Discovery of the Host Cluster for the Fundamental Cepheid Calibrator Zeta Geminorum (original) (raw)
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Odessa Astronomical Publications, 2020
We present the results of a spectroscopic and photometric investigation of 18 probable members of the open cluster Collinder 220, which contains the Cepheid UW Car. Besides the Cepheid, we studied tree K-giants, two B-giants, and twelve B-A-F main sequence stars. Radial velocities (RV), v sin i, T eff , and log g were determined using spectroscopic model fitting and atmosphere models. We have derived color-excesses, reddenings, and intrinsic colors for these stars using their T eff and log g from comparison to the atmosphere models, especially for hot stars. Proper motions, RV, and the GAIA DR2 2018 parallaxes allowed us to determine their membership in the cluster and absolute magnitudes. We found that seven stars along with the Cepheid can be the cluster members with a high confidence. The parallaxes and reddenings of the 7 confident cluster members led to the distances in a range of 1900-2800 pc, while the other stars are probably foreground objects. All the members have [Fe/H] near 0.1 dex. The B-giant HD 90435 located near the cluster's "turn-off" point has a low projected rotational velocity, and this fact allows us to determine its chemical composition to compare with that of the Cepheid UW Car. The CNO abundances of HD 90435 are nearly solar, while the Cepheid shows a deficit of carbon, an overabundance of nitrogen, a nearly solar oxygen, and an overabundance of sodium. The confident cluster member, K-supergiant CPD −57 • 3199, has CNO-and Na abundances close to those of UW Car.
Some Characteristics of Galactic Cepheids Relevant to the Calibration of the Distance Scale
arXiv: Astrophysics, 1998
An analysis of the observed characteristics of the Galactic Cepheid variables is carried out in the framework of their period-luminosity relation being used as a standard candle for distance measurement. The variation of the observed number density of Galactic Cepheids as function of their period and amplitude along with stellar pulsation characteristics is used to divide the population into two groups: one with low periods, probably multi-mode or higher mode oscillators, and another of high period variables which should be dominantly fundamental mode radial pulsators. Methods to obtain extinction-corrected colors from multi-wavelength observations of the second group of variables are described and templates of the (V-I) light curves are obtained from the V light curves. Colors computed from the model atmospheres are compared with the extinction-corrected colors to determine the Cepheid instability strip in the mean surface gravity--effective temperature diagram, and relations are d...
ANCHORING THE DISTANCE SCALE VIA X-RAY/INFRARED DATA FOR CEPHEID CLUSTERS: SU Cas
The Astrophysical Journal, 2012
New X-ray (XMM-Newton) and JHK s (Observatoire du Mont-Mégantic) observations for members of the star cluster Alessi 95, which Turner et al. discovered hosts the classical Cepheid SU Cas, were used in tandem with UCAC3 (proper motion) and Two Micron All Sky Survey observations to determine precise cluster parameters: E(J − H) = 0.08 ± 0.02 and d = 405 ± 15 pc. The ensuing consensus among cluster, pulsation, and trigonometric distances (d = 414 ± 5(σx) ± 10(σ) pc) places SU Cas in a select group of nearby fundamental Cepheid calibrators (δ Cep, ζ Gem). High-resolution X-ray observations may be employed to expand that sample as the data proved pertinent for identifying numerous stars associated with SU Cas. Acquiring X-ray observations of additional fields may foster efforts to refine Cepheid calibrations used to constrain H 0 .
Studies of Galactic Cepheids: The Inasan/Sai Integrated Program
Odessa Astronomical Publications, 2007
We present the review of the main results of more than two decades of the Moscow program of Cepheid studies, carried out at the Institute of Astronomy (INASAN) and Sternberg Astronomical Institute (SAI). This program consists of extensive photometry and radial velocity measurements (the contribution from our team being the largest among observations of comparable precision), studies of period variations (permitting identification of the number of a particular star's current instability-strip crossing), detection of spectroscopic binaries among Cepheids, determinations of Cepheid radii, discoveries of double-mode Cepheids, studies of galactic structure, kinematics, and dynamics, etc.
Galactic clusters with associated Cepheid variables – VII. Berkeley 58 and CG Cassiopeiae
Monthly Notices of the Royal Astronomical Society, 2008
Photoelectric, photographic and CCD UBV photometry, spectroscopic observations and star counts are presented for the open cluster Berkeley 58 to examine a possible association with the 4.37 d Cepheid CG Cas. The cluster is difficult to separate from the early-type stars belonging to the Perseus spiral arm, in which it is located, but has reasonably well-defined parameters: an evolutionary age of ∼108yr, a mean reddening of E(B−V) (B0) = 0.70 ± 0.03 s.e. and a distance of 3.03 ± 0.17 kpc (V0−MV= 12.40 ± 0.12 s.d.). CG Cas is a likely cluster coronal member on the basis of radial velocity, and its period increase of +0.170 ± 0.014 s yr−1 and large light amplitude describe a Cepheid in the third crossing of the instability strip lying slightly blueward of strip centre. Its inferred reddening and luminosity are E(B−V) = 0.64 ± 0.02 s.e. and 〈MV〉=−3.06 ± 0.12. A possible K supergiant may also be a cluster member.
