J. A. Thorburn

STUDIES OF THE DIFFUSE INTERSTELLAR BANDS. III. HD 183143

Echelle spectra of HD 183143 [B7Iae, E(B − V) = 1.27] were obtained on three nights, at a resolving power R = 38,000 and with a signal-to-noise ratio ≈ 1000 at 6400 A in the final, combined spectrum. A catalog is presented of 414 diffuse interstellar bands (DIBs) measured between 3900 and 8100 A in this spectrum. The central wavelengths, the widths (FWHM), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. Among the 414 bands, 135 (or 33%) were not reported in four previous, modern surveys of the DIBs in the spectra of various stars, including HD 183143. The principal result of this study is that the great majority of the bands in the catalog are very weak and fairly narrow. Typical equivalent widths amount to a few mA, and the bandwidths (FWHM) are most often near 0.7 A. No preferred wavenumber spacings among the 414 bands are identified which could provide clues to the identities of the large molecules thought to cause the DIBs. At generally comparable detection limits in both spectra, the population of DIBs observed toward HD 183143 is systematically redder, broader, and stronger than that seen toward HD 204827 (Paper II). In addition, interstellar lines of C2 molecules have not been detected toward HD 183143, while a very high value of N(C2)/E(B − V )i s observed toward HD 204827. Therefore, either the abundances of the large molecules presumed to give rise to the DIBs, or the physical conditions in the absorbing clouds, or both, must differ significantly between the two cases.

Lithium in the hyades I. New observations

Sixty-eight main-sequence members of the Hyades cluster with spectral types ranging from F7 to K1 V have been observed at Li I λ6707 to determine whether solar-like stars of a given mass, age, and initial composition have a unique surface lithium abundance Li/H. The data are compared with previous Li I observations in the Hyades, and similarities and differences are discussed. For the sample as a whole, the observed distribution of lithium abundances is inconsistent with a unique relationship between lithium abundance and mass. Furthermore, there is strong observational evidence for main-sequence lithium depletion when the data are compared with observations in younger clusters, even for masses that are not predicted to experience such depletion in standard stellar models

VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds

We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds, derived from spectra of seven SMC and 13 LMC stars obtained (mostly) with the VLT UVES. CH and/or CH+ have now been detected toward three SMC and nine LMC stars; CN is detected toward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly all the optical detections of these molecular species in interstellar media beyond the Milky Way. In the LMC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in four sight lines but is lower by factors of 2.5-4.0 in two others. In the SMC, the CH/H2 ratio is comparable to the local Galactic value in one sight line but is lower by factors of 10-15 in two others. The abundance of CH in the Magellanic Clouds thus appears to depend on local physical conditions and not just on metallicity. In both the SMC and the LMC, the observed relationships between the column density of CH and those of CN, CH+, Na I, and K I are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2 in these sight lines, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. The inferred temperatures range from about 45 to 90 K, the radiation fields range from about 1 to 900 times the typical local Galactic field, and the densities (in most cases) lie between 100 and 600 cm-3. Densities estimated from the observed N(CH), under the assumption that CH is produced via steady state gas-phase reactions, are considerably higher than those derived from H2. Much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH+. A significant fraction of the CH and CH+ in diffuse molecular material in the SMC and LMC may be produced in photon-dominated regions. The excitation temperature obtained from the populations of the two lowest CN rotational levels toward Sk -67 2 is quite consistent with the temperature of the cosmic microwave background radiation measured with COBE. Toward most of our targets, the UVES spectra also reveal absorption at velocities corresponding to the Magellanic Clouds ISM from several of the strongest of the diffuse interstellar bands (DIBs; at 5780, 5797, and 6284 A). On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those typically observed in Galactic sight lines with similar N(H I), presumably due to the lower metallicities and stronger radiation fields in the LMC and SMC. The three DIBs are also weaker (on average, but with some exceptions), by factors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in the Magellanic Clouds. The detection of several of the so-called C2 DIBs toward Sk 143 and Sk -67 2 with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or for all sight lines) in the Magellanic Clouds.

Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines*

Using UV spectra obtained with FUSE, HST, and/or IUE together with higher resolution optical spectra, we determine interstellar column densities of 12 CO, 13 CO, and/or C2 for 10 Galactic sight lines with E(BV ) ranging from 0.37 to 0.72. The N(CO)/N(H2) ratio varies over a factor of 100 in this sample, due primarily to differences in N(CO). For a given N(H2), published models of diffuse and translucent clouds predict less CO than is observed. The J ¼ 1Y3 rotational levels of 12 CO are subthermally populated in these sight lines, with Tex typically between 3 and 7 K. In general, there appears to be no significant difference between the excitation temperatures of 12 CO and 13 CO. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. WeobtainC2columndensitiesfrom theFYX(0Y0)bandat13418(threesight lines;J ¼ 0Y12), theFYX(1Y0)band at1314 8(one sight line;J ¼ 0Y12), theDYX(0Y0) band at 2313 8(four sight lines; J ¼ 0Y18), and the AYX(3Y0) and (2Y0) bands at 7719 and 8757 8 (seven sight lines; J ¼ 0Y12). Comparisons among those column densities yield a set of mutually consistent band f-values for the UV and optical C2 bands, but also reveal some apparent anomalies within the FYX (0Y0) band. Both the kinetic temperature Tk inferred from the C2 rotational populations and the excitation temperature T02(C2) are generally smaller than the corresponding T01(H2)—suggesting that C2 is concentratedincolder,densergasthanH2.IncorporatingadditionalcolumndensitydataforKi,HD,CH,C2,C3,CN, andCOfromtheliterature(foratotalsampleof 74sightlines),wefindthat(1)COismosttightlycorrelatedwithCN; (2) the ratios 12 CO/H2 and 13 CO/H2 both are fairly tightly correlated with the density indicator CN/CH (but C2/H2 is not);and(3)theratio 12 CO/ 13 COissomewhatanticorrelatedwithbothCN/CHandN(CO).Sightlineswith 12 CO/ 13 CO below the average local Galactic value of 12 C/ 13 C appear to sample colder, denser gas in which isotope exchange re- actions have enhanced 13 CO, relative to 12 CO. Subject headingg ISM: clouds — ISM: molecules — ultraviolet: ISM

Observations of C3 in Translucent Sight Lines

The A 1 � u X 1 � þ transition of the simplest polyatomic carbon chain molecule, C3, at 4051.6 Ahas been searched for toward reddened stars where abundant C2 had been reported and toward other stars with high color excess. Absorption from C3 has been detected toward 15 stars with color excess E(BV ) from 0.33 to 1.12. The observed C3 column densities, ranging from 10 12 to 10 13 cm � 2 , are well correlated with the corresponding C2 column densities, with NðC2Þ=NðC3Þ� 40, indicating their close chemical relation. The carbon-rich sight line toward HD 204827 (for which no previous C2 observation had been reported) has by far the highest C3 and C2 column densities. The chemistry of formation of C3 from C2 is discussed. A search for the next strongest 020-000 vibronic band was unsuccessful as a result of the low Franck-Condon factor and interference with a stellar line. Searches for C4 and C5 were negative. Subject headings: astrochemistry — ISM: lines and bands — ISM: molecules

Studies of the Diffuse Interstellar Bands. IV. The Nearly Perfect Correlation Between λλ6196.0 and 6613.6

In a sample of 114 diffuse cloud sightlines spanning a wide range of interstellar environments, we find the equivalent widths of the diffuse interstellar bands (DIBs) λ6196.0 and λ6613.6 to be extremely well correlated, with a correlation coefficient of 0.986. A maximum likelihood functional relationship analysis shows that the observations are consistent with a perfect correlation if the observational errors, which are dominated by continuum placement and other systematics such as interfering lines, have been underestimated by a factor of 2. The quality of this correlation far exceeds other previously studied correlations, such as that between the λ5780.5 DIB and either the color excess or the atomic hydrogen column density. The unusually tight correlation between these two DIBs would seem to suggest that they might represent the first pair of DIBs known to be due to the same molecular carrier. However, further theoretical work will be required to determine whether the different linewidths and band shapes of these two DIBs can be consistent with a common carrier. If the two DIBs do not in fact share the same molecular carrier, their two carriers must be chemically very closely related.

