Cepheid Variable Stars & Distance
Discovery of the Period - Luminosity Relationship; Calculating Distances Using He used his data on nearby Cepheids to calculate the distance to the. These relations can be used directly as reference for extra-galactic We employ the near-IR surface brightness (IRSB) method to determine. PERIOD-LUMINOSITY RELATION AND ITS IMPLICATIONS cannot attend the meetings to present some results from this Thesis Dissertation. . The coefficients of the Fourier Interrelations, as determined from "calibrating set", .. Cepheid are useful tools and probes for studying astrophysics, which mainly come in two.
However, although neither the magnitude of the effect nor its wavelength dependence have yet been firmly established, the observational and theoretical evidence for an effect is steadily growing. Published empirical values for the index see Equation 5 in Section 3.
Since the effects of metallicity are wavelength-dependent, it is critical that the appropriate correction for a given dataset be applied. Some recent theoretical models e.
Unfortunately, the sign of the effect is still uncertain. Thus, for the present, calibrating the metallicity effect based on models alone is not feasible. As part of the Key Project, we carried out a second differential test comparing two fields in the face-on galaxy, M Kennicutt et al. These two studies are consistent with there being a shallow metallicity dependence, but the statistical significance of each test is individually low. A comparison of the VIH photometry for the inner and outer field is consistent with a metallicity sensitivity of the PL relations, but artificial star tests in the inner field indicate that crowding is significant, and precludes an accurate determination of the magnitude of the effect.
Other recent studies e. This sample has extensive phase coverage at BVI magnitudes and covers the period range of 0. The Sebo et al. These 10 Cepheids are all well fit by, and all lie within 1- of the period-luminosity slopes defined by the Udalski et al. The Udalski et al. With this modulus and the reddening-corrected Udalski et al.
New Revised Cepheid Distances Over the 6 years that we have been publishing data from the Key Project, our analysis methods, as well as the photometric calibration, have evolved and improved. Hence, the sample of published Key Project distances has not been analyzed completely homogeneously. In this paper, we have redetermined the true moduli to each galaxy used in the Key Project. These distances are calculated with the new calibration described above, and with attention to minimizing bias at the short-period end of the PL relation, as described below and by Freedman et al.
PHY / The Cepheid Period-Luminosity Relation
In this analysis we have 1 consistently adopted only the published Cepheid photometry which were reduced using the ALLFRAME stellar photometry reduction package, whose phase points were converted to mean magnitudes using intensity-weighted averages or their template-fitted equivalents.
In two cases NGC and NGCa single long-period Cepheid was also dropped because of stochastic effects at the bright sparsely populated end of the PL relation, which can similarly bias solutions. Finally, 4 we have adopted the published slopes of the Udalski et al.
The adoption of the new Udalski et al.
HST Key Project Summary - W. Freedman et al.
The astrophysical significance of studying variable stars in the LMC is that all the stars in the LMC are roughly the same distance from us. Distances are one of the most fundamental, but most difficult to measure, quantities in astronomy.
Therefore, one can compare distance dependent quantities like luminositieswhich one cannot do easily for the brighter stars in our own Milky Way galaxy. Most, if not all, stars are variable. Some, like the novae, are spectacularly variable, while others are barely noticeable even upon close inspection.
Among the plethora of types of variables are: Eclipsing variables, which are binary systems we observe from in or close to the plane of the orbit. They vary because one star gets in front to the other each orbit, thereby diminishing total brightness. This is simply a geometrical obscuration. Pulsating variables, which are stars that lie in the instability strip in the Hertzsprung-Russell H-R diagram a plot of luminosity or magnitude vs.
Most stars are stable against adiabatic perturbations. Perturb the star to a larger radius. The temperature will fall and the opacity will increase. Then let the star contract under its gravity. The enhanced opacity will result in an enhanced radiation pressure, and a damping of the oscillation.
- Astrophysics > Cosmology and Nongalactic Astrophysics
- Cepheid Variable Stars & Distance Determination
However, in the instability strip, hydrogen is partially ionized in the outer radiative envelope of the star. As gravity makes the star contract, the low opacity does not lead to an increased radiation pressure, so the oscillations do not damp out.
The instability strip runs diagonally through the H-R diagram, with temperatures near 10,K. All stars are natural pulsators at low amplitudes. The atmosphere of the Sun oscillates with a fundamental period of about 5 minutes. The study of these periods helioseismology provides a probe of the interior of the Sun, and provides temperature and density profiles accurate to a few percent within the convective zone.
Asteroseismology, now possible for some of the brighter stars, reveals their internal characteristics temperatures, densities, etc. Intrinsic variables All convective stars spectral types F, G, K, Mincluding the Sun, display both periodic and irregular variability resulting from stellar magnetic activity in the outer atmosphere.
Asymmetric distributions of starspots reveal themselves as periodic rotational modulation of the brightness, with amplitudes up to 0. Flaring due to magnetic recombination is irregular, and is common among the younger, more rapidly rotating convective stars. Flaring is most noticeable in the X-rays and UV, and among the M stars, where contrast with the photosphere is enhanced.
Explosive variables include novae. The buildup of hydrogen-rich matter on the surface of a white dwarf, drawn fron a Roche-lobe-filling companion, will undergo a runaway thermonuclear detonation once enough builds up on the surface so that the lower layers become degenerate. Novae occur irregulary, at intervals from millions of years to a few years, depending on the accretion rate and the mass of the white dwarf.
Cataclysmic variables CVs are white dwarf binaries undergoing accretion. These vary by a few magnitudes irregularly due to changes in the mass accretion rate. In the Polars, or AM Her stars, the accretion stream impacts the surface of the white dwarf directly. Variations in the mass accretion rate lead directly to brightness changes as the gravitational potential energy released heats the accretion colun and the impact zone.
In the dwarf novae an accretion disk forms, and the brightness variations in the disk reflect the viscous heating of the disk. All CVs eventually become novae. In an analogous set of variables, the X-ray binaries, the white dwarf is replaced by a neutron star of stellar-mass black hole. Artist's conception of a Polar, showing the disruption of the accretion stream by the MG magnetic field of the white dwarf Image copyright M.
Garlick Ellipsoidal variables are stars that are not round, and present different aspects to us as they rotate. Ellipsoidal variables are all in close binary systems, where they are tidally-distorted by their companions. Stars pulsating in an overtone are more luminous and larger than a fundamental mode pulsator with the same period. When the helium core ignites in an IMS, it may execute a blue loop and crosses the instability strip again, once while evolving to high temperatures and again evolving back towards the asymptotic giant branch.
The duration and even existence of blue loops is very sensitive to the mass, metallicity, and helium abundance of the star. In some cases, stars may cross the instability strip for a fourth and fifth time when helium shell burning starts.
Classical Cepheid variable
More massive and hotter stars develop into more luminous Cepheids with longer periods, although it is expected that young stars within our own galaxy, at near solar metallicity, will generally lose sufficient mass by the time they first reach the instability strip that they will have periods of 50 days or less. Very massive stars never cool sufficiently to reach the instability strip and do not ever become Cepheids. At low metallicity, for example in the Magellanic Clouds, stars can retain more mass and become more luminous Cepheids with longer periods.
This is due to the phase difference between the radius and temperature variations and is considered characteristic of a fundamental mode pulsator, the most common type of type I Cepheid. In some cases the smooth pseudo-sinusoidal light curve shows a "bump", a brief slowing of the decline or even a small rise in brightness, thought to be due to a resonance between the fundamental and second overtone.
The bump is most commonly seen on the descending branch for stars with periods around 6 days e.