Assigned ID: 1065
----- Attempting Submission 1 (Sat Feb 25 00:53:51 GMT 2012) -----
HST Phase I Proposal 1065 (mcmol20v1.apt) successfully submitted.----- Attempting Submission 1 (Fri Jul 06 15:04:32 GMT 2012) -----Properties of Diffuse Molecular Gas in the Magellanic CloudsStudies of the interstellar medium in the lower-metallicity Magellanic Clouds explore somewhat different environmental conditions from those typically probed in our own Galactic ISM. Recent studies based on optical/UV spectra of SMC and LMC targets, for example, have revealed unexpected differences in gas-phase abundance patterns (for various atomic and molecular species) and have begun to explore the effects of differences in metallicity on the atomic-to-molecular transition and resulting molecular fraction f(H_2) -- a key aspect in the formation of molecular clouds. We propose a more detailed study of the abundances, depletions, and local physical conditions characterizing diffuse molecular material in the Magellanic Clouds, using STIS E140H and E230M spectra of two sight lines with N(H_2) > 10^20 cm^-2 (both probing the outskirts of molecular clouds seen in CO emission). The two STIS settings will include lines from various neutral and ionized species (with a range in depletion behavior), several C I multiplets, and several bands of CO and C_2. By probing and characterizing the atomic-to-molecular transition in the Magellanic Clouds, we will address key issues regarding the effects of differences in metallicity on the relationship between the atomic and molecular gas in galaxies; on cloud structure, physical conditions, and diffuse cloud chemistry; and on the composition and properties of interstellar dust. The results of this project should thus aid in the interpretation of observations of atomic and molecular material in more distant low-metallicity systems.We propose to obtain high-to-moderate resolution (~2.7-10 km/s), moderate-to-high
S/N (~25-70) STIS spectra of interstellar absorption lines from a variety of atomic and
molecular species toward 2 moderately reddened LMC and SMC stars with appreciable
N(H_2).
Targets:
The targets are the UV-brightest stars with measured N(H_2) > 10^{20} cm^{-2} in
each of the SMC and LMC; both lie near molecular material seen in CO emission.
Sk18 lies near the prominent molecular and star-forming regions (strong radiation
field?) in the SW part of the main SMC bar.
Sk-68d73 lies within the edge of a CO complex, north of the center of the LMC, and
shows CO absorption in FUSE spectra.
Both have been observed with VLT/UVES (4.5-5.0 km/s; very high S/N) and FUSE;
both have high resolution (1.2-2.0 km/s) Na~I spectra.
Star & Flux & MC & MC & Location & Other Data
& (1500/2050) &E(B-V)& N(H2)& &
Sk18 & 6.0e-13/5.0e-13 & 0.14 & 20.6 & SMC - SW bar & FUSE, opt
Sk-68d73& 4.0e-13/3.0e-13 & 0.34 & 20.1 & LMC - central & FUSE, HST, opt
Wavelength settings:
We will observe both stars with STIS using the E140H/1307 and E230M/1978
settings, which include numerous lines from both neutral and singly ionized species
of both depleted and undepleted elements, as well as several bands of CO and C_2.
Several C I multiplets (important for estimating local thermal pressures and densities)
will be included. The high resolution of the E140H spectra is needed to discern the
complex component structure known from the existing optical spectra and to resolve
the CO rotational structure (though the optical data for CH suggest that only a single
component for CO will be detectable in each sight line). While higher resolution would
be desirable as well for the near-UV spectra, the NUV-MAMA dark current makes
observations with E230H difficult for these faint targets. We have elected to use
STIS/E230M instead of COS, to enable somewhat higher resolution and broad
wavelength coverage in a single setting. Many of the lines will be detectable, but not
saturated -- so that reliable column densities can be derived via simultaneous,
multi-component fits to the line profiles. We will use the profiles in the higher
resolution E140H spectra -- together with higher-resolution, higher-S/N optical
spectra -- to define the component structures for the lines in the lower resolution
(but higher S/N) E230M spectra. While the Si II 1808 line may be strong, use of
component information from weaker lines (e.g., Mg II 1240) should still enable us
to discern deviations from Galactic depletion patterns. Only HST can cover these
UV lines.
Grating & Setting & Range & Species
E140H & 1307 & 1206-1408 & CO, C_2, C~I, C~II, O~I, Mg~II, S~II, P~II,
& & & Cl~I, S~I, Cu~II, Ni~II, Si~II, Ge~II
E230M & 1978 & 1574-2382 & C~I, C_2, Mg~I, Si~I, S~I, Zn~II, Al~II, Al~III,
& & & Si~II, Ti~II, Cr~II, Fe~II, Ni~II
Exposure times:
For both stars, we aim to achieve a S/N of 25-30 per resolution element in the
extracted E140H spectra near 1300\AA\ (comparable to the values obtained for
our previous STIS echelle spectra of MC stars), and S/N of 60-70 for the E230M
spectra near 2000 A. These S/N should enable detections (or interesting limits)
for several of the stronger CO A-X bands, as well as adequate definition of the line
profiles for the simultaneous fits. We will use the 0.2x0.2 arcsec slit for all
exposures, in order to maximize S/N.
We will take advantage of the one CVZ opportunity for Sk$-$68~73 to minimize
the total orbits needed.
We have used the on-line STIS exposure calculator, with IUE low-dispersion spectra,
for these estimates; none of the targets violates any STIS count rate limits.
Each visit (<=5 orbits) will have the following overheads:
6 min for initial guide star acquisition (4 min for re-acquisition in subsequent
non-CVZ orbits);
6 min for target acquisition (no peakup, with the 0.2x0.2 slit);
8 min for each new MAMA spectroscopic exposure (1 min for subsequent identical
exposures).
For Sk18, each orbit will have about 51 min visibility (reduced for scheduling
flexibility); for Sk-68d73 the CVZ orbits will have 96 min visibility, and the
regular orbits will have 60 min visibility.
The following table gives the plan for the 4 visits (19 orbits):
Star &Grating &Setting & Range&Resolution& Flux &Orbits &Exposure&S/N
& & & & (\kms) & & & (min)
Sk18 &E140H & 1307 & 1206-1408 & 2.7 & 6.0e-13 & 5 & 226 & 30
&E230M & 1978 & 1574-2382 & 10.0 & 5.0e-13 & 5 & 226 & 68
Sk-68d73 &E140H & 1307 & 1206-1408 & 2.7 & 4.0e-13 & 4CVZ & 346 & 26
&E230M & 1978 & 1574-2382 & 10.0 & 3.0e-13 & 5 & 269 & 60
Acquisition:
The stars have accurate coordinates, and are bright enough and isolated enough
that a standard STIS CCD point source acquisition should be sufficient, with the
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