We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, η Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate (Ṁ ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high density, low filling factor accretion spots co-exist with low density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near infrared. Comparing our estimates ofṀ to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for theṀ estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II] and Mg II) and accretion properties obtained simultaneously.

We present a far-ultraviolet (FUV) spectral atlas consisting of spectra of 91 pre-main sequence stars. Most stars in this sample were observed with the Space Telescope Imaging Spectrograph (STIS) and Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST ). A few archival spectra from International Ultraviolet Explorer (IUE ) and the Goddard High Resolution Spectrograph (GHRS) on the HST are included for completeness. We find strong correlations among the O I λ1304 triplet, the Si IV λλ1394/1403 doublet, the C IV λ1549 doublet, and the He II λ1640 line luminosities. For classical T Tauri stars (CTTSs), we also find strong correlations between these lines and the accretion luminosity, suggesting that these lines form in processes related to accretion. These FUV line fluxes and X-ray luminosity correlate loosely with large scatters. The FUV emission also correlates well with Hα, Hβ, and Ca II K line luminosities. These correlations between FUV and optical diagostics can be used to obtain rough estimates of FUV line fluxes from optical observations. Molecular hydrogen (H 2 ) emission is generally present in the spectra of actively accreting CTTSs but not the weak-lined T Tauri stars (WTTSs) that are not accreting. The presence of H 2 emission in the spectrum of HD 98800 N suggests that the disk should be classified as actively accreting rather than a debris disk.We discuss the importance of FUV radiation, including the hydrogen Lyα line, on the photoevaporation of exoplanet atmospheres. We find that the Ca II/C IV flux ratios for more evolved stars are lower than those for less evolved accretors, indicating preferential depletion of refactory metals into dust grains. (2003) published the PMS archive of IUE far-and near-UV spectra of 137 TTSs and 97 Herbig Ae/Be (HAeBe) stars, although only 50 of the TTSs were observed in the FUV. Together, this trilogy has laid out the foundation for our understanding of UV radiation fields from young stars, including the strength and the physical processes responsible for such emission. However, the IUE survey of FUV emission from young stars was limited to the FUV-brightest CTTSs and included only three WTTSs, of which the latest spectral type is K0. The survey is also limited by S/N, evident in the low detection rate of H 2 line emission (13/32 CTTSs, Valenti et al. 2000) despite subsequent observations showing that such emission is common to all CTTSs (Herczeg et al. 2006; Ingleby et al. 2009). Analysis of the location and kinematics of the gas that produces the FUV emission lines was also limited by the large aperture (∼ 10 ′′ × 20 ′′ ) and low spectral resolution (6Å) of the IUE SWP camera. Since the launch of the Hubble Space Telescope (HST ) 20 years ago, the Goddard High Resolution Spectrograph (GHRS), Space Telescope Imaging Spectrograph (STIS) and the Advanded Camera for Surveys (ACS) prisms have observed over 80 PMS stars. Analyses of small subsets of these observations (< 10 objects, and often only one) have been used to address specific issues, including ...

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ∼1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of H 2 to study the evolution of the gas in the inner disk. We find that the H 2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting H 2 in the disk, the fact that the non-accreting sources show no H 2 emission implies that the H 2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600Å, we estimate that the column density of H 2 left in the inner regions of the debris disks in our sample is less than ∼ 3 × 10 −6 g cm −2 , nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.

Two decades ago "transitional disks" described spectral energy distributions (SEDs) of T Tauri stars with small near-IR excesses, but significant mid-and far-IR excesses. Many inferred this indicated dust-free holes in disks, possibly cleared by planets. Recently, this term has been applied disparately to objects whose Spitzer SEDs diverge from the expectations for a typical full disk. Here we use irradiated accretion disk models to fit the SEDs of 15 such disks in NGC 2068 and IC 348. One group has a "dip" in infrared emission while the others' continuum emission decreases steadily at all wavelengths. We find that the former have an inner disk hole or gap at intermediate radii in the disk and we call these objects "transitional" and "pre-transitional" disks, respectively. For the latter group, we can fit these SEDs with full disk models and find that millimeter data are necessary to break the degeneracy between dust settling and disk mass. We suggest the term "transitional" only be applied to objects that display evidence for a radical change in the disk's radial structure. Using this definition, we find that transitional and pre-transitional disks tend to have lower mass accretion rates than full disks and that transitional disks have lower accretion rates than pre-transitional disks. These reduced accretion rates onto the star could be linked to forming planets. Future observations of transitional and pre-transitional disks will allow us to better quantify the signatures of planet formation in young disks.

There is much debate on how high-mass star formation varies with environment, and whether the sparsest star-forming environments are capable of forming massive stars. To address this issue, we have observed eight apparently isolated OB stars in the SMC using HST's Advanced Camera for Surveys. Five of these objects appear as isolated stars, two of which are confirmed to be runaways. The remaining three objects are found to exist in sparse clusters, with 10 companion stars revealed, having masses of 1 -4 M ⊙ . Stochastic effects dominate in these sparse clusters, so we perform Monte Carlo simulations to explore how our observations fit within the framework of empirical, galactic cluster properties. We generate clusters using a simplistic -2 power-law distribution for either the number of stars per cluster (N * ) or cluster mass (M cl ). These clusters are then populated with stars randomly chosen from a Kroupa IMF. We find that simulations with cluster lower-mass limits of M cl,lo ≥ 20M ⊙ and N * ,lo ≥ 40 match best with observations of SMC and Galactic OB star populations. We examine the mass ratio of the second-most massive and most massive stars m max,2 /m max , finding that our observations all exist below the 20th percentile of our simulated clusters. However, all of our observed clusters lie within the parameter space spanned by the simulated clusters, although some are in the lowest 5th percentile frequency. These results suggest that clusters are built stochastically by randomly sampling stars from a universal IMF with a fixed stellar upper-mass limit. In particular, we see no evidence to suggest a m max − M cl relation. Our results may be more consistent with core accretion models of star formation than with competitive accretion models, and they are inconsistent with the proposed steepening of the integrated galaxy IMF (IGIMF).