41 research outputs found
Maia variables and other anomalies among pulsating stars
From TESS photometry, 493 mid- to late-B stars with high frequencies (Maia
variables) have been identified. The distribution of projected rotational
velocities shows that the rotation rates of Maia variables are no different
from those of SPB stars. Moreover, many Maia stars pulsate with frequencies
exceeding 60 c/d. Rapid rotation is ruled out as a possible factor in
understanding the Maia variables. There is clearly a serious problem with
current pulsational models. Not only are the models unable to account for the
Maia stars, but they also fail to account for the fact that SPB and gamma Dor
variables form one continuous instability strip from the cool end of the delta
Sct region to the hot end of the beta Cep instability strip. Likewise, there is
continuity between the distributions of delta Sct, Maia, and beta Cep
variables. In fact, Maia stars seem to be an extension of delta Sct stars to
the mid-B type. These observations suggest an interplay between multiple
driving mechanisms rather than separate dominant mechanisms for each
variability group.Comment: 5 pages, 1 table, 3 figure
Identification and classification of TESS variable stars
Visual classification of the variability classes of over 120,000 Kepler, K2
and TESS stars is presented. The sample is mainly based on stars with known
spectral types. Since variability classification often requires the location of
the star in the H--R diagram, a catalogue of effective temperatures was
compiled. Luminosities were estimated from Gaia DR3 parallaxes. The different
classes of variable found in this survey are discussed. Examples of light
curves and periodograms for common variability classes are shown. A catalogue
of projected rotational velocities is also included.Comment: 11 pages, 2 figures, 6 table
The extraordinary frequency pattern variation in {\delta} Scuti stars
Inspection of the periodograms of TESS delta Scuti stars indicates that there
is little, if any, similarity between the frequencies of stars in the same
region of the H-R diagram. This is difficult to understand because pulsation
models predict that stars with similar physical parameters should have similar
frequencies. To investigate the problem, a quantitative measure of similarity
between frequency patterns is described. When applied to non-adiabatic
pulsation models with similar temperatures and luminosities, a strong
correlation is found between the frequency patterns, as expected. The
correlation increases when rotational frequency splitting is included. When
applied to observations, no correlation can be found, confirming the impression
from visual inspection. It seems that each star has its own unique frequency
pattern, unrelated to its position in the H-R diagram. This presents a problem
in our understanding of stellar pulsation. The existence of a period-luminosity
law and the effect of combination frequencies in delta Scuti stars is briefly
discussed.Comment: 8 pages, 7 figures, 1 tabl
Metal-Rich SX Phe Stars in theKeplerField
High-resolution spectroscopic observations have been made for 32 of the 34 candidate SX Phe stars identiļ¬ed in the Kepler ļ¬eld by Balona & Nemec (2012). All available long- and short-cadence Q0-Q17 Kepler photometry has been analyzed for the 34 candidates. Radial velocities (RVs), space motions (U, V, W), projected rotation veloc-
ities (v sin i), spectral types, and atmospheric characteristics (Teļ¬ , log g, [M/H], vmic, etc.) were derived from ā¼160 spectra taken with the ESPaDOnS spectrograph on the Canada-
France-Hawaii 3.6-m telescope and with the ARCES spectrograph on the Apache Point Observatory 3.5-m telescope. Two thirds of the stars are fast rotators with v sin i > 50 km/s, including four stars with v sin i > 200 km/s. Three of the stars have (negative) RVs > 250 km/s and retrograde space motions, and seven stars have total space motions > 400 km/s. All the spectroscopically measured SX Phe candidates have positions in a Toomre diagram that are consistent with being bona ļ¬de halo and thick-disk stars. Although several stars
show a marked metal weakness, the mean [Fe/H] of the sample is near 0.0 dex (Ļ ā¼ 0.25
dex), which is considerably more metal-rich than is normally expected for a sample of Pop. II stars. Observed pulsation frequency modulations and optical time delays suggest that at least eight of the SX Phe stars are in binary systems, some of which show signif- icant RV variations. Six of the time-delay binaries have secondary masses ranging from
0.05 to 0.70 Mo and orbital periods in the range 9 to 1570 days. Another star appears to be an ellipsoidal variable with a 2.3-day orbital period; and two other systems have orbital
periods longer than the ā¼4-year sampling interval of the Kepler data
Pulsation in Intermediate-Mass Stars
A new perspective of pulsation in stars within the Ī“ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all Ī“ Scuti stars contain low frequencies. Because Ī³ Doradus stars co-exist with Ī“ Sct stars in the same region of the instability strip, it follows that Ī³ Dor stars are driven by the same mechanism as Ī“ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot Ī³ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6,300ā8,300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same massātemperatureāluminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere