Patrick Hartigan

Mass-loss rates, ionization fractions, shock velocities, and magnetic fields of stellar jets

In this paper we calculate emission-line ratios from a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields. The models cover the initial conditions relevant to stellar jets, and we show how to estimate the ionization fractions and shock velocities in jets directly from observations of the strong emission lines in these flows. The ionization fractions in the HH 34, HH 47, and HH 111 jets are approximately 2%, considerably smaller than previous estimates, and the shock velocities are approximately 30 km/s. For each jet the ionization fractions were found from five different line ratios, and the estimates agree to within a factor of approximately 2. The scatter in the estimates of the shock velocities is also small (+/- 4 km/s). The low ionization fractions of stellar jets imply that the observed electron densities are much lower than the total densities, so the mass-loss rates in these flows are correspondingly higher (approximately greater than 2 x 10(exp -7) solar mass/yr). The mass-loss rates in jets are a significant fraction (1%-10%) of the disk accretion rates onto young stellar objects that drive the outflows. The momentum and energy supplied by the visible portion of a typical stellar jet are sufficient to drive a weak molecular outflow. Magnetic fields in stellar jets are difficult to measure because the line ratios from a radiative shock with a magnetic field resemble those of a lower velocity shock without a field. The observed line fluxes can in principle indicate the strength of the field if the geometry of the shocks in the jet is well known.

Balmer-dominated Spectra of Nonradiative Shocks in the Cygnus Loop, RCW 86, and Tycho Supernova Remnants

We present an observational and theoretical study of the optical emission from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86, and Tycho. The spectra of these shocks are dominated by collisionally excited hydrogen Balmer lines, which have both a broad component caused by proton-neutral charge exchange and a narrow component caused by excitation of cold neutrals entering the shock. In each remnant, we have obtained the broad-to-narrow flux ratios of the H? and H? lines and measured the H? broad component width. A new numerical shock code computes the broad and narrow Balmer line emission from nonradiative shocks in partially neutral gas. The Balmer line fluxes are sensitive to Lyman line trapping and the degree of electron-proton temperature equilibration. The code calculates the density, temperature, and size of the postshock ionization layer and uses a Monte Carlo simulation to compute narrow Balmer line enhancement from Lyman line trapping. The initial fraction of the shock energy allocated to the electrons and protons (the equilibration) is a free parameter. Our models show that variations in electron-proton temperature equilibration and Lyman line trapping can reproduce the observed range of broad-to-narrow ratios. The results give 80%-100% equilibration in nonradiative portions of the northeast Cygnus Loop (vS ~ 300 km s-1), 40%-50% equilibration in nonradiative portions of RCW 86 (vS ~ 600 km s-1), and 20% equilibration in Tycho (vS ~ 2000 km s-1). Our results suggest an inverse correlation between magnetosonic Mach number and equilibration in the observed remnants.

A Spectroscopic survey of subarcsecond binaries in the Taurus-Auriga dark cloud with the Hubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low- and medium-resolution Space Telescope Imaging Spectrograph spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission-line luminosities for each star in each pair. We examine the ability of IR color excesses, Hα equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one classical T Tauri star (CTTS) and one weak-lined T Tauri star (WTTS); the CTTS is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primarys disk. Mass accretion rates of primaries and secondaries are strongly correlated, and Hα luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do and are typically slightly older than their secondaries according to three different sets of modern pre-main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking: the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks. This discrepancy can be resolved in several ways, among them a 10% closer distance for the Taurus-Auriga dark cloud.

Are wide pre-main-sequence binaries coeval?

We have observed a sample of 39 wide (projected separations 400-6000 AU) pre-main-sequence binary pairs spectroscopically and with optical and near-infrared images. The observations enable us to place 26 of the pairs in an H-R diagram and to determine masses and ages of the primary and secondary according to three sets of pre-main-sequence evolutionary tracks. In two-thirds of the cases the primary and secondary lie along the same isochrone to within the observational errors. However, real age differences appear for about one-third of our sample pairs-there is no set of nonintersecting theoretical isochrones that can make the primary and secondary have the same age for all pairs in our sample. In the cases where there are significant age differences between the component stars, the less massive star is usually younger than the more massive star. There is no correlation of the age differences with the presence or absence of accretion disks around the young stars. Hence, while disk accretion may affect the evolutionary tracks of the pre-main-sequence stars H-R diagram, we see no clear evidence of this effect among the pairs in our sample. The age differences also do not depend systematically on the apparent separation, the mass ratio, or the ages of stars.

