Anita Krishnamurthi

Theoretical Models of the Angular Momentum Evolution of Solar-Type Stars

We examine the effects of different assumptions about the initial conditions, angular momentum loss law, and angular momentum transport on the angular momentum evolution of 0.5-1.2 solar mass stars. We first perform a parameter variation study to test the sensitivity of the surface rotation rate as a function of mass and age to changes in the initial conditions and input physics. We then check to see if the distribution of initial conditions for a given physical scenario is consistent for open clusters of different ages. The behavior of the rapid rotators is highly sensitive to the saturation threshold for angular momentum loss (ωcrit), above which angular momentum loss scales linearly with the rotation rate. Very high values for ωcrit suppress rapid rotation prior to the main sequence, and very low values permit rapid rotation to survive for too long. For solid-body (SB) and differential rotation (DR) models, higher mass models rotate more rapidly than lower mass models for the same initial conditions and ωcrit. DR models differ from SB models in both the direct effect of core-envelope decoupling and a change in the calibration of the angular momentum loss law needed to reproduce the solar rotation at the age of the Sun; the effects of both are discussed. Slow rotation in young clusters can be achieved with modest disk lifetimes (3-10 Myr) for the DR models and longer disk lifetimes for the SB models (10 or more Myr). In addition, the slowly rotating DR models spin down during the early main sequence more than the slowly rotating SB models do. When compared with the cluster data, the observed mass dependence of the rapid rotator phenomenon can be reproduced only with a mass-dependent ωcrit for both the SB and DR models. A scaling of ωcrit inversely proportional to the convective overturn timescale can reproduce the observed mass-dependent spindown. The observed spindown of the slow rotators in the young open clusters is in better agreement with the DR than the SB models. We also discuss observational tests to distinguish different classes of models using low-mass stars and rotation periods in open clusters.

New Rotation Periods in the Pleiades: Interpreting Activity Indicators

We present results of photometric monitoring campaigns of G, K, and M dwarfs in the Pleiades carried out in 1994-1996. We have determined rotation periods for 18 stars in this cluster. In this paper we examine the validity of using observables such as X-ray activity and the amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds.

Rotational Velocities of Low-Mass Stars in the Pleiades and Hyades

We have obtained high-resolution spectra of 89 M dwarf members of the Pleiades and Hyades and have derived radial velocities, Hα equivalent widths, and spectroscopic rotational velocities for these stars. Typical masses of the newly observed Pleiades and Hyades stars are ~0.4 M⊙ and ~0.2 M⊙, respectively. We combine our new observations with previously published data to explore the rotational evolution of young stars with M ≤ 0.4 M⊙. The average rotation rate in the Hyades (age 600 Myr) is about 0.4 times that in the Pleiades (110 Myr), and the mean equivalent widths of Hα are also lower. As found in previous studies, the correlation between rotation and chromospheric activity is identical in both clusters, implying that the lower activity in the Hyades is a result of the lower rotation rates. We show that a simple scaling of the Pleiades rotational distribution for M ≤ 0.4 M⊙, corrected for the effects of structural evolution, matches that of the Hyades if the average angular momentum loss from the Pleiades to the Hyades age is factor of ≈6. This suggests that the distribution of initial angular momenta and disk-locking lifetimes for the lowest mass stars was similar in both clusters. We argue that this result provides further evidence for a saturation of the angular momentum loss rate at high rotational velocities.

Rotational Velocities and Chromospheric Activity of M Dwarfs in the Hyades

We have obtained high-resolution spectra of 52 K and M dwarf members of the Hyades open cluster. We have used these spectra to derive radial velocities, rotational velocities, and H? equivalent widths. Seven of the stars in our sample are double-lined spectroscopic binaries, and one is a spectroscopic triple. Of the apparently single stars, 13 have rotational velocities above our detection limit of 6 km s-1, with the maximum rotational velocity being about 25 km s-1. Given the relatively low mass for these stars, this modest spectroscopic rotational velocity of 25 km s-1 corresponds to a rotational period of order 17 hr, and thus the most rapidly rotating Hyades M dwarfs actually are quite rapid rotators. Somewhat surprisingly, the components of the spectroscopic binaries are, if anything, on average apparently more slowly rotating than the single stars. Forty-nine of the stars in our sample are M dwarfs. There is a good rotation-activity correlation for this sample, with stars with v sin i > 12 km s-1 having log (LX/LBol) ~ -3 and H? equivalent widths saturating at ~4 ?. There appear to be two branches of the rotation-activity relation as one approaches saturation, however?one set of stars approaches the saturation level at a very slow rotation rate (v sin i ? 6 km s-1), whereas the second set does not reach saturation until about v sin i = 12 km s-1. The components of the double-lined spectroscopic binaries are generally members of the first set since they are quite active but generally slowly rotating.

THE LITHIUM-ROTATION CORRELATION IN THE PLEIADES REVISITED

The dispersion in lithium abundance at fixed effective temperature in young cool stars like the Pleiades has proved a difficult challenge for stellar evolution theory. We propose that Li abundances relative to a mean temperature trend, rather than the absolute abundances, should be used to analyze the spread in abundance. We present evidence that the dispersion in Li equivalent widths at fixed color in cool single Pleiades stars is at least partially caused by stellar atmosphere effects (most likely departures from ionization predictions of model photospheres) rather than being completely explained by genuine abundance differences. We find that effective temperature estimates from different colors yield systematically different values for active stars. There is also a strong correlation between stellar activity and Li excess, but not a one-to-one mapping between unprojected stellar rotation (from photometric periods) and Li excess. Thus, it is unlikely that rotation is the main cause for the dispersion in the Li abundances. Finally, there is a significant correlation between detrended Li excess and potassium excess but not calcium—perhaps supporting incomplete radiative transfer calculations (and overionization effects in particular) as an important source of the Li scatter. Other mechanisms, such as very small metallicity variations and magnetic fields, which influence pre–main-sequence Li burning may also play a role. Finally, we find no statistical evidence for a decrease in dispersion in the coolest Pleiades stars, contrary to some previous work.

A Search for Photometric Rotation Periods in Low-Mass Stars and Brown Dwarfs in the Pleiades

We have photometrically monitored (Cousins IC) eight low-mass stars and brown dwarfs that are probable members of the Pleiades. We derived rotation periods for two of the stars—HHJ 409 and CFHT-PL 8—to be 0.258 and 0.401 days, respectively. The masses of these stars are near 0.4 and 0.08 M⊙, respectively; the latter is the second such object near the hydrogen-burning boundary for which a rotation period has been measured. We also observed HHJ 409 in V. The relative amplitude in the two bands shows that the spots in that star are about 200 K cooler than the stellar effective temperature of 3560 K and have a filling factor on the order of 13%. With one possible exception, the remaining stars in the sample do not show photometric variations larger than the mean error of measurement. We also examined the M9.5 V disk star 2MASS J0149090+295613, which had previously exhibited a strong flare event, but we did not detect any photometric variation.

Chandra, Extreme Ultraviolet Explorer, and Very Large Array Observations of the Active Binary System σ2 Coronae Borealis

We present the results of a coordinated observing campaign on the short-period RS CVn binary σ2 Coronae Borealis (F6V + G0V; Porb = 1.14 days) with the Very Large Array, the Extreme Ultraviolet Explorer, and the Chandra X-Ray Observatory High-Energy Transmission Grating Spectrometer. The radio emission is consistent with previously determined quiescent gyrosynchrotron properties. Multiple flares were seen with Extreme Ultraviolet Explorer, five occurring within two consecutive orbital periods. The first of these flares was observed with Chandra. The Chandra observations of σ2 CrB showed no systematic variations of line fluxes, widths, or Doppler shifts with orbital phase, nor any response in line width or offset due to the flare. This is consistent with both stars being equally active coronal emitters. We have developed a self-consistent method of spectral analysis to derive information from the line and continuum emissions concerning the distribution of plasma with temperature and elemental abundances. A bimodal temperature distribution is appropriate for both quiescent and flare intervals, with a stable peak at 6-8 MK and another variable enhancement at higher temperatures, with evidence for significant contribution from temperatures up to 50 MK during the flare, compared to 30 MK during quiescence. The iron abundance is subsolar during quiescence but is enriched by about a factor of 2 during a large flare seen with Chandra. The noble gas elements neon and argon show elevated abundances with respect to iron, but there is no clear evidence for any first ionization potential-based abundance pattern during quiescence or the flare. We have determined coronal electron densities from the helium-like ions O VII, Ne IX, Mg XI, and Si XIII, which imply densities ≥1010 cm-3. There is a small enhancement in the electron densities derived for the flare, but it is not statistically significant. We call attention to electron temperature constraints provided by the ratios of 1s2 1S0-1snp 1P1 transitions of the helium-like ions O VII, Ne IX, Mg XI, and Si XIII. The derived coronal electron pressures change by 1-2 orders of magnitude over a 25% change in temperature, implying nonisobaric coronal conditions. We find no evidence for significant departures from the effectively thin coronal assumption. The electron densities inferred from the soft X-ray spectra are inconsistent with cospatial gyrosynchrotron emission; further observations are necessary to discriminate the relative locations of the radio and soft X-ray-emitting plasma.

Observations of the Core of the Pleiades with theChandra X-Ray Observatory

We present results from a 36 ks observation of the core of the Pleiades open cluster using ACIS-I on the Chandra X-Ray Observatory. We have detected 57 sources, most of which do not have previously known optical counterparts. Follow-up photometry indicates that many of the detections are likely to be active galactic nuclei, in accordance with extragalactic source counts, but some of the sources may be previously undiscovered low-mass members of the Pleiades. We discuss our data set and our findings about X-ray emission from early-type stars, as well as very late-type stars. In particular, the large X-ray fluxes, lack of variability, and hardness ratios of the four Pleiades B6 IV–F4 V stars suggest a tentative conclusion that Pleiades stars in this spectral type range are intrinsic X-ray sources rather than previously unknown binaries in which the X-ray emission is from a late-type companion. Also, the sensitivity of Chandra allowed us to detect nonflare X-ray emission from late M stars.

A Search for Radio Emission at the Bottom of the Main Sequence and Beyond

We have used the VLA to conduct a deep search for 3.6 cm radio emission from nearby very low mass stars and brown dwarfs. The G?del-Benz relation is used to predict radio luminosities for some very low mass stars and candidate brown dwarfs with measured X-ray fluxes. The predicted radio fluxes are quite small, whereas the measured radio flux from the brown dwarf candidate GY 31 in the ? Oph cloud is relatively strong. In light of our new observations, this object remains an anomaly. We present upper limits for our measured radio fluxes at 3.6 cm for our targets.