Cepheid variable - Wikipedia
Periods for the luminosity variation of these RSVs were determined from 20 hints of a red supergiant variable (RSV) period-luminosity relation (PLR) suitable for . to M raises questions about the accuracy of stellar evolution models if the. Period - luminosity - color - radius relationships of Cepheids as a function of on period–magnitude relation- uncertainties inherent in stellar evolution models . criterion used to determine the red edge. servatory (Baraffe & El Eid ). , — () Period—luminosity relations for red supergiant variables The usefulness of Cepheids as distance indicators arises because they are . the late stages of stellar evolution by determining some properties of asymptotic.
The temperature and spectral type vary as they pulsate.
Their radii are a few tens to a few hundred times that of the sun. More luminous Cepheids are cooler and larger and have longer periods. Along with the temperature changes their radii also change during each pulsation e.
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.
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. Using the four Cepheids they derive an extinction-corrected distance modulus of This Key Project has produced two Cepheid-based distances — one for Cepheids in an outer region of M by Kelson et al. The distance modulus reported by Kelson et al. There are at least two other recent measurements that agree with the Cepheid result.
It has a read noise of 3. At the WIYN 3. Since M spans an angular size that is larger than the field of view of the CCDs, it was split into two fields for imaging purposes — a north-east field and a south-east field.
The central coordinates of the fields used are shown in Table 1. The positions of the two M field centres. The size of each field about the centres was either 5.
View large Download slide The positions of the two M field centres.
Classical Cepheid variable
The bulk of the observations were taken in the R band to find the LPVs in M because this bandpass represents a good compromise between shorter wavelengths, where their amplitudes are higher, and longer wavelengths, where the LPVs are more prominent.
By imaging the fields in the R band at a number of epochs, the variable stars and their periods were found. At almost all epochs, two or more exposures were obtained of each field to aid in the removal of cosmic-ray hits. Exposure times for individual images were mostly 10 min long, though occasionally 15 min.
Classical Cepheid variable - Wikipedia
The image sets at each epoch were summed to produce images with total integration times of 20 to 30 min. Photometric observations of each field were made on the night of December 1, in both the R and I bands.
The I-band observations allow single-epoch random-phase PL relations to be derived. Details of the R-band observations listing the date of observation, telescope, detector used and seeing conditions for each field are given in Tables 2 and 3. To calibrate the R and I photometry of the M fields, images of Landolt's standard star fields were obtained on the same nights as the M fields, provided conditions were photometric.
For each night of telescope observations a set of calibration images was obtained for the reduction procedure. These calibration images usually consisted of ten bias images and five dome flat images in each filter taken on each night of observation.
The calibration, program and standard star images were reduced with the iraf ccdproc package. In brief, the procedure entailed performing an overscan correction, a bias level subtraction and a flatfield correction.
The program fields with multiple observations at each epoch, which includes most of the data, had all the images of a field from a particular night averaged together to improve the signal-to-noise ratio in the image. This was achieved by performing a fractional pixel alignment with respect to a reference image on all the images of a field from one night with imalign and then averaging the aligned images together with imcombine.
The imcombine process also carried out a cosmic-ray rejection while averaging the images. The final set of reduced images used to find the LPVs consisted of an R-band image for each of the nights shown in Tables 2 and 3 and an I-band image taken on December 1 of each field. Specifically, these programs were used to find stars in all the images, perform aperture photometry, derive the point spread function PSF for each image, and use the PSF to obtain photometry for all the stars in each field.