The period lumosity relationship is useful in determining cost

LMC variables used to establish the slope for the Mira PL(K) relation. . The slope determined above for the LMC O-rich Miras, i.e. ρ=− ± can be compared with the value ), DIRBE (Smith, Price & Baker ), Noguchi et al. It is the same technique used later by Clyde make the marginal cost (in time) of analyzing one more star small. This study is aimed at investigating the period–luminosity relation of SX The cluster distances from Dotter et al. are used to calibrate an SX Phe period-- luminosity relation based . These are often calibrated using nearby pulsators with well-determined Hence, we use simpler, linear relations, at the cost of precision.

The brightness changes are more pronounced at shorter wavelengths. Pulsations in an overtone higher than first are rare but interesting. 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.

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In some cases, stars may cross the instability strip for a fourth and fifth time when helium shell burning starts. 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.

Classical Cepheid variable - Wikipedia

As the period increases, the location of the bump moves closer to the maximum and may cause a double maximum, or become indistinguishable from the primary maximum, for stars having periods around 10 days e. At longer periods the bump can be seen on the ascending branch of the light curve e. X Cygnibut for period longer than 20 days the resonance disappears. A minority of classical Cepheids show nearly symmetric sinusoidal light curves. The cluster distances from Dotter et al.

Classical Cepheid variable

We explore the possibility that, at least in GGCs, they represent blue stragglers which have enhanced helium content that was either inherited from second-generation progenitors or gained as a result of the blue straggler formation process. Accordingly, grids of linear non-adiabatic SX Phe pulsational models have been computed by Santolamazza et al. The astrophysics of SX Phe variables is of particular interest because in GGCs, the region of the instability strip which they occupy in a colour—magnitude diagram is coincident with the blue straggler region e.

Information on the interiors of SX Phe stars ascertained through an analysis of their pulsation frequencies has been proposed as a viable avenue to gain further clues to the as-yet-unsolved issues of blue straggler star BSS formation and evolution e.

PHY / The Cepheid Period-Luminosity Relation

However, aside from the lack of a large global helium enhancement for a large sample of SX Phe stars Olech et al. The number of known short-period instability strip pulsators has more than doubled in recent years. Recent photometric variability surveys of Local Group galaxies have revealed short-period instability strip pulsators in Fornax P08its cluster Fornax 5 Greco et al.

Rather than investigating individual cases in detail, we use the known SX Phe population as an ensemble to explore the connection between SX Phe and blue stragglers.

The ability to use abundances of blue stragglers to differentiate between their formation mechanisms, predicted by Bailynrecently gained strong observational support in the spectroscopic work of Ferraro et al.