Variable stars in nearby galaxies I. Search for Cepheids in field A of IC 1613
Astron Astrophys, 1999
The first results are presented of a four-year program dedicated to the CCD observations of Cepheids in the nearby galaxy IC 1613. The goal was to obtain good light curves for Fourier decomposition and to detect shorter period Cepheids. Since the program was carried out with a relatively small telescope, the Dutch 0.9 m at ESO-La Silla, the observations were performed without filter (white light), or Wh-band; the advantage of this technique is that the photon statistics correspond to that of V-band observations made with larger telescopes than 2 m and similar exposure time. The effective wavelength of the Wh-band is intermediate between that of V and R bands for stars of A-G spectral type, for back-illuminated CCD detectors, therefore the photometric characteristics of variable stars (e.g. amplitudes) are generally analogous to those obtained with the standard technique. Field A in IC 1613 has size 3.8'x3.8'. A total of 67 images were obtained and the reduction was performed with DAOPHOT. More than 2900 stars were measured, and for about 1700 stars there are from 67 to 24 Wh data points. Indications on the color of 739 bright stars were obtained also from V and R additional data. The analysis revealed the presence of about 110 variable stars. The detected population I Cepheids are 43; 9 Cepheids were already known from previous works, while most of the new stars have a short period P. We remark the following results: a) for stars with P >~ 5 d and sufficient phase coverage it is possible to perform good Fourier decomposition with resulting standard deviation of the fit of 0.02-0.04 mag; b) there are several Cepheids with relatively small amplitude, and most of them are (probable) first overtone mode pulsators; c) the faintest detected Cepheids have m_V ~ 23. No double-mode Cepheid has been found, probably because the precision and sampling of the data are not sufficient for the detection. Furthermore, at least 5 population II Cepheids and at least 8 eclipsing binaries have been observed. The other variable stars are probable long period, semiregular and irregular variables. A comparison with results of other massive CCD photometric projects dedicated to the detection of variable stars shows some advantages of the observations in white light for fully exploiting the capabilities of relatively small telescopes. A suggestion is made on how to use these results for distance determinations. Based on observations collected at ESO-La Silla}
Discovery of a Pair of Classical Cepheids in an Invisible Cluster Beyond the Galactic Bulge
The Astrophysical Journal, 2015
We report the discovery of a pair of extremely reddened classical Cepheid variable stars located in the Galactic plane behind the bulge, using near-infrared (NIR) time-series photometry from the VISTA Variables in the Vía Láctea Survey. This is the first time that such objects have ever been found in the opposite side of the Galactic plane. The Cepheids have almost identical periods, apparent brightnesses, and colors. From the NIR Leavitt law, we determine their distances with~1.5% precision and~8% accuracy. We find that they have a same total extinction of A V () 32 mag, and are located at the same heliocentric distance of á ñ = d 11.4 0.9 kpc, and less than 1 pc from the true Galactic plane. Their similar periods indicate that the Cepheids are also coeval, with an age of~ 48 3 Myr, according to theoretical models. They are separated by an angular distance of only 18″ .3, corresponding to a projected separation of ∼1 pc. Their position coincides with the expected location of the Far 3 kpc Arm behind the bulge. Such a tight pair of similar classical Cepheids indicates the presence of an underlying young open cluster that is both hidden behind heavy extinction and disguised by the dense stellar field of the bulge. All our attempts to directly detect this "invisible cluster" have failed, and deeper observations are needed.
Cepheid Variable Stars: An Overview of Their Nature and Their Importance In Astrophysics
Properties of Cepheid variables are briefly outlined, including their characteristic 'light curves'. The archetype Cepheid, -Cephei is discussed, including its historical context. The Distance Modulus Equation is derived and the Period-Luminosity relation for Type 1 Cepheids is reviewed. A non-rigorous but physically sound derivation of an approximate Period-Luminosity relation for pulsating variable stars is presented. Cepheids are important as distance indicators within the Milky Way and beyond. This importance is described and illustrated with calculations; in particular, the distance to the Large Magellanic Cloud (LMC) is examined, data from the Hubble Space Telescope (HST) is used to verify the distance to Hubble's first Cepheid in Andromeda and further HST data elucidates the distance to the furthest Cepheids observed to date in the spiral galaxy NGC 4603. Finally, the importance of Cepheid data to cosmology is considered and some areas of current research are summarised, including the tension between Cepheid and CMBR determinations of H0 and a proposal (July 2017) to use HST to determine Ho to within 1%.
The Baade–Becker–Wesselink technique and the fundamental astrophysical parameters of Cepheids
Proceedings of the International Astronomical Union, 2012
The BBW method remains one of most demanded tool to derive full set of Cepheid astrophysical parameters. Surface brightness version of the BBW technique was preferentially used during last decades to calculate Cepheid radii and to improve PLC relations. Its implementation requires a priory knowledge of Cepheid reddening value. We propose a new version of the Baade-Becker-Wesselink technique, which allows one to independently determine the colour excess and the intrinsic colour of a radially pulsating star, in addition to its radius, luminosity, and distance. It is considered to be a generalization of the Balona light curve modelling approach. The method also allows calibration of the function F (CI0) = BC(CI0) + 10 × log(T ef f (CI0)) for the class of pulsating stars considered. We apply this technique to a number of classical Cepheids with very accurate light and radial-velocity curves. The new technique can also be applied to other pulsating variables, e.g. RR Lyraes. We also discuss the possible dependence of the projection factor on the pulsation phase.