M. W. Feast

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.

HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR PARALLAXES OF GALACTIC CEPHEID VARIABLE STARS: PERIOD-LUMINOSITY RELATIONS ∗

We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: l Car, ζ Gem, β Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on the Hubble Space Telescope. We find absolute parallaxes in milliarcseconds: l Car, 2.01 ± 0.20 mas; ζ Gem, 2.78 ± 0.18 mas; β Dor, 3.14 ± 0.16 mas; W Sgr, 2.28 ± 0.20 mas; X Sgr, 3.00 ± 0.18 mas; Y Sgr, 2.13 ± 0.29 mas; FF Aql, 2.81 ± 0.18 mas; T Vul, 1.90 ± 0.23 mas; and RT Aur, 2.40 ± 0.19 mas; average σπ/π = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit WVI bandpasses, corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct period-luminosity relations (PLRs) for 10 Galactic Cepheids. We compare our new PLRs with those adopted by several recent investigations, including the Freedman and Sandage H0 projects. Adopting our PLR would tend to increase the Sandage H0 value, but leave the Freedman H0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and WVI to be identical in the two galaxies within their respective errors. Our data lead to a WVI distance modulus for the LMC m - M = 18.50 ± 0.03, uncorrected for any metallicity effects. Applying recently derived metallicity corrections yields a corrected LMC distance modulus of (m - M)0 = 18.40 ± 0.05. Comparing our PLR to solar-metallicity Cepheids in NGC 4258 results in a distance modulus 29.28 ± 0.08 that agrees with one derived from maser studies.

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.

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.

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.

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space telescopespectroscopic 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 C2H2 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 � 6 × 10 5 yr. For the LMC, this lifetime is � 3 × 10 5 yr. The superwind phase traced with Spitzer lasts for � 10 4 yr. Spitzer spectra of a K supergiant and a compact HII region are also given.

The 2003 shell event in η Carinae

Near-infrared, JHKL, photometry of η Car is reported covering the period 2000-2004. This includes the 2003 shell event which was the subject of an international multiwavelength campaign. The fading that accompanied this event was similar to, although slightly deeper than, that which accompanied the previous one. The period between these events is 2023 ± 3 d and they are strictly periodic. Their cause, as well as that of the quasi-periodic variations and secular brightening, are discussed. It seems possible that all three types of variability are consequences of the binary nature of the star.