When observations of these stars are made in two colours, so that their distance and reddening can be determined simultaneously, the problems of dust obscuration are minimized. Here we report that five of the candidates are classical Cepheid stars. These five stars are distributed from approximately one to two kiloparsecs above and below the plane of the Galaxy, at radial distances of about 13 to 22 kiloparsecs from the centre. The presence of these relatively young (less than 130 million years old) stars so far from the Galactic plane is puzzling, unless they are in the flared outer disk. If so, they may be associated with the outer molecular arm.

astro-ph.GA↗

arXiv2013

Variable Stars and Galactic Structure

Variable stars have a unique part to play in Galactic astronomy. Among the most important of these variables are the Cepheids (types I and II), the RR Lyraes and the Miras (O- and C-rich). The current status of the basic calibration of these stars in their roles as distance, structure and population indicators is outlined and some examples of recent applications of these stars to Galactic and extragalactic problems is reviewed. The expected impact of Gaia on this type of work is discussed and the need for complementary ground based observations, particularly large scale near-infrared photometry, is stressed.

astro-ph.GA↗

arXiv2012

Cepheids and other short-period variables near the Galactic Centre

We report the result of our near-infrared survey of short-period variable stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre. Forty-five variables are discovered and we classify the variables based on their light curve shapes and other evidence. In addition to 3 classical Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one Cepheid-like variable whose nature is uncertain. Eclipsing binaries are separated into the foreground objects and those significantly obscured by interstellar extinction. One of the reddened binaries contains an O-type supergiant and its light curve indicates an eccentric orbit. We discuss the nature and distribution of type II Cepheids as well as the distance to the Galactic Centre based on these Cepheids and other distance indicators. The estimates of R0(GC) we obtained based on photometric data agree with previous results obtained with kinematics of objects around the GC. Furthermore, our result gives a support to the reddening law obtained by Nishiyama and collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would result in a rather different distance estimate.

astro-ph.SR↗

arXiv2012

The Local Group Galaxy NGC 6822 and its Asymptotic Giant Branch Stars

JHKs photometry is presented from a 3.5 year survey of the central regions of the irregular galaxy NGC6822. The morphology of the colour-magnitude and colour-colour diagrams is discussed with particular reference to M, S and C-type AGB stars and to M-supergiants. Mean JHKs magnitudes and periods are given for 11 O-rich and 50 presumed C-rich Miras. Data are also listed for 27 large amplitude AGB stars without periods and for 69 small amplitude AGB variables. The slope of the bolometric period-luminosity relation for the C-rich Miras is in good agreement with that in the LMC. Distance moduli derived from the C- and O-rich Miras are in agreement with other estimates. The period distribution of C-rich Miras in NGC6822 is similar to that in the Magellanic Clouds, but differs from that in the dwarf spheroidals in the Local Group. In the latter there is a significant proportion of large amplitude, short period variables indicating a population producing old carbon-rich AGB stars.

astro-ph.GA↗

arXiv2012

A Carbon-rich Mira variable in a globular cluster: A stellar merger

The membership of Matsunaga's variable 1, a carbon-rich, mass-losing, Mira variable, in the globular cluster Lynga 7 is discussed on the basis of radial velocities. We conclude that it is a member, the first known C-Mira in a globular cluster. Since such a variable is expected to have an age of $\sim 1-2$ Gyr and an initial mass of $\sim 1.5$ solar masses, we conclude that this star must be the product of a stellar merger.