ARCES: an echelle spectrograph for the Astrophysical Research Consortium (ARC) 3.5m telescope

A new echelle spectrograph was commissioned in 1999 for the ARC 3.5 meter telescope. The key features of the instrument are that it has a resolution of 9 km/sec, limited by the pixel size of the CCD; has no moving parts behind the slit during observation; provides complete spectral coverage from 3200A to 10000A, limited by the prism cross disperser material on the blue side and by the CCD sensitivity on the red side; provides blazeless spectra; achieves S/N>3000; and is remotely operable. The instrument is being used for studies of abundances in stars and for a large survey of diffuse interstellar bands.

Rejection of the C7- Diffuse Interstellar Band Hypothesis

Using the new high-resolution (~8 km s-1) echelle spectrograph on the 3.5 m telescope at the Apache Point Observatory, we have begun a high-sensitivity survey of the diffuse interstellar bands (DIBs) in a large sample of reddened stars. Now that we are 2 years into this long-term survey, our sample includes over 20 reddened stars that show at least one of the DIBs that had been suggested to be caused by C, based on the gas-phase measurement of the C spectrum by J. P. Maiers group. The high-quality astronomical data from this larger sample of stars, along with the spectroscopic constants from the new laboratory work recently reported by Maiers group, have enabled us to examine more carefully the agreement between C and the DIBs. We find that none of the C bands match the DIBs in wavelength or expected profile. One of the DIBs (λ5748) attributed to C is actually a stellar line. The two strongest DIBs attributed to C (λ6270 and λ4964) are not correlated in strength, so they cannot share the same carrier. On the whole, we find no evidence supporting the hypothesis that C is a carrier of the DIBs.

ATOMIC AND MOLECULAR EMISSION LINES FROM THE RED RECTANGLE

HD 44179 is the binary, post-asymptotic giant branch, central star of the Red Rectangle Nebula. Echelle spectra of the star have been obtained over a wavelength range from about 3800 to 10000 A at a resolving power R = 38,000. A maximum S/N of 850 was achieved near 6800 A. Fifty-seven identified atomic or ionic emission lines of 12 elements are detected in the stars spectrum, along with 76 emission lines of CH, CH+, or CN. Three other CN lines are also present in absorption. Fewer than 30 of these 136 lines apparently have been previously reported, and the newly detected species include N II, Mg I, S II, K I, Fe I, Fe II, Rb I, Ba II, CH, and CN. On the basis of their shapes and widths, the line profiles of the various species can be classified into three groups: narrow, broad, or double-peaked. The emission apparently originates in an unusual, compact H II region and in associated neutral gas, both concentrated primarily within the small, dusty torus that optically obscures the central star. The detection of the Ba II emission from the gas may support the hypothesis of Waelkens et al. that HD 44179 is destined to become a barium star.

Hubble Space Telescope Beryllium Abundances in the α Centauri System

High signal-to-noise ratio Hubble Space Telescope Goddard High-Resolution Spectrograph spectra of α Centauri A (spectral type G2 V) and α Centauri B (spectral type K1 V) have been analyzed in the Be II λ3130 spectral region. Both stars offer an excellent opportunity for testing predictions of 9Be destruction since they are nearby, have a well-determined orbit and parallax, and thus have very well known physical parameters. A detailed spectrum synthesis has been made using a line list that has been carefully tested by comparison with the Sun and with metal-poor stars of different temperature and metallicity. Our analysis gives [Be/H] = +0.06 ± 0.09 dex for α Cen A and -0.54 ± 0.28 dex for α Cen B, using a model atmosphere with a metallicity [M/H] = +0.10. The implications of the new beryllium abundances and the previous lithium studies for models of stellar light-element depletion are then studied. Both the Sun and α Cen A are more highly depleted in lithium than younger stars; this is not consistent with standard stellar models. The Sun, α Cen A, and α Cen B have photospheric 9Be abundances lower than the current accepted solar meteoritic value. Because the initial 9Be abundance for the α Cen system is not known, and the depletion is much smaller than that for lithium, it is difficult to determine precise beryllium depletion factors for this system. However, because neither star is expected to deplete beryllium in standard models, the lower relative abundance of α Cen B might be evidence for main-sequence beryllium depletion. The theoretical implications of beryllium depletion are briefly discussed.