New Links Between Pulsation and Stellar History

X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and l Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected (δ Cep, β Dor, and Polaris). X-rays have also been detected from the low-mass F, G, and K companions of 4 Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr), and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28% have low mass companions. This fraction of low-mass companions in intermediate mass Cepheids is significantly lower than expected from random pairing with the field IMF. Combining the companion fraction from X-rays with than from ultraviolet observations results in a binary/multiple fraction of 57% ±12% for Cepheids with the ratios q > 0.1 and separations a > 1 au. This is a lower limit since M stars are not included. X-ray observations detect less massive companions than other existing studies of intermediate mass stars. Our measured occurrence rate of unresolved, low-mass companions to Cepheids suggests that intermediate-period binaries derive from a combination of disk and core fragmentation and accretion. This yields a hybrid mass-ratio distribution that is skewed toward small values compared to a uniform distribution but is still top-heavy compared to random pairings drawn from the IMF.

The Astrophysical Journal, 2017

From our Secret Lives of Cepheids program, the prototype Classical Cepheid, δ Cep, is found to be an X-ray source with periodic pulsation-modulated X-ray variations. This finding complements our earlier reported phasedependent FUV-UV emissions of the star that increase ∼10-20 times with highest fluxes at f-0.90 0.95 , just prior to maximum brightness. Previously δ Cepwas found as potentially X-ray variable, using XMM-Newton observations. Additional phase-constrained data were secured with Chandra near X-ray emission peak, to determine if the emission and variability were pulsation-phase-specific to δ Cepand not transient or due to a possible coronally active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and were found to very closely match the previously observed X-ray behavior. From the combined data set, a ∼4 increase in X-ray flux is measured, reaching a peak L X =1.7×10 29 erg s −1 near 0.45f. The precise X-ray flux phasing with the star's pulsation indicates that the emissions arise from the Cepheid and not from a companion. However, it is puzzling that the maximum X-ray flux occurs ∼0.5f (∼3 days) later than the FUV-UV maximum. There are several other potential Cepheid X-ray detections with properties similar to δ Cep, and comparable X-ray variability is indicated for two other Cepheids: β Dorand V473 Lyr. X-ray generating mechanisms in δ Cepand other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then δ Cepwill be the prototype of a new class of pulsation-induced X-ray variables.

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%.

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}

The Astronomical Journal, 1997

Pulsating convective models and evolutionary tracks are used to constrain mass, luminosity, and effective temperature of metal-poor (Z in the range of 0.0001 to 0.001) low-mass (0.52 M -0.80 M ) fundamental pulsators connected with the horizontal branch and asymptotic branch phase. On this basis the fundamental period P (F ) and the absolute blue magnitude B of the pulsators are calculated. We show that, by scaling the blue magnitude of the pulsator to the value B(3.83) of the zero age horizontal branch at the mean effective temperature of the RR Lyrae strip (log T e = 3.83), the distribution of theoretical fundamental pulsators in the B(3.83) -B vs P (F ) plane is independent of metallicity. The limits of this distribution, which depend on the adopted edges of the instability strip, conform well with data of known type II Cepheids and show that these variables are fundamental pulsators with mass in the range of 0.59 M to 0.52 M . Moreover, since for fundamental pulsating models there is a linear correlation of the blue magnitude with period and blue amplitude A(B), we are able to predict that the periodluminosity-blue amplitude relation for type II Cepheids with P ≤ 15 days is B(3.83) -B = -0.12 + 1.39 log P + 0.57A(B). The comparison with the theoretical evolutionary tracks yields that the mass of type II Cepheids decreases with increasing the period and that the brightest variables with P ≥10 days should likely belong to the very rapid phases of the 0.52 M and 0.53 M evolution. The agreement between observations and calculated expectations turns out to be almost satisfactory, suggesting that the pulsational and evolutionary scenario is capable to match the properties of metal-poor pulsating stars.

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.

Monthly Notices of the Royal Astronomical Society, 2011

We report results of initial work done on selected candidate Cepheids to be observed with the Kepler space telescope. Prior to the launch 40 candidates were selected from previous surveys and databases. The analysis of the first 322 days of Kepler photometry, and recent ground-based follow-up multicolour photometry and spectroscopy allowed us to confirm that one of these stars, V1154 Cyg (KIC 7548061), is indeed a 4.9-d Cepheid. Using the phase lag method we show that this star pulsates in the fundamental mode. New radial velocity data are consistent with previous measurements, suggesting that a long-period binary component is unlikely. No evidence is seen in the ultra-precise, nearly uninterrupted Kepler photometry for nonradial or stochastically excited modes at the micromagnitude level. The other candidates are not Cepheids but an interesting mix of possible spotted stars, eclipsing systems and flare stars.

2013

2014

Classical Cepheids are useful tracers of the Galactic young stellar population because their distances and ages can be determined from their period-luminosity and period-age relations. In addition, the radial velocities and chemical abundance of the Cepheids can be derived from spectroscopic observations, providing further insights into the structure and evolution of the Galaxy. Here, we report the radial velocities of classical Cepheids near the Galactic Center, three of which were reported in 2011, the other reported for the first time. The velocities of these Cepheids suggest that the stars orbit within the Nuclear Stellar Disk, a group of stars and interstellar matter occupying a region of ∼ 200 pc around the Center, although the three-dimensional velocities cannot be determined until the proper motions are known. According to our simulation, these four Cepheids formed within the Nuclear Stellar Disk like younger stars and stellar clusters therein.

The Astrophysical Journal, 2001

We have selected a sample of Galactic Cepheids for which accurate estimates of radii, distances, and photometric parameters are available. The comparison between their pulsation masses, based on new Period-Mass-Radius (PMR) relations, and their evolutionary masses, based on both optical and NIR Color-Magnitude (CM) diagrams, suggests that pulsation masses are on average of the order of 10% smaller than the evolutionary masses. Current pulsation masses show, at fixed radius, a strongly reduced dispersion when compared with values published in literature. The increased precision in the pulsation masses is due to the fact that our predicted PMR relations based on nonlinear, convective Cepheid models present smaller standard deviations than PMR relations based on linear models. At the same time, the empirical radii of our Cepheid sample are typically accurate at the 5% level.