The stellar population of the Lupus clouds

We present photometric and spectroscopic observations of the H alpha emission stars in the Lupus dark cloud complex. We estimate the effective temperatures of the stars from their spectral types and calculate the reddening towards each object from the (R-I) colors. From these data, we derive mass and age distributions for the Lupus stars using a new set of pre-main sequence evolutionar tracks. We compare the results for the Lupus stars with those for a similar population of young stellar objects in Taurus-Auriga and Chamaeleon and with the initial mass function for field stars in the solar neighborhood. From the H-R diagrams, Lupus appears to contain older stars than Taurus. The Lupus dark clouds form a greater proportion of low mass stars than the Taurus complex. Also, the proportion of low mass stars in Lupus is higher than that predicted by the Miller-Scalo initial mass function, and the lowest mass stars in Lupus are less active than similar T Tauri stars in other regions.

Optical excess emission in T Tauri stars

We present a set of simultaneous high-resolution spectroscopic and spectrophotometric observations of 22 K7-M1 T Tauri stars in the Taurus-Auriga dark cloud. Analysis of the high-resolution data makes it possible to separate the optical excess (veiling) emission from the photospheric fluxes in these stars. The amount of optical excess emission at 5500 A ranges from undetectable (≤10% of the photosphere) to as much as 10 times the photospheric flux in the most extreme objects

Hubble Space Telescope Images of the HH 34 Jet and Bow Shock: Structure and Proper Motions

We present new, deep Hα and [S II] images of the HH 34 jet and bow shock obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST), which reveal the structure of this fine HH flow with unprecedented detail. Many of the knots in the jet appear to form small working surfaces with bright [S II] cores and thin Hα filaments where the mini–bow shocks extend into the surrounding medium. In combination with earlier, short-exposure HST images we have determined very precise proper-motion vectors for the various shock structures in the outflow. The jet becomes visible within about an arcsecond of the source, where a new knot has emerged between our two epoch images; it has a space velocity of at least 300 km s-1, as derived from the proper motions and correcting for the 30° angle of the flow to the line of sight. The jet rapidly slows down to a mean space velocity of about 220 km s-1, with a standard deviation of 20 km s-1 among the jet knots. Such low internal velocities lead to weak shocks, consistent with the high [S II]/Hα ratio along the jet body and in accordance with the internal working surface model for jets. The jet motion appears to be ballistic, with no evidence for a turbulent boundary layer. The jet is well resolved and steadily expands with a half-opening angle of 04. The large HH 34 working surface shows a multitude of knots, all of which are enveloped by a series of very thin, limb-brightened Hα-emitting filaments immediately behind the shock front where the flow faces into the preshock medium. One of these filaments developed four regularly spaced tiny knots between the two epochs, possibly due to a Rayleigh-Taylor instability along the filament or caused by the presence of small, dense clumps in the ambient medium. Proper motions of the HH 34 working surface show an obvious expansion due to material being squirted sideways. In addition to the large-scale S-shaped symmetry of the giant HH 34 flow, the jet shows a marked and surprisingly abrupt change in flow direction during a 65 yr interval that ended 10 yr ago, suggesting that the jet-disk system may have been influenced by powerful tidal effects by a companion star during a recent periastron passage. A second, smaller bowlike flow, called HH 534, possibly emanates from the HH 34 source region, and if so this supports the contention that the source is a binary. This data set is a testament to the unique abilities of the HST to follow morphological, photometric, and excitation changes on cooling timescales in the shocks of flows from young stars.

