Suzanne L. Hawley

Absolute Proper Motions to B ~ 22.5: Large-Scale Streaming Motions and the Structure and Origin of the Galactic Halo

With an enlarged sample of radial velocities, we explore the phase-space and metallicity distributions for a sample of dwarf stars with distances up to 8 kpc in a deep north Galactic pole proper motion survey. We find that the halo stars in the sample are concentrated into clumps in the combined phase-space and metallicity distribution, one of the most prominent clumps representing a retrograde, predominantly metal-poor ([Fe/H] < -0.8) stream moving toward the Galactic plane. These new data reveal that (1) the halo is not dynamically mixed but presently contains a significant fraction of stars with membership in correlated stellar streams, (2) phase-space substructure might account for differences in halo kinematics derived among surveys along different lines of sight, and (3) a significant fraction of the halo field star population may be derived from the accretion of stellar agglomerations (e.g., star clusters, satellite galaxies, or Searle & Zinn fragments).

Tidal Disruption and Tails from the Carina Dwarf Spheroidal Galaxy

New photometry of regions beyond the classical tidal radius of Carina exposes a Carina-like stellar distribution that is about 1% of the central surface density and which extends at least as far as 2° (3.5 kpc) from Carinas center. The detections of a spatially extended RR Lyrae distribution, and a significant Carina-like stellar population at large central distances confirm predictions of the time-dependent tidal interaction model and suggest that Carina is not in virial equilibrium.

Brown Dwarfs in the Hyades and Beyond

We have used both the Low-Resolution Imaging Spectrograph and the High Resolution Echelle Spectrograph on the Keck telescopes to obtain spectra of 12 candidate members of the Hyades identified by Leggett & Hawkins. All of the objects are chromospherically active, late-type M dwarfs, with Hα equivalent widths varying from 1 to 30 A. Based on our measured radial velocities, the level of stellar activity, and other spectroscopic features, only one of the 12 stars has properties consistent with cluster membership. We consider how this result affects estimates of the luminosity and mass function of the Hyades. Five of the 11 field stars have weak K I λλ7665, 7699 and CaH absorption as compared with M dwarf standards of the same spectral type, suggesting a lower surface gravity. Two of these sources, LH 0416+14 and LH 0419+15, exhibit significant lithium 6708 A absorption. Based partly on parallax measurements by the US Naval Observatory (Harris et al.), we identify all five as likely to be young, pre–main-sequence objects in or near the Taurus-Auriga association at distances of between 150 and 250 pc. A comparison with theoretical models of pre–main-sequence stars indicates masses of less than 0.05 M⊙.

Hamilton Echelle Spectroscopy of the 1993 March 6 Solar Flare

We report on a successful program that used the Hamilton echelle spectrograph and the coud{acute e} auxiliary telescope at Lick Observatory to take spectra of solar flares. Our observations consist of high-resolution ({lambda}/{delta}{lambda}{approximately}48,000) spectra covering the entire optical region from approximately 3800 to 9000 {Angstrom} in each exposure. These are the first time-resolved high-resolution optical spectra of this type obtained for a solar flare. On 1993 March 6 we observed a relatively large ({ital GOES} class M7.7) solar flare event. Our sequence of observations began before flare maximum and continued for more than 1 hr. We present our high signal-to-noise spectra and compare them with similar stellar flare observations. We find that the hydrogen-emitting layers in flares on the Sun differ markedly from those in flares on dMe stars, though the total energy emitted in various emission lines can be rather similar. We also find that the amount of energy released in the optical emission lines is similar to that emitted in soft X-rays. We find evidence for Stark broadening in the Balmer lines for members lower in the series than reported in earlier studies. This appears to have occurred because the optical depth in the Balmer lines is lowermorexa0» than in previously reported flares. Early in the flare, the H{alpha} and H{beta} line profiles appear to be asymmetric as a result of {ital absorption} by chromospheric material expanding upward into the corona. We also examine solar flare model atmospheres synthesized with the non-LTE code MULTI and find that our observations can be generally understood in terms of equilibrium models of electron-beam- and X-ray-heated chromospheres in equilibrium with coronal loops in which the pressure is rather high; however, there remain marked differences between the theoretical predictions and our observations, implying that substantial refinement of the models is in order. Several photospheric lines show flare enhancements as well. The temporal behavior of these line enhancements is identical to that of the chromospheric lines, but there is no indication that significant flare heating penetrates to continuum formation depths. {copyright} {ital 1997} {ital The American Astronomical Society}«xa0less

Extreme Ultraviolet Explorer Spectra of the 1993 March Flares on AD Leonis: The Differential Emission Measure and Implications for Coronal Structure

