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Register a new userOn the rotational velocity of Sirius A
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Yoichi Takeda
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With an aim of getting information on the equatorial rotation velocity (v_e) of Sirius A separated from the inclination effect (sin i), a detailed profile analysis based on the Fourier transform technique was carried out for a large number of spectral lines, while explicitly taking into account the line-by-line differences in the centre-limb behaviours and the gravity darkening effect (which depend on the physical properties of each line) based on model calculations. The simulations showed that how the 1st-zero frequencies (q_1) of Fourier transform amplitudes depends on v_e is essentially determined by the temperature-sensitivity parameter (K) differing from line to line, and that Fe I lines (especially those of very weak ones) are more sensitive to v_e than Fe II lines. The following conclusions were drawn by comparing the theoretical and observed q_1 values for many Fe I and Fe II lines: (1) The projected rotational velocity (vsini) for Sirius A is fairly well established at 16.3 (+/-0.1) km/s by requiring that both Fe I and Fe II lines yield consistent results. (2) Although precise separation of v_e and i is difficult, v_e is concluded to be in the range of 16 < v_e < 30-40 km/s, which corresponds to 25 < i(deg) < 90. Accordingly, Sirius A is an intrinsically slow rotator for an A-type star, being consistent with its surface chemical peculiarity.
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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.
Oxygen sequence Wolf-Rayet stars (WO) are thought to be the final evolution phase of some high mass stars, as such they may be the progenitors of type Ic SNe as well as potential progenitors of broad-lined Ic and long gamma-ray bursts. We present the first spectropolarimetric observations of the Galactic WO stars WR93b and WR102 obtained with FORS1 on the VLT. We find no sign of a line effect, which could be expected if these stars were rapid rotators. We also place constraints on the amplitude of a potentially undetected line effect. This allows us to derive upper limits on the possible intrinsic continuum polarisation, and find P$_{rm cont}$ < 0.077 percent and P$_{rm cont}$ < 0.057 percent for WR93b and WR102, respectively. Furthermore, we derive upper limits on the rotation of our WO stars by considering our results in the context of the wind compression effect. We estimate that for an edge-on case the rotational velocity of WR93b is v$_{rm rot}$ < 324 km/s while for WR102 v$_{rm rot}$ < 234 km/s. These correspond to values of v$_{rm rot}$/v$_{rm crit}$ <19 percent and <10 percent, respectively, and values of log(j)<18.0 cm$^2$/s for WR93b and <17.6 cm^2 /s for WR102. The upper limits found on v$_{rm rot}$/v$_{rm crit}$ and log(j) for our WO stars are therefore similar to the estimates calculated for Galactic WR stars that do show a line effect. Therefore, although the presence of a line effect in single WR stars is indicative of fast rotation, the absence of a line effect does not rule out significant rotation, even when considering the edge-on scenario.
We address the origin of the observed bimodal rotational distribution of stars in massive young and intermediate age stellar clusters. This bimodality is seen as split main sequences at young ages and also has been recently directly observed in the $Vsini$ distribution of stars within massive young and intermediate age clusters. Previous models have invoked binary interactions as the origin of this bimodality, although these models are unable to reproduce all of the observational constraints on the problem. Here we suggest that such a bimodal rotational distribution is set up early within a clusters life, i.e., within the first few Myr. Observations show that the period distribution of low-mass ($la 2 M_odot$) pre-main sequence (PMS) stars is bimodal in many young open clusters and we present a series of models to show that if such a bimodality exists for stars on the PMS that it is expected to manifest as a bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for stars with masses in excess of $sim1.5$~msun. Such a bimodal period distribution of PMS stars may be caused by whether stars have lost (rapid rotators) or been able to retain (slow rotators) their circumstellar discs throughout their PMS lifetimes. We conclude with a series of predictions for observables based on our model.
Context. Abundance anomalies observed in a fraction of A and B stars of both Pop I and II are apparently related to internal particle transport. Aims. Using available constraints from Sirius A, we wish to determine how well evolutionary models including atomic diffusion can explain observed abundance anomalies when either turbulence or mass loss is used as the main competitor to atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative accelerations, have been computed from the zero age main-sequence of 2.1Modot stars for metallicities of Z0 = 0.01 pm 0.001 and shown to agree with the observed parameters of Sirius A. Surface abundances were predicted for three values of the mass loss rate and for four values of the mixed surface zone. Results. A mixed mass of ~ 10^-6 Modot or a mass loss rate of 10^-13 Modot/yr were determined through comparison with observations. Of the 17 abundances determined observationally which are included in our calculations, up to 15 can be predicted within 2 sigmas and 3 of the 4 determined upper limits are compatible. Conclusions. While the abundance anomalies can be reproduced slightly better using turbulence as the process competing with atomic diffusion, mass loss probably ought to be preferred since the mass loss rate required to fit abundance anomalies is compatible with the observationally determined rate. A mass loss rate within a factor of 2 of 10^-13 Modot/yr is preferred. This restricts the range of the directly observed mass loss rate.
Modeling the submillimeter to centimeter emission of stars is challenging due to a lack of sensitive observations at these long wavelengths. We launched an ongoing campaign to obtain new observations entitled Measuring the Emission of Stellar Atmospheres at Submillimeter/millimeter wavelengths (MESAS). Here we present ALMA, GBT, and VLA observations of Sirius A, the closest main-sequence A-type star, that span from 1.4 to 9.0 millimeters. These observations complement our previous millimeter data on Sirius A and are entirely consistent with the PHOENIX stellar atmosphere models constructed to explain them. We note that accurate models of long wavelength emission from stars are essential not only to understand fundamental stellar processes, but also to determine the presence of dusty debris in spatially unresolved observations of circumstellar disks.
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