Space Telescope Imaging Spectrograph Observations of Interstellar ...

THE ASTROPHYSICAL JOURNAL, 562 : 394È399, 2001 November 20 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

SPACE TELESCOPE IMAGING SPECTROGRAPH OBSERVATIONS OF INTERSTELLAR OXYGEN AND KRYPTON IN TRANSLUCENT CLOUDS1 STEFAN I. B. CARTLEDGE, DAVID M. MEYER, AND J. T. LAUROESCH Department of Physics and Astronomy, Dearborn Observatory, 2131 Sheridan Road, Northwestern University, Evanston, IL 60208 ; cartledg=elvis.astro.nwu.edu, davemeyer=northwestern.edu, jtl=elvis.astro.nwu.edu

AND U. J. SOFIA Department of Astronomy, Whitman College, 345 Boyer Avenue, Walla Walla, WA 99362 ; soÐauj=whitman.edu Received 2001 April 26 ; accepted 2001 July 20

ABSTRACT We have obtained high-resolution Hubble Space T elescope (HST ) Space Telescope Imaging Spectrograph (STIS) observations of O I j1356 and Kr I j1236 absorption in 11 sight lines characterized by high extinction, large H I column densities, and/or long path lengths. Previous Goddard High Resolution Spectrograph (GHRS) measurements of these weak features in seven relatively nearby di†use clouds have shown no evidence for density-dependent depletion of either oxygen or krypton and have yielded a weighted mean gas-phase abundance ratio of log [N(O)/N(Kr)] \ 5.56 ^ 0.04. Our STIS GHRS measurements yield a lower weighted mean of log [N(O)/N(Kr)] \ 5.48 ; the di†erence is due primarily to several translucent sight lines in the STIS data set that STIS diverge from the GHRS value. These translucent cloud sight lines pass near dense, star-forming regions, notably the o Oph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas, should not be depleted much into grains, these cases suggest a trend toward the enhanced oxygen depletion predicted for denser ISM clouds. Subject headings : dust, extinction È ISM : abundances 1.

INTRODUCTION

dance of krypton. Meyer et al. (1998) demonstrated that the sight lines common to both oxygen and krypton studies exhibit a remarkable uniformity in the gas-phase O/Kr abundance ratio : all data points agree with the logarithmic mean gas-phase O/Kr abundance ratio of 5.56 ^ 0.04. The spread in the distribution, as identiÐed by its standard deviation, is less than the uncertainty in the mean. This Ñat dependency across a diverse range of di†use cloud densities affirms the value of Kr I j1236 as a benchmark for accurately determining ISM gas abundances along sight lines with sufficiently high column densities, but for which the total hydrogen column density is not known. In this paper, we present O I j1356 and Kr I j1236 Space Telescope Imaging Spectrograph (STIS) data for 11 sight lines characterized by high extinction, large H I column densities, and/or long path lengths. These sight lines supplement the group common to the GHRS studies, extending the study of oxygen depletion to translucent ISM clouds under the assumption that krypton remains a reliable Ðducial for total hydrogen column density in such conditions.

Oxygen, as the most abundant element in the interstellar medium (ISM) after hydrogen and helium, is important for constraining the contributions of stellar evolution, novae and supernovae, winds, mixing, and other processes to ISM evolution. In particular, isolated departures from any large length-scale trends in oxygen ISM abundance distributions can identify sites with markedly di†erent properties (e.g., high-density ISM clouds and supernova remnants ; Roy & Kunth 1995). Accurate analysis of conditions in such sites requires that large-scale trends be well characterized. Recent investigations using Goddard High Resolution Spectrograph (GHRS) observations of O I j1356 have shown that for di†use clouds in the nearby ISM, gas-phase oxygen abundances are very nearly constant (Meyer et al. 1994 ; Cardelli et al. 1996 ; Meyer, Jura, & Cardelli 1998 ; with an ionization potential of 13.618 eV, O I is the dominant form of oxygen in di†use clouds). These GHRS sight lines probed a wide range of di†use cloud conditions, sampling 5 orders of magnitude in f (H ) \ 2N(H )/[N(H I) 2 enhanced 2 oxygen ] 2N(H )]. Various ISM models predict 2 depletions for translucent sight lines (e.g., Spaans 1996) as cloud densities increase toward those of dense molecular clouds. The local homogeneity of the log [N(O )/N(H )] total ratio provides a backdrop against which suchgaspredictions can be tested. Krypton is a noble gas, suggesting that in its dominant neutral form (Kr I has an ionization potential of 13.999 eV), it should not be depleted into grains in a di†use ISM environment. Cardelli & Meyer (1997) found that local di†use ISM clouds exhibit a near constant gas-phase abun-

2.