The flare star AD Leonis was observed by the Extreme Ultraviolet Explorer (EUVE) from 1993 March 1 to 3 UT. Two flares were detected by the EUVE Deep Survey detector and spectrometer and also seen in optical photometry on 1993 March 2 UT. The DS Lexan/boron-band and optical results have been discussed in the previous paper by Hawley et al. In this paper, we describe the spectra observed by EUVE during quiescence, the peaks of the flares, and the decay phase following the first flare and analyze the spectra to investigate the stellar atmospheric structure during these time periods. The spectra show that the observed EUV emission from AD Leo is dominated by iron lines from a hot coronal plasma. Two methods were used to estimate the differential emission measure distribution (DEM) of the stellar corona. In the Pottasch method, we fitted Gaussian line profiles to the strongest lines in the spectra and estimated the DEM at the formation temperature of those lines. Upper limits to the DEM were obtained in the case of no detection. We also used a regularized inversion technique, together with a weighting scheme based on information contained in the plasma-emission model and on the signal-to-noise ratio of the data, to find the DEM. The weighting was designed to prevent the noisy pixels in our low-signal-to-noise ratio data from dominating the solution. The results produced by the two methods are consistent in the temperature regimes where strong lines are present. The inversion method provides additional information where no strong single lines dominate the spectra. The ability to use lines from the entire wavelength region covered by the spectra allowed us to investigate the hydrogen column NH and iron abundance [Fe/H]. We found that [Fe/H] in the corona of AD Leo was essentially unconstrained by our data, but NH was well determined, yielding NH ~ (3 ± 1) × 1018 cm-2. We assumed both a solar-coronal value of [Fe/H] and a value one tenth of this and computed the DEM distribution of the stellar corona for both cases. The DEM of the quiescent corona is dominated by a broad plateau of emission ranging from 106.8 to 107.2 K, with the DEM of plasma near 106.2 K about an order of magnitude less. We interpret the plateau of the DEM in terms of a broad distribution of loops with differing peak temperatures. We discuss and compare these results with those of Giampapa et al., who analyzed ROSAT soft X-ray data from AD Leo taken during a different time period. The DEM of the flare plasma is strongly peaked at temperatures greater than 107 K, indicative of hot flare loops, while that of the decay phase consists of a smaller peak at temperatures less than 107 K, as might be expected from the cooling and condensation of previously heated flare loops. These results are consistent with a flare model that includes strong evaporation and condensation as in our previous paper. The EUVE spectral analysis leads to lower peak flare temperatures than those used in our previous paper, but the basic conclusion reached—that the dominant flaring emission originates from long loops with L ~ R* and with peak flare densities ranging from 109 to 1011 cm-3—remains unchanged. This conclusion is not qualitatively affected by the value of [Fe/H] used in our DEM analysis.

Continued Analysis of EUVE and Optical Observations of a Flare on AD Leonis

The flare star AD Leo (dM3.5e, 4.9 pc) was observed by EUVE from 1993 March 1-March 3 UT. A flare was detected by the EUVE DS/S and seen in optical photometry on 1993 March 2 UT. We summarize an analysis of the flare’s physical parameters, and present differential emission measure (DEM) curves calculated for the quiescent, flare peak and flare decay phases of the observation.

The stellar luminosity function

The stellar luminosity function, Φ(M) — the number of stars per unit absolute magnitude per unit volume is one of the fundamental quantities required for understanding star formation and investigating the structure of our Galaxy. In recent years, most emphasis has been placed on determining the shape of Φ(M) at faint magnitude — and hence the mass spectrum, with the ultimate goal of assessing the likely dark-matter contribution of VLM stars and brown dwarfs. However, it was Galactic structure analyses, rather than an interest in the relative number of luminous and faint stars per se, which prompted the first derivation of the luminosity function.

The mass function

The number of stars per unit mass — the stellar mass function — describes how a molecular cloud redistributes its material to form stars. This parameter is also a necessary ingredient for determining the mass distribution and the total stellar mass in star clusters and external galaxies. It is therefore fundamental to both star-formation theory and Galactic structure.

A Galactic structure primer

The first chapters of this book consider M dwarfs as individual stars, and study the underlying astrophysics through observations of their atmospheric properties. Low-mass stars are also potentially highly effective as probes of Galactic structure, since they are the most populous constituent of each stellar population. Until recently, their utility in this role has been restricted both by difficulties in interpreting their emergent spectra in terms of luminosities, temperatures and abundances, and by their low intrinsic luminosities, which limit observations to relatively nearby stars. However, with the development of the atmosphere models described in Chapter 4, together with substantial improvements in instrumental efficiency and the consequent availability of extensive, statistically well-defined samples, it is now becoming possible to better exploit their potential.

The structure, formation and evolution of low-mass stars and brown dwarfs

The development of a self-consistent theoretical description of the internal structure of stars, and the consequent construction of models which trace evolutionary behaviour, represents one of the major achievements of twentieth-century astrophysics. Most studies have centred on intermediate- and high-mass stars, with little consideration of M dwarfs with masses below 0.6 M⊙. In part, this reflects the availability of more precise observational constraints, and also the greater analytic tractability in modelling higher-mass stars. Recently, however, the lower main sequence has attracted more attention, with a series of detailed models extending past the hydrogen-burning limit to the boundary between low-mass brown dwarfs and giant planets.