A colour magnitude diagram (CMD) extending to V=~19mag is given for 444 stars in the region of the galactic cluster Berkeley 66. The V and I photometry of a nearby field is also reported. This object appears very faint, highly contaminated by foreground stars and very reddened. The apparent distance modulus (m-M) and the colour excess E_V-I_ are guessed to be 17.5 and 1.1, respectively, with an uncertainty of at least 30%. Adopting these values the comparison of the CMD with theoretical isochrones from the Padova group provides an age around 1.0Gyr.
The ultra-long-period Cepheids (ULPCs) are classical Cepheids with pulsation periods exceeding ~80days. The intrinsic brightness of ULPCs are ~1 to ~3mag brighter than their shorter period counterparts. This makes them attractive in future distance scale work to derive distances beyond the limit set by the shorter period Cepheids. We have initiated a program to search for ULPCs in M31, using the single-band data taken from the Palomar Transient Factory, and identified eight possible candidates. In this work, we presented the VI-band follow-up observations of these eight candidates. Based on our VI-band light curves of these candidates and their locations in the color-magnitude diagram and the Period-Wesenheit diagram, we verify two candidates as being truly ULPCs. The six other candidates are most likely other kinds of long-period variables. With the two confirmed M31 ULPCs, we tested the applicability of ULPCs in distance scale work by deriving the distance modulus of M31. It was found to be {mu}_M31,ULPC_=24.30+/-0.76mag. The large error in the derived distance modulus, together with the large intrinsic dispersion of the Period-Wesenheit (PW) relation and the small number of ULPCs in a given host galaxy, means that the question of the suitability of ULPCs as standard candles is still open. Further work is needed to enlarge the sample of calibrating ULPCs and reduce the intrinsic dispersion of the PW relation before re-considering ULPCs as suitable distance indicators.
Cepheid-based distances to seven Type Ia supernovae (SNe) host galaxies have been derived using the standard Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale pipeline. For the first time, this allows for a transparent comparison of data accumulated as part of three different HST projects: the Key Project, the Sandage et al. Type Ia supernovae (SNe) program, and the Tanvir et al. Leo I Group study. Reanalyzing the Tanvir et al. galaxy and six Sandage et al. galaxies, we find a mean (weighted) offset in true distance moduli of 0.12+/-0.07mag; i.e., 6% in linear distance, in the sense of reducing the distance scale or increasing H_0_. Adopting the reddening-corrected Hubble relations of Suntzeff et al. tied to a zero point based on SNe 1990N, 1981B, 1998bu, 1989B, 1972E, and 1960F and the photometric calibration of Hill et al. leads to a Hubble constant of H_0_=68+/-2(random)+/-5(systematic)km/s/Mpc. Adopting the Kennicutt et al. Cepheid period-luminosity-metallicity dependency decreases the inferred H_0_ by 4%. The H0 result from Type Ia SNe is now in good agreement, to within their respective uncertainties, with that from the Tully-Fisher and surface brightness fluctuation relations.
We report on the discovery of 29 Cepheid variables in the galaxy M101 using the original Wide Field Camera (WFC) and the new Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. We observed a field in M101 at 17 independent epochs in V (F555W), five epochs in I (F785LP/F814W), and one epoch in B (F439W), with a time interval baseline of 381 days. We have found Cepheids with periods ranging from 10 to 60 days. The data have been calibrated using WFPC2 observations with zero points derived from Omega Cen, Pal 4, and NGC 2419 observations. This calibration has been verified by using the Medium Deep Survey (MDS) WFC photometric zero points, and ground-based secondary standards in V and I. The V calibrations agree to +/-0.06mag, and the I calibrations agree to +/-0.4mag. We have constructed V and I period-luminosity (PL) relations and have derived apparent distance moduli based on a distance modulus for the Large Magellanic Cloud (LMC) of 18.50mag and a reddening of E(B-V)=0.10mag to the LMC Cepheids. Period-residual minimization was used to minimize the effects of Malmquist bias on the period-luminosity relation fitting process. Using a Galactic extinction law and the apparent V and I distance moduli, we have found a mean reddening for the M101 sample of E(B-V)=0.03mag and a true distance modulus to M101 of 29.34+/-0.17mag, corresponding to a distance of 7.4+/-0.6Mpc. The sources of error have been rigorously tracked through an error budget; systematic and random errors contribute roughly equally to the quoted error. The mean gas-phase metal abundances in the LMC and in the M101 outer field are similar so we expect metallicity effects to be minimal. These Cepheids will be used in conjunction with results from a Key Project search for Cepheids in an inner field, where the metallicity is larger by a factor of 5, to probe the effects of abundance on the Cepheid period-luminosity relation.
We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheid variables observed with the Hubble Space Telescope. Twelve F555W (V) and six F814W (I) epochs of cosmic-ray-split WFPC2 observations were obtained. A total of 56 bona fide Cepheids were discovered, with periods ranging from 5 to ~50 days; five of these are likely Population II Cepheids of the W Virginis class, associated with the bulge or halo of NGC 5128.
We have obtained extensive photometry of the suspected cataclysmic variable CG Muscae, and find that it is in fact an RR Lyrae star. We measure its light curve parameters and derive physical parameters including metal abundance, reddening, and distance. We also have discovered three new variables in the nearby field. We suspect that two are contact binary stars, while the third is too faint to determine its variability type with certainty. (c)Astronomical Society of the Pacific
We present a VI photometric catalogue towards the open cluster Cr 121. XMM-Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sightline. de Zeeuw et al. (1999, Cat. <J/AJ/117/354>) have identified Cr 121 as a moving group, using Hipparcos data, at a distance of 592pc; we reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sightline, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050pc.
We present (V, I) photometry of two wide (~25x25arcmin^2^) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided [(m-M)_0_(UMi)=19.41+/-0.12 and (m-M)_0_(Dra)=19.84+/-0.14], and a comparative study of their evolved stellar population is presented.
Tables show the photometry and colour index for the observed galaxy, with the image parameter SHARP. This parameter is a measure of the difference between the observed width of the object and the psf model.
In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project and the Sandage-Tammann-Saha group. We use much of the Key Project methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean of a sparsely sampled Cepheid light curve, to published extra-galactic Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLE LMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale.