astro-ph.GA↗

arXiv2011

Distance Scale Zero-Points from Galactic RR Lyrae Star Parallaxes

We present new absolute trigonometric parallaxes and proper motions for seven Pop II variable stars: five RR Lyr variables; RZ Cep, XZ Cyg, SU Dra, RR Lyr, UV Oct; and two type 2 Cepheids; VY Pyx and κ Pav. We obtained these results with astrometric data from Fine Guidance Sensors, white-light interfer- ometers on Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: RZ Cep, 2.12\pm0.16 mas; XZ Cyg, 1.67\pm0.17 mas; SU Dra, 1.42\pm0.16 mas; RR Lyr, 3.77\pm0.13 mas; UV Oct, 1.71\pm0.10 mas; VY Pyx, 6.44\pm0.23 mas; and κ Pav, 5.57\pm0.28 mas; an average σπ/π = 5.4%. With these parallaxes we compute absolute magnitudes in V and K bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Using these RRL absolute magnitudes, we then derive zero-points for MV -[Fe/H] and MK -[Fe/H]-Log P relations. The technique of reduced parallaxes corroborates these results. We employ our new results to determine distances and ages of several Galactic globular clusters and the dis- tance of the LMC. The latter is close to that previously derived from Classical Cepheids uncorrected for any metallicity effect, indicating that any such effect is small. We also discuss the somewhat puzzling results obtained for our two type 2 Cepheids.

astro-ph.SR↗

arXiv2011

Asymptotic Giant Branch Stars in the Sculptor Dwarf Spheroidal Galaxy

JHK_S photometry is presented for a 35 arcmin square field centred on the Sculptor dwarf spheroidal galaxy. With the aid of published kinematic data definite galaxy members are identified and the width in J-K of the colour-magnitude diagram is shown to be consistent with an old population of stars with a large range in metal abundance. We identify two Asymptotic Giant Branch variables, both carbon Miras, with periods of 189 and 554 days, respectively, and discuss their ages, metallicities and mass loss as well as their positions in the Mira period-luminosity diagram. There is evidence for a general period-age relation for Local Group Miras. The mass-loss rate for the 554-day variable, MAG29, appears to be consistent with that found for Miras of comparable period in other Local Group galaxies.

astro-ph.CO↗

arXiv2010

Period-luminosity relations of type II Cepheids in the Magellanic Clouds

Period-luminosity relations (PLRs) of type II Cepheids (T2Cs) in the Small Magellanic Cloud are derived based on OGLE-III, IRSF/SIRIUS and other data, and these are compared with results for the Large Magellanic Cloud and Galactic globular clusters. Evidence is found for a change of the PLR slopes from system to system. Treating the longer period T2Cs (W Vir stars) separately gives an SMC-LMC modulus difference of 0.39+-0.05 mag without any metallicity corrections being applied. This agrees well with the difference in moduli based on different distance indicators, in particular the PLRs of classical Cepheids. The shorter period T2Cs (BL Her stars) give a smaller SMC-LMC difference suggesting that their absolute magnitudes might be affected either by metallicity or by age effects. It is shown that the frequency distribution of T2C periods also changes from system to system.

astro-ph.SR↗

arXiv2010

Asymptotic Giant Branch Variables in NGC 6822

Using multi-epoch JHK photometry obtained with the 1.4-m Japanese-South African Infrared Survey Facility at Sutherland we have identified large numbers of AGB variables in NGC 6822. This paper uses 30 large amplitude variables, with periods ranging from about 200 to 900 days, to provide a new calibration of the period-luminosity relation.

astro-ph.GA↗

arXiv2010

Asymptotic Giant Branch Stars in the Leo I Dwarf Spheroidal Galaxy

Twenty six Asymptotic Giant Branch (AGB) variables are identified in the Local Group galaxy Leo I. These include 7 Mira and 5 semi-regular variables for which periods, amplitudes and mean magnitudes are determined. The large range of periods for the Miras, 158<P<523 days, suggests an AGB spanning a significant age range. The youngest must be around 1.6 Gyr while the oldest could be 10 Gyr or more. Two of these old Miras are found in the outer regions of Leo I (over 490 arcsec from the centre) where stars on the extended AGB are rare. They could provide an interesting test of third dredge-up and mass loss in old stars with low metallicity and are worth further detailed investigation. At least two stars, one a Mira, the other an irregular variable, are undergoing obscuration events due to dust ejection. An application of the Mira period-luminosity relation to these stars yields a distance modulus for Leo I of (m-M)=21.80 \pm 0.11 mag (internal), \pm 0.12 (total) (on a scale that puts the LMC at 18.39 mag) in good agreement with other determinations.

astro-ph.GA↗