45 research outputs found
Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy
Carbon monoxide (CO) is the most commonly used tracer of molecular gas in the
inner regions of protoplanetary disks. CO can be used to constrain the
excitation and structure of the circumstellar environment. Absorption line
spectroscopy provides an accurate assessment of a single line-of-sight through
the protoplanetary disk system, giving more straightforward estimates of column
densities and temperatures than CO and molecular hydrogen emission line
studies. We analyze new observations of ultraviolet CO absorption from the
Hubble Space Telescope along the sightlines to six classical T Tauri stars. Gas
velocities consistent with the stellar velocities, combined with the
moderate-to-high disk inclinations, argue against the absorbing CO gas
originating in a fast-moving disk wind. We conclude that the far-ultraviolet
observations provide a direct measure of the disk atmosphere or possibly a slow
disk wind. The CO absorption lines are reproduced by model spectra with column
densities in the range N(^{12}CO) ~ 10^{16} - 10^{18} cm^{-2} and N(^{13}CO) ~
10^{15} - 10^{17} cm^{-2}, rotational temperatures T_{rot}(CO) ~ 300 - 700 K,
and Doppler b-values, b ~ 0.5 - 1.5 km s^{-1}. We use these results to
constrain the line-of-sight density of the warm molecular gas (n_{CO} ~ 70 -
4000 cm^{-3}) and put these observations in context with protoplanetary disk
models.Comment: 12 pages, 14 figures, ApJ - accepte
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FUV Spectroscopy of Interstellar and Circumstellar Materials
I have designed, assembled, and calibrated a sounding rocket payload to perform high-resolution FUV spectroscopy. Debate lingers over the existence of a pervasive 1 million K gas in our region of the interstellar medium, the Local Bubble. If this hot gas were present, it would produce O VI upon interacting with the cold cavity wall. As such, O VI serves as a diagnostic for the hot interstellar gas. I designed the Diffuse Interstellar Cloud Experiment (DICE) to measure the O VI (1032,1038 Angstrom) doublet in absorption against two stars on either side of the Local Bubble wall. The instrument is functionally a Cassegrain telescope followed by a modified Rowland-mount spectrograph. With a holographically-ruled grating and a secondary magnifying optic, the spectrograph can achieve high resolving power (R=60,000) in a compact space.
In addition, I have analyzed Carbon Monoxide (CO) emission from Classical T-Tauri star disks using spectra from the HST-Cosmic Origins Spectrograph. The CO is photo-excited by Lyman alpha from the magnetospheric shock. I model this incident Lyman alpha radiation to use as input for a simple CO fluorescence model. Fits to the data constrain CO temperatures and column densities, which can then be compared with other known properties of the disks in our target sample. I find no correlation between this UV-emitting CO and the CO studied at infrared and sub-mm wavelengths between targets
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Direct Measurement of Interstellar Extinction toward Young Stars Using Atomic Hydrogen Lyα Absorption
Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Lyα emission profiles in young stars can be used to obtain the total H i column density (N(H i)) along the line of sight. We measure N(H i) with new and archival ultraviolet observations from the Hubble Space Telescope (HST) of 31 classical T Tauri and Herbig Ae/Be stars. The H i column densities range from log_10(N(H i)) ≈19.6–21.1, with corresponding visual extinctions of A_V =0.02–0.72 mag, assuming an RV of 3.1. We find that the majority of the H i absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new HST blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H i) values by ~0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry
White Light Demonstration of One Hundred Parts per Billion Irradiance Suppression in Air by New Starshade Occulters
A new mission concept for the direct imaging of exo-solar planets called the New Worlds Observer (NWO) has been proposed. The concept involves flying a meter-class space telescope in formation with a newly-conceived, specially-shaped, deployable star-occulting shade several meters across at a separation of some tens of thousands of kilometers. The telescope would make its observations from behind the starshade in a volume of high suppression of incident irradiance from the star around which planets orbit. The required level of irradiance suppression created by the starshade for an efficacious mission is of order 0.1 to 10 parts per billion in broadband light. This paper discusses the experimental setup developed to accurately measure the suppression ratio of irradiance produced at the null position behind candidate starshade forms to these levels. It also presents results of broadband measurements which demonstrated suppression levels of just under 100 parts per billion in air using the Sun as a light source. Analytical modeling of spatial irradiance distributions surrounding the null are presented and compared with photographs of irradiance captured in situ behind candidate starshades
Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
Carbon Monoxide is a commonly used IR/sub-mm tracer of gas in protoplanetary
disks. We present an analysis of ultraviolet CO emission in {HST}-COS spectra
for 12 Classical T Tauri stars. Several ro-vibrational bands of the CO A^1\Pi -
X^1\Sigma^+ (Fourth Positive) electronic transition system are spectrally
resolved from emission of other atoms and H_2. The CO A^1\Pi v'=14 state is
populated by absorption of Ly\alpha photons, created at the accretion column on
the stellar surface. For targets with strong CO emission, we model the Ly\alpha
radiation field as an input for a simple fluorescence model to estimate CO
rotational excitation temperatures and column densities. Typical column
densities range from N_{CO} = 10^{18} - 10^{19} cm^{-2}. Our measured
excitation temperatures are mostly below T_{CO} = 600 K, cooler than typical
M-band CO emission. These temperatures and the emission line widths imply that
the UV emission originates in a different population of CO than that which is
IR-emitting. We also find a significant correlation between CO emission and the
disk accretion rate M_{acc} and age. Our analysis shows that ultraviolet CO
emission can be a useful diagnostic of CTTS disk gas
The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
We exploit the high sensitivity and moderate spectral resolution of the
-Cosmic Origins Spectrograph to detect far-ultraviolet spectral features
of carbon monoxide (CO) present in the inner regions of protoplanetary disks
for the first time. We present spectra of the classical T Tauri stars HN Tau,
RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN
Tau shows CO bands in absorption against the accretion continuum. We measure a
CO column density and rotational excitation temperature of N(CO) = 2 +/- 1
10 cm and T_rot(CO) 500 +/- 200 K for the absorbing gas.
We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by
ultraviolet line photons, predominantly HI LyA. All three objects show emission
from CO bands at 1560 \AA, which may be excited by a combination
of UV photons and collisions with non-thermal electrons. In previous
observations these emission processes were not accounted for due to blending
with emission from the accretion shock, collisionally excited H, and
photo-excited H2; all of which appeared as a "continuum" whose components could
not be separated. The CO emission spectrum is strongly dependent upon the shape
of the incident stellar LyA emission profile. We find CO parameters in the
range: N(CO) 10 cm, T_{rot}(CO) > 300 K for the LyA-pumped
emission. We combine these results with recent work on photo- and
collisionally-excited H emission, concluding that the observations of
ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We
suggest that the CO/H2 ratio in the inner disk is ~1, a transition between the
much lower interstellar value and the higher value observed in solar system
comets today, a result that will require future observational and theoretical
study to confirm.Comment: 12 pages, 7 figures, 3 tables. ApJ - accepte
A Hubble Space Telescope Survey of H2 Emission in the Circumstellar Environments of Young Stars
The formation timescale and final architecture of exoplanetary systems are
closely related to the properties of the molecular disks from which they form.
Observations of the spatial distribution and lifetime of the molecular gas at
planet-forming radii (r < 10 AU) are important for understanding the formation
and evolution of exoplanetary systems. Towards this end, we present the largest
spectrally resolved survey of H2 emission around low-mass pre-main sequence
stars compiled to date. We use a combination of new and archival
far-ultraviolet spectra from the COS and STIS instruments on the Hubble Space
Telescope to sample 34 T Tauri stars (27 actively accreting CTTSs and 7
non-accreting WTTSs) with ages ranging from roughly 1-10 Myr. We observe
fluorescent H2 emission, excited by LyA photons, in 100 of the accreting
sources, including all of the transitional disks in our sample (CS Cha, DM Tau,
GM Aur, UX Tau A, LkCa15, HD 135344B and TW Hya). The spatial distribution of
the emitting gas is inferred from spectrally resolved H2 line profiles. Some of
the emitting gas is produced in outflowing material, but the majority of H2
emission appears to originate in a rotating disk. For the disk-dominated
targets, the H2 emission originates predominately at r < 3 AU. The emission
line-widths and inner molecular radii are found to be roughly consistent with
those measured from mid-IR CO spectra.Comment: ApJ - accepted. 19 pages, 12 figure