Analysis of HST/STIS absorption line spectra for Perseus Molecular Cloud Sightlines Authors: C. Church (Harvey Mudd College), B. Penprase (Pomona College), P. Wannier (JPL) Abstract We present new results from the STIS instrument on the Hubble Space Telescope relating to the Perseus star association and molecular cloud. Spectra from the stars HD and HD in the wavelength region between 1160 and 1550 Å were used to determine models of the kinematics and relative abundances of ions and atoms within the cloud. The column densities, velocities, and line widths for several ionized and non-ionized species were obtained by fitting absorption lines present in the spectra to a Voigt profile with an interactive fitting program. The resulting column densities were used to determine abundances and depletions within the clouds. The Perseus Region Both HD and HD24190 are believed to be members of the Perseus OB2 star forming region, as distance estimates and radial velocity measurements place them in the vicinity of the OB2 association. (8,4) The interstellar medium toward the Perseus OB2 association has been described as ``patchy'' and and is thought to be distributed at various distances. The distance to the early-type members of the association has been estimated at 320 ± 30 pc, while the distance to the late-type members has been estimated at 280 ± 20 pc. The mean radial velocity of the association is estimated to be 20.1 km s -1. (4) The Perseus OB2 dark cloud complex is located at 260 ± 20 pc and extends over 80 pc. (7) Two cloud complexes traced by Ca II absorption have been found at 7 and 13 km s -1 with the component at 7 km s -1 linked to a cloud stripe extending from the Barnard B1 and the 13 km s -1 component linked to the open galactic cluster IC348. Column Densities Component A was determined to have a velocity of 11.5 ± 0.5 km/s, and component B was determined to have a velocity of 15.3 ± 0.5 km/s. The standard deviation of the velocities of S I was on the order of the standard deviation of the velocities for all different species for both components, implying that the velocity dispersion is tracing nothing more exciting than an imperfect wavelength calibration. The column densities and b values for components A and B are displayed in Table 2. References Observations The spectra analyzed here are from observations taken as part of the General Observer program on the Hubble Space Telescope. The observations include 12 orbits awarded in Cycle 11 to Peter Wannier, Bryan Penprase, Ed Jenkins, Andersson and Federman, of which the present work is a subset. The data were acquired using the Space Telescope Imaging Spectrograph, or STIS instrument, aboard the Hubble Space Telescope. The high-resolution echelle gratings ECH-A and ECH-B, along with the small 0.1 to 0.03 arcsecond aperture were used to acquire the spectra. These spectra were taken with the Hubble STIS instrument, and spanned between 1160 and 1550 Å in wavelength. The above plots show a comparison of depletions with typical values for depletions in the diffuse medium (n H ~ 10 cm 3 ). The numbers on the horizontal axis correspond to elements as follows: 1, S; 2, O; 3, Kr; 4, Mg; 5, Mn; 6, Ge; 7, Ni; 8, P; 9, Cu; 10, Ga. The depletions seem to follow the pattern seen for the cold and warm diffuse medium with the possible exception of sulfur. Sulfur is 2.5 orders of magnitude more depleted than it would be expected to be in the cold and warm diffuse medium, when S I is assumed to be the dominant state. If S II is assumed to be part of the medium traced by the other species, a depletion of -0.8 ± 0.2 and -1.1 ± 0.3 are obtained for components A and B in the HD spectrum,respectively, and -0.7 ± 0.5 and -0.8 ± 0.6 for components A and B in the HD spectrum, respectively. Components A and B in table 2 are thought to trace cold, dense molecular gas. S II lines reveal a more complex velocity structure which may be tracing warmer, perhaps even slightly ionized gas. The velocity structure in S II was not found to be the same for both stars. The figures above show the Voigt profile fits for absorption lines in the spectra of HD and HD For species where more than one absorption line was present, an average of the fits for the different lines was plotted over the strongest absorption line. The depletions for different species are exhibited in Table 4. Abundances were assumed to be solar and were taken from Anders and Grevesse 1989 and Grevesse and Noels 1993; O abundances were taken from Holweger Krypton was assumed to be undepleted and to exist at solar levels and was used to determine the hydrogen column density for each component. (2, 3) The species listed in Table 2 were assumed to be the dominant form in the gas phase. Abundances and Depletions