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NASA NTRS1986

The mass of the classical cepheid SU Cygni

Radial velocity differences between the classical Cepheid SU Cyg and a hot companion were measured from IUE high dispersion spectra. The blue star is itself a close binary. The triple system solution provides the mass ratio of the Cepheid to the masses of the pair of companions. The masses of the companions are inferred from the low dispersion IUE spectra. The mass derived for the Cepheid is 6.3 solar masses, with a range from 7.0 to 5.9 solar masses, which is very close to the evolutionary mass.

Evans, Nancy Remage↗

NASA NTRS1988

Are most short-period Cepheids overtone pulsators?

Data given and analyzed recently by Coulson et al. (1986) and Gieren (1982) provide a consistent set of Wesselink radii and Wesselink masses, for which the internal error bars appear to be small enough that a quantitative discussion appears to be warranted. When it is assumed that all Cepheids pulsate in the fundamental mode, a steep discontinuity in mass is found for periods around 6.5 days. When short-period Cepheids are interpreted as first-overtone pulsators as required theoretically (Christy, 1966; Stobie, 1969), the discontinuity disappears and the mass-period relation passes through the bump and beat masses for Cepheids.

Bohm-Vitense, Erika↗

NASA NTRS1987

The mass of the classical Cepheid SU Cygni

Evans, Nancy Remage↗

NASA NTRS1988

Abundance effects on the Cepheid distance scale

A new investigation of the dependence of the period-luminosity (PL) and period-luminosity-color (PLC) relations on the helium and metal abundances in classical Cepheids is presented. Composition dependences are assigned to all quantitites under study, so that the results represent a true theoretical prediction. It is found that the PLC relation in B,V magnitudes is at least five times more sensitive to chemical composition than is the PL relation. If the PL relation is adopted and the helium abundance is held fixed, a Cepheid with Z = 0.005 should be fainter than one with Z = 0.02 by about 0.04 mag. Since the slope of the predicted PL relation is independent of chemical composition, the PL relation can be considered to be effectively universal. Additional supporting observational evidence is described, and theoretical results are also presented for the bolometric PL relation, whose slope and dependence on metal abundance should be roughly applicable to infrared observations of Cepheids.

Stothers, Richard B.↗

NASA NTRS1988

Cepheid binaries with large mass ratios (M1/M2)

The IUE observations of 3 Cepheid systems (Polaris, FF Aql, and S Sge) are used to derive, or set limits on, the temperatures and masses of the companions. Light from the companions of FF Aql and S Sge from 1700 to 2000 A is consistent with an A5 to A7 main sequence companion for both Cepheids, with a mass of 1.8 solar mass. This mass for the companion of S Sge is smaller than required by the orbital mass function and an evolutionary mass of the Cepheid, suggesting that the companion may itself be a binary. For Polaris, the mass of the companion must be less than 1.8 solar mass.

Evans, Nancy Remage↗

NASA NTRS1988

The orbit of the Cepheid AW Per

An orbit for the classical Cepheid AW Per was derived. Phase residuals from the light curve are consistent with the light-time effect from the orbit. The companion was studied using IUE spectra. The flux distribution from 1300 to 1700 A is unusual, probably an extreme PbSi star, comparable to a B7V or B8V star. The flux of the composite spectrum from 1200 A through V is well matched by F7Ib and B8V standard stars with Delta M(sub upsilon) = 3(m) multiplied by 1. The mass function from the orbit indicates that the mass of the Cepheid must be greater that 4.7 solar mass if it is the more massive component. A B7V to B8V companion is compatible with the 1 sigma lower limit (3.5 solar mass) from the mass function. This implies that the Cepheid has the same mass, but the large magnitude difference rules this out. It is likely that the companion is itself a binary.

Evans, Nancy Remage↗

NASA NTRS1991

New Cepheid distances to nearby galaxies based on BVRI CCD photometry. II - The local group galaxy M33

A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.

