Patricia A. Whitelock

Galactic kinematics of cepheids from hipparcos proper motions

The Hipparcos proper motions of 220 galactic Cepheids, together with relevant ground-based photometry, have been analysed. The effects of galactic rotation are very clearly seen. Mean values of the Oort constants, A = 14.82 +/- 0.84 km/s/kpc, and B = -12.37 +/- 0.64 km/s/kpc, and of the angular velocity of circular rotation at the Sun, Omega_0 = 27.19 +/- 0.87 km/s/kpc, are derived. Comparison of the value of A with values derived from recent radial velocity solutions confirm, within the errors, the zero-points of the PL and PLC relations derived directly from the Hipparcos trigonometrical parallaxes of the same stars. The proper motion results suggest that the galactic rotation curve is declining slowly at the solar distance from the galactic centre (dTheta/dR = -2.4 +/- 1.2 km/s/kpc). The component of the solar motion towards the North Galactic Pole is found to be +7.61 +/- 0.64 km/s. Based on the increased distance scale deduced in the present paper the distance to the galactic centre derived in a previous radial velocity study is increased to R_0 = 8.5 +/- 0.5 kpc.

Cepheid parallaxes and the Hubble constant

Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 Hubble Space Telescope (HST)-based parallaxes. In a reddening-free V, I relation we find that the coefficient of log P is the same within the uncertainties in our Galaxy as in the Large Magellanic Cloud (LMC), contrary to some previous suggestions. Cepheids in the inner region of NGC 4258 with near solar metallicities confirm this result. We obtain a zero-point for the reddening-free relation and apply it to the Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubble constant. We revise their result for H0 from 62 to 70 ± 5k m s −1 Mpc −1 . The Freedman et al. value is revised from 72 to 76 ± 8k m s −1 Mpc −1 . These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC 4258 yield a modulus of 29.22 ± 0.03 (int.) compared with a maser-based modulus of 29.29 ± 0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are 18.52 ± 0.03 from a reddening-free relation in V, I; 18.47 ± 0.03 from a period‐luminosity relation at K; 18.45 ± 0.04 from a period‐luminosity‐ colour relation in J, K. Adopting a metallicity correction in V, I from Macri et al. leads to a true LMC modulus of 18.39 ± 0.05.

AGB variables and the Mira period–luminosity relation

Published data for large-amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re-analysed to establish the constants for an infrared (K) period-luminosity relation of the form M K = ρ[logP - 2.38] + ρ. A slope of p = -3.51 + 0.20 and a zero-point of δ = -7.15 ± 0.06 are found for oxygen-rich Miras (if a distance modulus of 18.39 + 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero-point of δ = -7.25 ± 0.07 for O-rich Galactic Miras. The zero-point for Miras in the Galactic bulge is not significantly different from this value. Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period-luminosity relation zero-points for systems with different metallicity.

The global gas and dust budget of the Large Magellanic Cloud: AGB stars and supernovae, and the impact on the ISM evolution

We report on an analysis of the gas and dust budget in the the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). Recent observations from the Spitzer Space Telescope enable us to study the mid-infrared dust excess of asymptotic giant branch (AGB) stars in the LMC. This is the first time we can quantitatively assess the gas and dust input from AGB stars over a complete galaxy, fully based on observations. The integrated mass-loss rate over all intermediate and high mass-loss rate carbon-rich AGB candidates in the LMC is 8.5 × 10 −3 M⊙ yr −1 , up to 2.1 × 10 −2 M⊙ yr −1 . This number could be increased up to 2.7×10 −2 M⊙ yr −1 if oxygen-rich stars are included. This is overall consistent with theoretical expectations, considering the star formation rate when these low- and intermediate-mass stars where formed, and the initial mass functions. AGB stars are one of the most important gas sources in the LMC, with supernovae (SNe), which produces about 2–4×10 −2 M⊙ yr −1 . At the moment, the star formation rate exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the star formation rate will eventually decline in the LMC, unless gas is supplied externally. Our estimates suggest ‘a missing dust-mass problem’ in the LMC, which is similarly found in high-z galaxies: the accumulated dust mass from AGB stars and possibly SNe over the dust life time (400–800Myrs) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star-forming regions.

