اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدData Analysis of Bright Main-Sequence A- and B-type Stars Observed Using the TESS and BRITE Spacecraft
96
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
During the last two years we have received long time-series photometric observations of bright (V mag < 8) main-sequence A- and B-type stars observed by the NASA TESS spacecraft and the Austria-Poland-Canada BRITE satellites. Using TESS observations of metallic-line A (Am) stars having peculiar element abundances, our goal is to determine whether and why these stars pulsate in multiple radial and non-radial modes, as do the delta Scuti stars in the same region of the H-R diagram. The BRITE data were requested to investigate pulsations in bright (V around 6 mag) A- and B-type stars in the Cygnus-Lyra field of view that had been proposed for observations during the now-retired NASA Kepler mission. Of the 21 (out of 62 proposed) Am stars observed by TESS so far, we find one delta Sct star and two delta Sct / gamma Dor hybrid candidates. Of the remaining stars, we find three gamma Dor candidates, six stars showing photometric variations that may or may not be associated with pulsations, and eight stars without apparent significant photometric variability. For the A- and B-type stars observed by BRITE, one star (HR 7403) shows low amplitude low frequency modes that likely are associated with its B(emission) star properties; one star (HR 7179) shows SPB variability that is also found in prior Kepler data, and two stars (HR 7284 and HR 7591) show no variability in BRITE data, although very low amplitude variability was found in TESS or Kepler data. For the TESS and BRITE targets discussed here, follow-up ground- and space-based photometric and spectroscopic observations combined with stellar modeling will be needed to constrain stellar parameters and to understand the nature of the variability.
قيم البحث
اقرأ أيضاً
Metallic-line A (Am) stars are main-sequence stars of around twice the mass of the Sun that show element abundance peculiarities in their spectra. The radiative levitation and diffusive settling processes responsible for these abundance anomalies sho
uld also deplete helium from the region of the envelope that drives delta Scuti pulsations. Therefore, these stars are not expected to be delta Scuti stars, which pulsate in multiple radial and nonradial modes with periods of around 2 hours. As part of the NASA TESS Guest Investigator Program, we proposed photometric observations in 2-minute cadence for samples of bright (visual magnitudes around 7-8) Am stars. Our 2020 SAS meeting paper reported on observations of 21 stars, finding one delta Scuti star and two delta Scuti / gamma Doradus hybrid candidates, as well as many stars with photometric variability possibly caused by rotation and starspots. Here we present an update including 34 additional stars observed up to February 2021, among them three delta Scuti stars and two delta Scuti / gamma Doradus hybrid candidates. Confirming the pulsations in these stars requires further data analysis and follow-up observations, because of possible background stars or contamination in the TESS CCD pixels with scale 21 arc sec per pixel. Asteroseismic modeling of these stars will be important to understand the reasons for their pulsations.
From sector 1--40 {em TESS} observations, 20 new roAp stars, 97 ostensibly non-peculiar stars with roAp-like frequencies (the roA variables) and 617 $delta$~Scuti stars with independent frequencies typical of roAp stars were found. There is no criter
ion that can distinguish roAp/roA stars from $delta$~Sct stars. For expediency, an arbitrary low frequency of 60,d$^{-1}$ was chosen as the boundary between the $delta$~Sct and roAp/roA classes. Because an unknown mode selection process is clearly present in $delta$~Sct stars, the roAp/roA stars may be considered as $delta$~Sct stars in which high frequencies are preferentially selected. This interpretation is supported by the fact that the combined proportion of $delta$~Sct and roAp stars among Ap stars is the same as among non-Ap stars. Contrary to models, observations show that low frequencies in Ap stars are not suppressed. One of the most puzzling aspects of roAp stars is the large fraction which have short mode lifetimes. The failure of current models to explain these results may be due to an incorrect treatment of the outer layers of these stars.
Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into Centrifugal Magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type s
tars to yield detectable emission in H$alpha$ -- the principal diagnostic of these structures -- are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with H$alpha$ emission originating in a CM are 1) rapid rotators, 2) strongly magnetic, and 3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.
Magnetic B-type stars exhibit photometric variability due to diverse causes, and consequently on a variety of timescales. In this paper we describe interpretation of BRITE photometry and related ground-based observations of 4 magnetic B-type systems:
$epsilon$ Lupi, $tau$ Sco, a Cen and $epsilon$ CMa.
We present examples of an extended asteroseismic modelling in which we aim at fitting not only pulsational frequencies but also certain complex parameter related to each frequency. This kind of studies, called textbf{complex asteroseismology}, has be
en successfully applied to a few main sequence B-type pulsators and provided, in particular, plausible constraints on textbf{stellar opacities}. Here, we briefly describe our results for three early B-type stars.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
