24 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
Pulsation in hot main sequence stars: comparison of observations with models
TESS observations of pulsating hot main sequence stars paint a very different
picture from what is currently accepted. There are large numbers of delta Scuti
(DSCT) stars hotter than the theoretical hot edge of the instability strip,
continuing to what appear to be DSCT stars of mid-B type (historically known as
Maia variables). The frequencies of maximum amplitude in DSCT stars are in poor
agreement with unstable frequencies from the models. There is a well-defined
upper envelope in the frequencies of maximum amplitude as a function of
effective temperature for DSCT and MAIA stars which requires an explanation.
The gamma Doradus (GDOR) stars should be regarded as DSCT stars with suppressed
high frequencies rather than a separate class. They are found mostly among the
cool DSCT stars, but occur throughout the DSCT instability strip and as early
A-type stars, where they merge with the SPB variables. The mixture of DSCT and
GDOR stars throughout the instability strip is one example of the unexplained
large variation of frequency patterns in DSCT stars. The location of beta
Cephei stars in the H-R diagram agrees quite well with the models, but the
observed frequencies are generally higher than predicted. There are no
discernible boundaries between the traditional classes of pulsating stars.
Existing pulsation models do not describe the observations at all well and a
re-evaluation is required.Comment: 9 pages, 7 figure
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 identified in the Kepler field 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 (Teff , log g, [M/H], vmic, etc.) were derived from ∼160 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 fide 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 (σ ∼ 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 ∼4-year sampling interval of the Kepler data