Freedman, Wendy L.↗

NASA NTRS1993

The distance to the Large Magellanic Cloud - Constraints from Cepheids in Large Magellanic Cloud star clusters

We have used recent observational data for the Cepheids in the rich, young LMC clusters NGC 1866 and NGC 2031 to constrain the cluster distances with the mass-equivalency (ME) method. The basis of this approach is to fix the cluster distances by requiring the Cepheid evolutionary and pulsational masses to be equal. Using evolutionary models incorporating a mild amount of core and envelope overshooting along with recent pulsational models, we derive distance moduli of 18.51 +/- 0.21 and 18.32 +/- 0.20 for NGC 1866 and NGC 2031, respectively. The quoted errors are dominated by the uncertainties in the heavy element abundances of the clusters (assumed to be 0.3 dex for both clusters), with a smaller contribution due to the apparently intrinsic spread in the masses of the Cepheids in each cluster. For the ME method, we find that Delta(m-M)(0)/Delta sub Z(1) = 0.69, where Z(1) = log (Z/0.016). This result implies that the cluster distances can be determined to better than +/- 5 percent if the cluster abundances can be measured to better than about +/- 0.15 dex.

Bertelli, Gianpaolo↗

NASA NTRS1993

On Cepheids at maximum and minimum light

It has been known for many decades that the spectral type of Cepheids at light maximum is constant with period. We use hydrodynamic pulsation models to explain this result in terms of the outward reach of the hydrogen ionization front. On the other hand, we show that, at minimum light, the Cepheid photospheric temperature is mainly a function of amplitude. A number of observed Cepheids have published temperatures that seem too hot at both maximum and minimum. We attribute this to an overestimation of the reddening for these stars. A list is given.

Simon, Norman R.↗

NASA NTRS1993

The orbit and companion of the Cepheid S Sge - A probable triple system

New radial velocities for the classical Cepheid S Sge have been obtained and combined with previous observations to derive a new orbit. The revised orbital elements are: gamma, -10.3 +/- 0.4 km/s; K, 15.5 +/- 0.2 km/s; e, 0.23 +/- 0.02; omega, 203.1 +/- 4.2 deg; T0, 39902.3 +/- 6.6 JD; P, 675.79 +/- 0.18 days; f(m), 0.239 +/- 0.010 solar masses; a sin i, 0.935 AU = 139.9 +/- 2.0 x 10 exp 6 km; s.e., 1.2 km/s. The revised elements differ very little from the orbit determined by Herbig and Moore (1952). We have also obtained low resolution IUE spectra to search for the companion. The IUE spectra show excess flux at 1800 A when compared with spectra of the single Cepheid Delta Cep at the same (B-V)0. The spectral type of the companion determined from this flux excess is A7 V to F0 V. However, the mass of such a companion (1.7 to 1.5 solar masses) is smaller than the minimum mass (2.8 solar masses) required by the mass function and an evolutionary mass of the Cepheid. We infer that the companion is itself a short period binary.

Evans, Nancy R.↗

NASA NTRS1994

The two period-luminosity relations for population I Cepheids

We summarize the evidence that most Population I Cepheids with periods less than approximately 8 days pulsate in the first overtone mode. Fundamental model and first overtone pulsators must follow different period-luminosity (P - L) relations. We demonstrate these different relations for different stellar systems, especially for the calibrating Cepheids in clusters and for Cepheids in the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), M31, and IC 4182.

Bohm-Vitense, Erika↗

NASA NTRS1995

Discovery of Cepheids in NGC 5253: Absolute peak brightness of SN Ia 1895B and SN Ia 1972E and the value of H(sub 0)

Observations of the Hubble Space Telescope (HST) between 1993 May 31 and 1993 July 19 in 20 epochs in the F555W passband and five epochs in the F785LP passband have led to the discovery of 14 Cepheids in the Amorphous galaxy NGC 5253. The apparent V distance modulus is (m-M)(sub AV) = 28.08 +/- 0.10 determined from the 12 Cepheids with normal amplitudes. The distance modulus using the F785LP data is consistent with the V value to within the errors. Five methods used to determine the internal reddening are consistent with zero differential reddening, accurate to a level of E(B-V) less than 0.05 mag, over the region occupied by Cepheids and the two supernovae (SNe) produced by NGC 5253. The apparent magnitudes at maximum for the two SNe in NGC 5253 are adopted as B(sub max) = 8.33 +/- 0.2 mag for SN 1895B, and B(sub max) = 8.56 +/- 0.1 and V(sub max) = 8.60 +/- 0.1 for SN 1972E which is a prototype SN of Type Ia. The apparent magnitude system used by Walker (1923) for SN 1859B has been corrected to the modern B scale and zero point to determine its adopted B(sub max) value.

Saha, A.↗

NASA NTRS2016

The Carnegie Hubble Program: The Distance and Structure of the SMC as Revealed by Mid-Infrared Observations of Cepheids

Using Spitzer observations of classical Cepheids we have measured the true average distance modulus of the Small Magellanic Cloud (SMC) to be18.96 +/- 0.01 stat +/- 0.03sys mag (corresponding to 62+/- 0.3kpc), which is 0.48 +/- 0.01 mag more distant than the LMC. This is in agreement with previous results from Cepheid observations, as well as with measurements from other indicators such as RR Lyrae stars and the tip of the red giant branch. Utilizing the properties of the mid-infrared Leavitt Law we measured precise distances to individual Cepheids in the SMC, and have confirmed that the galaxy is tilted and elongated such that its eastern side is up to20 kpc closer than its western side. This is in agreement with the results from red clump stars and dynamical simulations of the Magellanic Clouds and Stream.

Scowcroft, Victoria↗

NASA NTRS2016

The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of Cepheids and the Effect of Metallicity on the CO Band-Head at 4.6 Micron

We compare mid-infrared (IR) 3.6 and 4.5 micron Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the CO rotation-vibration band-head at 4.6 micron on the mid-IR photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 micron and find no evidence for an effect at 3.6 micron. We compare the ([3.6]-[4.5]) period-colour relations in the MW, LMC and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al., demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.

Scowcroft, Victoria↗

NASA NTRS2018

The Mass of the Cepheid V350 Sgr

V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot companion which is prominent in the ultraviolet (UV) and which is not itself a binary. We have obtained two high-resolution echelle spectra of the companion at orbital velocity maximum and minimum with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope in the 1320 to 1510 Å region. By cross-correlating these spectra we obtained the orbital velocity amplitude of the companion with an uncertainty in the companion amplitude of 1.9 km s(exp −1). This provides a mass ratio of the Cepheid to the companion of 2.1. The UV energy distribution of the companion provides the mass of the companion, yielding a Cepheid mass of 5.2 ± 0.3 solar mass. This mass requires some combination of moderate main sequence core convective overshoot and rotation to match evolutionary tracks.

V350 Sgr↗

NASA NTRS1997

Hipparcos Parallaxes and the Cepheid Distance Scale

Hipparcos parallaxes have recently become available for a sample of Galactic Cepheids. We have used these new distances to calibrate the Cepheid period-luminosity (PL) relation at six wavelengths (BVIJHK).

Hipparcos Parallaxes Cepheid Multiwavelength↗

NASA NTRS1996

The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331

The distance to NGC 7331 has been derived from Cepheid variables observed with HST/WFPC2, as part of the Extragalactic Distance Scale Key Project. Multi-epoch exposures in F555W(~V) and F814W (~1), with photometry derived independently from DoPHOT and DAOPHOT/ALLFRAME programs, which were used to detect a total of 14 reliable Cepheids, with periods between 11 and 42 days.

Cepheids galaxies distances redshifts NGC 7331 ear↗

NASA NTRS1971

Cepheid variables and the Magellanic Clouds.

The Magellanic Cloud variables differ from variables in the Galaxy by the number of variables at different periods, by the period-amplitude relation, and by the mean color. Schematic evolutionary tracks are used to demonstrate how a proposed difference between Galactic evolution and Small Magellanic Cloud evolution might be responsible for the observed mean colors and frequencies of occurrence of cepheid variables with different periods. The frequency of short-period cep heids is seen to be a very sensitive indicator of some, not yet fully understood, aspect of abundances. It is demonstrated that the characteristic second-bump feature appearing in the light curves of cepheids with the period range from 7 to 15 days can be used to determine stellar mass and radius on the basis of calculations of interior motions.-

Christy, R. F.↗