Near-infrared photometry of carbon stars*

Near-infrared, JHKL, photometry of 239 Galactic carbon-rich variable stars is presented and discussed. From these and published data the stars were classified as Mira or non-Mira variables and amplitudes and pulsation periods, ranging from 222 to 948 days for the Miras, were determined for most of them. A comparison of the colour and period relations with those of similar stars in the Large Magellanic Cloud indicates minor differences, which may be the consequence of sample selection effects. Apparent bolometric magnitudes were determined by combining the mean JHKL fluxes with mid-infrared photometry from IRAS and MSX. Then, using the Mira period luminosity relation to set the absolute magnitudes, distances were determined – to greater accuracy than has hitherto been possible for this type of star. Bolometric corrections to the K magnitude were calculated and prescriptions derived for calculating these from various colours. Mass-loss rates were also calculated and compared to values in the literature. Approximately one third of the C-rich Miras and an unknown fraction of the non-Miras exhibit apparently random obscuration events that are reminiscent of the phenomena exhibited by the hydrogen deficient RCB stars. The underlying cause of this is unclear, but it may be that mass loss, and consequently dust formation, is very easily triggered from these very extended atmospheres.

Obscured asymptotic giant branch variables in the Large Magellanic Cloud and the period–luminosity relation

The characteristics of oxygen-rich and carbon-rich, large amplitude (�K > 0.4 mag), asymptotic giant branch variables in the Large Magellanic Clouds are discussed, with an emphasis on those obscured by dust. Near-infrared photometry, obtained over about 8 years, is combined with published mid-infrared observations from IRAS and ISO to determine bolometric magnitudes for 42 stars. Pulsation periods of the O-rich stars are in the range 116 0.6 mag, secular or very long period variations which may be associated with changes in their mass-loss rates. We discuss and compare various methods of determining the bolometric magnitudes and show, perhaps surprisingly, that most of the very long period stars seem to follow an extrapolation of the periodluminosity relation determined for stars with shorter periods - although the details do depend on how the bolometric magnitudes are calculated. Three stars with thin shells, which are clearly more luminous than the obscured AGB stars, are undergoing hot bottom burning, while other stars with similar luminosities have yet to be investigated in sufficient detail to determine their status in this regard. We suggest that an apparent change in slope of the period luminosity relation around 400-420 days is caused by variables with luminosities brighter than the predictions of the core-mass luminosity relation, due to excess flux from hot bottom burning.

Asymptotic giant branch superwind speed at low metallicity

We present the results of a survey for OH maser emission at 1612 MHz from dustenshrouded AGB stars and supergiants in the LMC and SMC, with the Parkes radio telescope, aimed at deriving the speed of the superwind from the double-peaked OH maser profiles. Out of 8 targets in the LMC we detected 5, of which 3 are new detections — no maser emission was detected in the two SMC targets. We detected for the first time the redshifted components of the OH maser profile in the extreme red supergiant IRAS 04553−6825, confirming the suspicion that its wind speed had been severely underestimated. Despite a much improved spectrum for IRAS 04407−7000, which was known to exhibit a single-peaked OH maser, no complementary peak could be detected. The new detection in IRAS 05003−6712 was also single-peaked, but for two other new detections, IRAS 04498−6842 and IRAS 05558−7000, wind speeds could be determined from their double-peaked maser profiles. The complete sample of known OH/IR stars in the LMC is compared with a sample of OH/IR stars in the galactic centre. The LMC sources generally show a pronounced asymmetry between the bright blueshifted maser emission and weaker redshifted emission, which we attribute to the greater contribution of amplification of radiation coming directly from the star itself as the LMC sources are both more luminous and less dusty than their galactic centre counterparts. We confirm that the OH maser strength is a measure of the dust (rather than gas) mass-loss rate. At a given luminosity or pulsation period, the wind speed in LMC sources is lower than in galactic centre sources, and the observed trends confirm simple radiation-driven wind theory if the dust-to-gas ratio is approximately proportional to the metallicity.