OBSERVATIONS

The STIS observations of O I j1356 and Kr I j1236 absorption toward 11 Galactic O and B stars were acquired through two Hubble Space T elescope (HST ) Cycle 8 observing programs. All spectra were acquired using the E140H echelle grating with a 202 A spectral window centered at 1271 A . Observations of HD 75309, HD 152590, HD 175360, HD 185418, and HD 203532 were made using the 0A. 2 ] 0A. 2 STIS aperture, while the remainder were observed with the 0A. 2 ] 0A. 9 aperture. The raw data were run through the standard STSDAS STIS data reduction package to produce geometrically corrected two-dimensional Ñat-Ðelded spectra. The Howk & Sembach (2000) scattered-light correction algorithm was then applied to generate one-dimensional extracted spectra.

1 Based on observations with the NASA/ESA Hubble Space T elescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

394

OXYGEN AND KRYPTON GAS IN TRANSLUCENT CLOUDS

1.0

1.0

0.8

HD 75309

0.6

0.8

HD 37061

0.6

Kr I 1236 O I 1356

0.4 Normalized Flux

395

0.4

1.0

1.0

0.8

HD 152590

HD 147888

0.8

0.6

0.6

0.4

0.4

1.0

1.0

0.8

HD 185418

HD 207198

0.8

0.6

0.6

0.4

0.4

-100

-50

0

50

100 -100

-50

0

50

100

Heliocentric Velocity (km/s) FIG. 1.ÈLine proÐles for O I j1356 (dashed line) and Kr I j1236 (solid line), expressed as a function of heliocentric velocity. HD 37061, HD 75309, and HD 185418 are representative of sight lines along which the gas-phase O/Kr value lies within 1 p of the GHRS mean. HD 147888, HD 152590, and HD 207198 are representative of sight lines that deviate from this mean by more than 1.5 p.

In general, O I j1356 and Kr I j1236 absorption were present in the overlap region between successive spectral orders, which were combined to improve the S/N ratio. The Ðnal S/N per pixel ranged from 30È60 near O I j1356 and from 25È40 near Kr I j1236. In Figure 1, examples of O I j1356 and Kr I j1236 features are displayed. The data include proÐles ranging in character from narrow, probably single-component absorption (e.g., HD 147888) to broad, multiple-component absorption (e.g., HD 207198). Both proÐle-Ðtting (Mar & Bailey 1995 ; Welty, Hobbs, & York 1991) and apparentÈoptical-depth methods (Savage & Sembach 1991) for deriving column densities were applied to the O I j1356 and Kr I j1236 absorption proÐles in order to evaluate and correct for uncertainties due to line satura-

tion. The process of proÐle Ðtting employed an interactive routine to determine the number of components required to approximate the absorption proÐles for each sight line. In particular, twelve S I lines in four multiplets between 1247 and 1316 A and the Mg II doublet at 1239 A provided a set of lines of diverse strengths with which to construct an appropriate absorption model. A semiautomated routine was then used to optimize the model componentsÏ widths and relative velocities, with emphasis placed on deriving consistent b-values for the diverse species open to investigation within this STIS wavelength band. The results of both proÐle-Ðtting and apparentÈoptical-depth methods are presented in Table 1. Notably, the column densities derived by the di†erent methods di†er by less than 0.05 dex for each

TABLE 1 DETAILS OF STIS COLUMN DENSITY DETERMINATIONS

Star HD HD HD HD HD HD HD HD HD HD HD

27778 . . . . . . . 37021 . . . . . . . 37061 . . . . . . . 37903 . . . . . . . 75309 . . . . . . . 147888 . . . . . . 152590 . . . . . . 175360 . . . . . . 185418 . . . . . . 203532 . . . . . . 207198 . . . . . .

Velocities (km s~1)

b-values (km s~1)

14, 18 19, 24 24 28 20, 23 [7 [10, [3, 1 [7 [13, [9 3, 15 [20, [16, [12, [9

1.9, 2.2 1.9, 1.3 2.2 1.6 2.9, 1.9 2.1 1.0, 2.0, 5.0 4.2 1.9, 2.3 1.1, 3.2 1.2, 1.4, 1.2, 0.7

log[N(O I) ]a P 17.83 (0.04) 18.09 (0.03) 18.23 (0.03) 17.88 (0.02) 17.73 (0.05) 18.18 (0.02) 18.06 (0.05) 17.59 (0.06) 18.06 (0.05) 17.85 (0.02) 18.13 (0.04)

log[N(O I) ]a O 17.82 (0.02) 18.07 (0.02) 18.21 (0.01) 17.88 (0.02) 17.73 (0.03) 18.14 (0.02) 18.07 (0.03) 17.56 (0.04) 18.04 (0.02) 17.83 (0.02) 18.13 (0.02)

log[N(Kr I) ]a P 12.37 (0.05) 12.63 (0.04) 12.72 (0.02) 12.31 (0.05) 12.21 (0.08) 12.73 (0.02) 12.71 (0.09) 12.01 (0.05) 12.50 (0.06) 12.26 (0.04) 12.68 (0.08)

log[N(Kr I) ]a O 12.35 (0.05) 12.61 (0.03) 12.69 (0.02) 12.29 (0.05) 12.17 (0.07) 12.72 (0.02) 12.71 (0.05) 12.00 (0.09) 12.48 (0.04) 12.23 (0.08) 12.70 (0.04)

a The oxygen and krypton column densities are determined by proÐle Ðtting (subscript P) and apparent optical depth (subscript O).

396

CARTLEDGE ET AL.

Vol. 562

TABLE 2 STIS AND GHRS GAS-PHASE O AND Kr ABUNDANCES

Star

E(B[V )a (mag)

HD 27778 . . . . . . . HD 37021 . . . . . . . HD 37061 . . . . . . . HD 37903 . . . . . . . HD 75309 . . . . . . . HD 147888 . . . . . . HD 152590 . . . . . . HD 175360 . . . . . . HD 185418 . . . . . . HD 203532 . . . . . . HD 207198 . . . . . . q CMac . . . . . . . . . . i Oric . . . . . . . . . . . . v Oric . . . . . . . . . . . . j Oric . . . . . . . . . . . . v Perc . . . . . . . . . . . . f Perc . . . . . . . . . . . . f Ophc . . . . . . . . . . .

0.36 0.54 0.45 0.32 0.17 0.52 0.39 0.12 0.43 0.29 0.59 0.14 0.10 0.09 0.12 0.11 0.27 0.33

W (1356)b j (mA ) 10.9 17.4 20.5 10.7 9.3 17.9 19.3 6.4 17.1 11.0 21.3 4.0 2.1 1.8 4.0 2.1 8.0 6.4

(0.5) (0.7) (0.4) (0.3) (0.7) (0.7) (1.1) (0.6) (0.8) (0.6) (1.0) (0.3) (0.2) (0.2) (0.5) (0.2) (0.5) (0.6)

N(O I) (cm~2)

N(Kr I) (cm~2)

W (1236)b j (mA )

6.78 (0.74) ] 1017 1.24 (0.09) ] 1018 1.68 (0.12) ] 1018 7.66 (0.43) ] 1017 5.35 (0.69) ] 1017 1.52 (0.08) ] 1018 1.15 (0.15) ] 1018 3.92 (0.60) ] 1017 1.14 (0.15) ] 1018 7.06 (0.39) ] 1017 1.36 (0.13) ] 1018 2.1 (0.2) ] 1017 1.1 (0.1) ] 1017 9.6 (1.1) ] 1016 2.1 (0.3) ] 1017 1.1 (0.1) ] 1017 5.2 (0.6) ] 1017 4.3 (0.4) ] 1017

