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Register a new userFourier analysis of He 4471/Mg 4481 line profiles for separating rotational velocity and axial inclination in rapidly-rotating B-type stars
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While the effect of rotation on spectral lines is complicated in rapidly-rotating stars because of the appreciable gravity-darkening effect differing from line to line, it is possible to make use of this line-dependent complexity to separately determine the equatorial rotation velocity (ve) and the inclination angle (i) of rotational axis. Although line-widths of spectral lines were traditionally used for this aim, we tried in this study to apply the Fourier method, which utilizes the unambiguously determinable first-zero frequency (sigma1) in the Fourier transform of line profile. Equipped with this technique, we analyzed the profiles of HeI 4471 and MgII 4481 lines of six rapidly-rotating (vesini~150-300km/s) late B-type stars, while comparing them with the theoretical profiles simulated on a grid of models computed for various combination of (ve, i). According to our calculation, sigma1 tends to be larger than the classical value for given vesini. This excess progressively grows with an increase in ve, and is larger for the He line than the Mg line, which leads to sigma1He > sigma1Mg. It was shown that ve and i are separately determinable from the intersection of two loci (sets of solutions reproducing the observed sigma1 for each line) on the ve vs. i plane. Yet, line profiles alone are not sufficient for their unique discrimination, for which photometric information (such as colors) needs to be simultaneously employed.
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While it is known that the sharp-line star Vega (vsini ~ 20km/s) is actually a rapid rotator seen nearly pole-on with low i (< 10 deg), no consensus has yet been accomplished regarding its intrinsic rotational velocity (v_e), for which rather different values have been reported so far. Methodologically, detailed analysis of spectral line profiles is useful for this purpose, since they reflect more or less the v_e-dependent gravitational darkening effect. However, direct comparison of observed and theoretically simulated line profiles is not necessarily effective in practice, where the solution is sensitively affected by various conditions and the scope for combining many lines is lacking. In this study, determination of Vegas v_e was attempted based on an alternative approach making use of the first zero (q_1) of the Fourier transform of each line profile, which depends upon K (temperature sensitivity parameter differing from line to line) and v_e. It turned out that v_e and vsini could be separately established by comparing the observed q_1^obs and calculated q_1^cal values for a number of lines of different K. Actually, independent analysis applied to two line sets (49 Fe I lines and 41 Fe II lines) yielded results reasonably consistent with each other. The final parameters of Vegas rotation were concluded as vsini = 21.6 (+/- 0.3) km/s, v_e = 195 (+/- 15) km/s, and i = 6.4 (+/- 0.5) deg.
We have searched for short periodicities in the light curves of stars with $T_{rm eff}$ cooler than 4000 K made from 2-minute cadence data obtained in TESS sectors 1 and 2. Herein we report the discovery of 10 rapidly rotating M-dwarfs with highly structured rotational modulation patterns among 10 M dwarfs found to have rotation periods less than 1 day. Star-spot models cannot explain the highly structured periodic variations which typically exhibit between 10 and 40 Fourier harmonics. A similar set of objects was previously reported following K2 observations of the Upper Scorpius association (Stauffer et al. 2017). We examine the possibility that the unusual structured light-curves could stem from absorption by charged dust particles that are trapped in or near the stellar magnetosphere. We also briefly explore the possibilities that the sharp structured features in the lightcurves are produced by extinction by coronal gas, by beaming of the radiation emitted from the stellar surface, or by occultations of spots by a dusty ring that surrounds the star. The latter is perhaps the most promising of these scenarios. Most of the structured rotators display flaring activity, and we investigate changes in the modulation pattern following the largest flares. As part of this study, we also report the discovery of 371 rapidly rotating M-dwarfs with rotational periods below 4 hr, of which the shortest period is 1.63 hr.
Radial-velocities for the early-type stars in the Pleiades cluster have always been challenging to measure because of the significant rotational broadening of the spectral lines. The large scatter in published velocities has led to claims that many are spectroscopic binaries, and in several cases preliminary orbital solutions have been proposed. To investigate these claims, we obtained and report here velocity measurements for 33 rapidly-rotating B, A, and early F stars in the Pleiades region, improving significantly on the precision of the historical velocities for most objects. With one or two exceptions, we do not confirm any of the previous claims of variability, and we also rule out all four of the previously published orbital solutions, for HD 22637, HD 23302, HD 23338, and HD 23410. We do find HD 22637 to be a binary, but with a different period (71.8 days). HD 23338 is likely a binary as well, with a preliminary 8.7 yr period also different from the one published. Additionally, we report a 3635 day orbit for HD 24899, another new spectroscopic binary in the cluster. From the 32 bona fide members in our sample we determine a mean radial velocity for the Pleiades of 5.79 +/- 0.24 km/s, or 5.52 +/- 0.31 km/s when objects with known visual companions are excluded. Adding these astrometric binaries to the new spectroscopic ones, we find a lower limit to the binary fraction among the B and A stars of 37%. In addition to the velocities, we measure v sin i for all stars, ranging between 69 and 317 km/s.
Rapidly rotating giant stars are relatively rare and may represent important stages of stellar evolution, resulting from stellar coalescence of close binary systems or accretion of sub-stellar companions by their hosting stars. In the present letter we report 17 giant stars observed in the scope of the Kepler space mission exhibiting rapid rotation behavior. For the first time the abnormal rotational behavior for this puzzling family of stars is revealed by direct measurements of rotation, namely from photometric rotation period, exhibiting very short rotation period with values ranging from 13 to 55 days. This finding points for remarkable surface rotation rates, up to 18 times the Sun rotation. These giants are combined with 6 other recently listed in the literature for mid-IR diagnostic based on WISE information, from which a trend for an infrared excess is revealed for at least a half of the stars, but at a level far lower than the dust excess emission shown by planet-bearing main-sequence stars.
A new two dimensional non-perturbative code to compute accurate oscillation modes of rapidly rotating stars is presented. The 2D calculations fully take into account the centrifugal distorsion of the star while the non perturbative method includes the full influence of the Coriolis acceleration. This 2D non-perturbative code is used to study pulsational spectra of highly distorted evolved models of stars. 2D models of stars are obtained by a self consistent method which distorts spherically averaged stellar models a posteriori. We are also able to compute gravito-acoustic modes for the first time in rapidly rotating stars. We present the dynamics of pulsation modes in such models, and show regularities in their frequency spectra.
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