Proper Motions of the HH 111 Jet Observed with the Hubble Space Telescope

New Hα and [S II] images of the HH 111 jet taken with the Hubble Space Telescope reveal marked proper motions and morphological changes when compared with similar images obtained 4 years earlier. Knots in the jet, which are dominated by emission from nested bow shocks, generally move ballistically, with no evidence for turbulent motions even in regions where the emission has a complex morphology. These bow shocks sometimes overtake one another; the new images show this occurred in knot L about 80 years ago. Photometric variability, clearly visible for the first time at subarcsecond scales, can confuse ground-based measurements that require many years between epochs to detect reliable proper motions. With the exception of the bow shock L, whose wings expand laterally, the jet moves mainly along its long axis. Because HH 111 lies nearly in the plane of the sky, the proper motions translate accurately to space velocities, which range from 220 to 330 km s-1 with a typical uncertainty of ±5 km s-1. The fastest knots are associated with object E at the base of the visible jet, where a cooling layer is in the process of forming behind one of the shocks. Velocity differences between adjacent knots within the optically bright part of the jet are typically 40 km s-1, in line with predictions of nonmagnetic shock models based on emission-line fluxes. This agreement limits the component of the magnetic field perpendicular to the axis of the jet to be 1 mG.

GOING SLITLESS: IMAGES OF FORBIDDEN-LINE EMISSION REGIONS OF CLASSICAL T TAURI STARS OBSERVED WITH THE HUBBLE SPACE TELESCOPE

We have observed five classical T Tauri stars known to have strong forbidden-line emission with STIS in slitless mode on the Hubble Space Telescope. This technique makes it possible to image jets within a few tens of AU of their exciting sources, a region of great interest for models of accretion disks and jets. Slitless images generate emission-line images at all wavelengths, including those where no narrowband filters exist. Images of the forbidden-line regions around each object, constructed by subtracting the stellar continuum and combining observations taken at different orientations, show [O I] jets from CW Tau, HN Tau, UZ Tau E, DF Tau, and the primary of DD Tau. Jets exist on both sides of the close binary DF Tau, either as a jet and its counterjet or as separate jets from the primary and secondary. Several emission lines not previously seen in jets close to the star exist in the HN Tau jet; the [Fe II] λ7155/λ8617 ratio is particularly useful because it measures the electron density in the densest regions of stellar jets, where log Ne 6. Electron densities in the inner 30 AU of the HN Tau jet range from log Ne = 6.2 to 6.9. We construct diagnostic diagrams for the density, temperature, and ionization fraction in jets close to their stars, using various emission lines of O I and O II. The red auroral [O II] lines are bright close to HN Tau, indicating that the emitting regions of the inner 35 AU of the jet have a substantial ionized component—20% if the emission comes from a shock and 50% for an isothermal flow. We discuss mass-loss rates and filling factors for these two cases. The intensity of the HN Tau jet in [O I] λ6300 declines exponentially with distance beyond ~15 AU. The superior continuum subtraction with slitless data, as compared with narrowband images, makes it possible to resolve the widths of jets at distances as close as ~15 AU from the star. The two best examples, HN Tau and UZ Tau E, have jets that expand with distance. When projected back to the source, the width of the jet in HN Tau is a few AU at the 3 σ level, while the jet in UZ Tau E is spatially unresolved. The new images of CW Tau reveal proper motions in this jet, which has ejected at least two knots since 1980. There is no indication that CW Tau brightened when it ejected the largest of these knots, but the photometric record of this star over the last two decades is fragmentary.

Collimation, Proper Motions, and Physical Conditions in the HH 30 Jet from Hubble Space Telescope Slitless Spectroscopy

We present STIS spectral images of the HH 30 stellar jet taken through a wide slit over two epochs. The jet is unresolved spectrally, so the observations produce emission-line images for each line in the spectrum. This rich data set shows how physical conditions in the jet vary with distance and time, produces precise proper motions of knots within the jet, resolves the jet width close to the star, and gives a spectrum of the reflected light from the disk over a large wavelength range at several positions. We introduce a new method for analyzing a set of line ratios based on minimizing a quadratic form between models and data. The method generates images of the density, temperature, and ionization fraction computed using all the possible line ratios appropriately weighted. In HH 30, the density declines with distance from the source in a manner consistent with an expanding flow and is larger by a factor of 2 along the axis of the jet than it is at the periphery. Ionization in the jet ranges from ~5% to 40%, and high-ionization/excitation knots form at about 100 AU from the star and propagate outward with the flow. These high-excitation knots are not accompanied by corresponding increases in the density, so if formed by velocity variations the knots must have a strong internal magnetic pressure to smooth out density increases while lengthening recombination times.