5.61 9.84 10.92 4.77 3.87 11.41 12.55 2.60 7.46 4.39 12.37 1.55 0.90 0.70 1.75 0.80 4.05 3.40

log

2.32 (0.26) ] 1012 4.28 (0.38) ] 1012 5.19 (0.35) ] 1012 2.06 (0.27) ] 1012 1.61 (0.37) ] 1012 5.42 (0.36) ] 1012 5.15 (1.30) ] 1012 1.03 (0.20) ] 1012 3.17 (0.46) ] 1012 1.80 (0.40) ] 1012 4.82 (1.02) ] 1012 5.6 (0.7) ] 1011 3.2 (0.6) ] 1011 2.5 (0.5) ] 1011 6.3 (0.8) ] 1011 2.9 (0.5) ] 1011 1.5 (0.1) ] 1012 1.1 (0.2) ] 1012

(0.64) (0.73) (0.64) (0.56) (0.64) (0.62) (1.61) (0.57) (0.78) (0.88) (1.10) (0.20) (0.15) (0.15) (0.20) (0.15) (0.30) (0.62)

(O/Kr) 10 (dex)

5.47 5.46 5.51 5.57 5.52 5.45 5.35 5.58 5.56 5.59 5.45 5.58 5.52 5.58 5.53 5.57 5.54 5.58

(0.06) (0.05) (0.04) (0.06) (0.10) (0.04) (0.11) (0.10) (0.08) (0.09) (0.09) (0.06) (0.08) (0.09) (0.08) (0.08) (0.06) (0.07)

a Derived using observed B[V and spectral class (Mihalas & Binney 1981). b Equivalent width for the STIS data ; uncertainties include contributions from noise, as described by Jenkins et al. 1973, and the rms continuum shift Ðtting error detailed in Sembach & Savage 1992 with a prefactor of 0.4. GHRS data are from Meyer et al. 1998 and Cardelli & Meyer 1997. c GHRS data are taken from Meyer et al. 1998 and Cardelli & Meyer 1997, and are adjusted to reÑect revised f-values.

3.

GAS-PHASE O/Kr ABUNDANCE RATIO

Based on the assumption that krypton is undepleted in this sample of di†use and translucent sight lines, any variations in the O/Kr gas-phase abundance ratio are expected to be due to changes in the depletion or the total ISM abundance of oxygen. As shown in Figure 2, the general trend of the STIS data extends the near constant O/Kr abundance ratio to oxygen column densities roughly 3 times larger than for GHRS sight lines. However, the spread of O/Kr ratios for higher oxygen column densities widens appreciably with multiple 2 p deviations from the GHRS

mean. The STIS sample diverges into two groups : six STIS sight lines are in reasonable agreement with the GHRS mean, and Ðve STIS sight lines fall 0.09È0.21 dex below it. The STIS sample contains several sight lines with large E(B[V ). Intriguingly, of the seven with E(B[V ) [ 0.35, only the sight lines toward HD 37061 and HD 185418 agree with the GHRS mean O/Kr ratio. Several of these are translucent cloud sight lines associated with well-known molecular cloud or star-formation complexes. HD 27778 lies within the Taurus dark cloud complex ; HD 37021 is an Orion Trapezium star ; HD 37061 is a member of the Orion Nebula Cluster ; HD 37903 excites the reÑection nebula 6 5.9

STIS Targets GHRS Targets

5.8 log(N(O)/N(Kr))

sight line, and these results are 0.05È0.23 dex larger than values derived under the assumption of no saturation. Unresolved saturation is a concern, since our measurements would then underestimate the column densities of oxygen and krypton. However, we assert that any unresolved saturation would have only a second-order e†ect on the gasphase O/Kr abundance ratio, since the O I j1356 and Kr I j1236 lines are similarly weak. Table 2 presents the new STIS data and previous GHRS data (Cardelli & Meyer 1997 ; Meyer et al. 1998). We have adopted the f-values used by Welty et al. (1999). For all measurements of O I j1356, the f-value was 1.161 ] 10~6, derived using the mean experimental transition lifetime from Johnson (1972), Wells & Zipf (1974), Nowak, Borst, & Fricke (1978), and Mason (1990) and the calculated branching factor of Biemont & Zeippen (1992). This value is slightly lower than the Ðgure of 1.248 ] 10~6 (Zeippen, Seaton, & Morton 1977) adopted by Meyer et al. (1998). All Kr I j1236 measurements used an f-value of 0.2042 (Lang et al. 1999), marginally di†erent from the value of 0.20 adopted by Cardelli & Meyer (1997), based on the bracketing determinations of 0.187 ^ 0.006 by Gri†en & Hutchenson (1969) and 0.214 ^ 0.011 by Chan et al. (1992). The GHRS results have been adjusted to reÑect these revised f-values.