ON THE NATURE OF THE PROTOTYPE LUMINOUS BLUE VARIABLE AG CARINAE. I. FUNDAMENTAL PARAMETERS DURING VISUAL MINIMUM PHASES AND CHANGES IN THE BOLOMETRIC LUMINOSITY DURING THE S-Dor CYCLE

We present a detailed spectroscopic analysis of the luminous blue variable AG Carinae during the last two visual minimum phases of its S-Dor cycle (1985-1990 and 2000-2003). The analysis reveals an overabundance of He, N, and Na, and a depletion of H, C, and O, on the surface of AG Car, indicating the presence of CNO-processed material. Furthermore, the ratio N/O is higher on the stellar surface than in the nebula. We found that the minimum phases of AG Car are not equal to each other, since we derived a noticeable difference between the maximum effective temperature achieved during 1985-1990 (22,800 K) and 2000-2001 (17,000 K). While the wind terminal velocity was 300 km/s in 1985-1990, it was as low as 105 km/s in 2001. The mass-loss rate, however, was lower from 1985-1990 (1.5 x 10^(-5) Msun/yr) than from 2000-2001 (3.7 x 10^(-5) Msun/yr). We found that the wind of AG Car is significantly clumped (f=0.10 - 0.25) and that clumps must be formed deep in the wind. We derived a bolometric luminosity of 1.5 x 10^6 Lsun during both minimum phases which, contrary to the common assumption, decreases to 1.0 x 10^6 Lsun as the star moves towards maximum flux in the V band. Assuming that the decrease in the bolometric luminosity of AG Car is due to the energy used to expand the outer layers of the star (Lamers 1995), we found that the expanding layers contain roughly 0.6 - 2 Msun. Such an amount of mass is an order of magnitude lower than the nebular mass around AG Car, but is comparable to the nebular mass found around lower-luminosity LBVs and to that of the Little Homunculus of Eta Car. If such a large amount of mass is indeed involved in the S Dor-type variability, we speculate that such instability could be a failed Giant Eruption, with several solar masses never becoming unbound from the star.(abridged)

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

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. 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 r elations 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-� m 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-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these 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. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. 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 compared to the Galaxy.

The luminosities and distance scales of type II Cepheid and RR Lyrae variables

Infrared and optical absolute magnitudes are derived for the type II Cepheids K Pav and VY Pyx using revised Hipparcos parallaxes and for K Pav, V553 Cen and SW Tau from pulsational parallaxes. Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorily and are combined in deriving absolute magnitudes. Phase-corrected J, H and K s mags are given for 142 Hipparcos RR Lyraes based on Two-Micron All-Sky Survey observations. Pulsation and trigonometrical parallaxes for classical Cepheids are compared to establish the best value for the projection factor (p) used in pulsational analyses. The M v of RR Lyrae itself is 0.16 ± 0.12 mag brighter than predicted from an M v -[Fe/H] relation based on RR Lyrae stars in the Large Magellanic Cloud (LMC) at a modulus of 18.39 ± 0.05 as found from classical Cepheids. This is consistent with the prediction of Catelan & Cortes that it is overluminous for its metallicity. The M Ks results for the metal- and carbon-rich Galactic disc stars, V553 Cen and SW Tau, each with small internal errors (±0.08 mag) have a mean deviation of only 0.02 mag from the period-luminosity (PL) relation established by Matsunaga et al. for type II Cepheids in globular clusters and with a zero-point based on the same LMC-scale. Comparing directly the luminosities of these two stars with published data on type II Cepheids in the LMC and in the Galactic bulge leads to an LMC modulus of 18.37 ± 0.09 and a distance to the Galactic Centre of R 0 = 7.64 ± 0.21 kpc. The data for VY Pyx agree with these results within the uncertainties set by its parallax. Evidence is presented that K Pav may have a close companion and possible implications of this are discussed. If the pulsational parallax of this star is incorporated in the analyses, the distance scales just discussed will be increased by ∼0.15 ± 0.15 mag. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters. This is probably due to a narrower range of masses in the latter case.