5.7

Solar ratio

5.6 5.5

GHRS mean

5.4 5.3 5.2 5.1 5

17

17.5

18

log N(O) FIG. 2.ÈGas-phase O/Kr abundance ratios for the STIS and GHRS sight lines listed in Table 1. The dashed line represents the GHRS mean reported by Meyer et al. (1998). Although the majority of STIS sight lines lie along or near this mean, several at high oxygen column densities di†er by more than 1.5 p. The solar O/Kr ratio is represented by the dotted line, and is derived from solar Kr/H (Anders & Grevesse 1989) and O/H (Holweger 2001) ratios.

No. 1, 2001


397

TABLE 3 STIS AND GHRS NEUTRAL AND MOLECULAR H ABUNDANCES N(H I) (cm~2)

Star HD 27778 . . . . . . . HD 37021a . . . . . . HD 37061a . . . . . . HD 75309 . . . . . . . HD 147888 . . . . . . HD 185418 . . . . . . HD 207198 . . . . . . q CMab . . . . . . . . . . i Orib . . . . . . . . . . . . v Orib . . . . . . . . . . . . j Orib . . . . . . . . . . . . v Perb . . . . . . . . . . . . f Perb . . . . . . . . . . . . f Ophb . . . . . . . . . . .

1.3 4.8 6.2 1.2 5.2 1.6 3.4 5.2 3.4 2.9 6.2 2.6 6.4 5.2

N(H ) 2 (cm~2)

(0.3) ] 1021 (1.1) ] 1021 (1.6) ] 1021 (0.2) ] 1021 (0.8) ] 1021 (0.3) ] 1021 (1.0) ] 1021 (0.5) ] 1020 (0.3) ] 1020 (0.5) ] 1020 (1.2) ] 1020 (0.4) ] 1020 (0.6) ] 1020 (0.3) ] 1020

5.2 (1.1) ] 1020 ... ... 1.6 (0.5) ] 1020 3.7 (1.5) ] 1020 5.1 (1.6) ] 1020 6.8 (1.8) ] 1020 3.0 ] 1015 4.8 ] 1015 3.7 ] 1016 1.3 (0.4) ] 1019 3.4 (1.4) ] 1019 4.7 (1.2) ] 1020 4.5 (0.5) ] 1020

N(H) (cm~2) 2.3 4.8 6.2 1.5 5.9 2.6 4.8 5.2 3.4 2.9 6.5 3.3 1.6 1.4

log

(0.4) ] 1021 (1.1) ] 1021 (1.6) ] 1021 (0.3) ] 1021 (0.9) ] 1021 (0.5) ] 1021 (1.1) ] 1021 (0.5) ] 1020 (0.3) ] 1020 (0.5) ] 1020 (1.2) ] 1020 (0.5) ] 1020 (0.2) ] 1021 (0.1) ] 1021

10

[N(O)/N(H)] (dex)

[3.52 [3.59 [3.56 [3.45 [3.59 [3.36 [3.55 [3.40 [3.50 [3.48 [3.48 [3.49 [3.49 [3.51

(0.08) (0.09) (0.10) (0.08) (0.08) (0.08) (0.09) (0.05) (0.05) (0.07) (0.09) (0.07) (0.07) (0.05)

log [N(Kr)/N(H)] 10 (dex) [8.99 [9.05 [9.07 [8.97 [9.03 [8.92 [9.00 [8.97 [9.02 [9.06 [9.01 [9.06 [9.03 [9.10

(0.08) (0.10) (0.10) (0.11) (0.08) (0.08) (0.09) (0.06) (0.08) (0.10) (0.09) (0.09) (0.05) (0.05)

log [ f (H )] 2 [0.34 ... ... [0.68 [0.89 [0.42 [0.40 [4.94 [4.55 [3.59 [1.40 [0.68 [0.23 [0.20

a For HD 37021 and HD 37061, only H I column densities are available. However, since Cl I absorption is very weak or absent along these sight lines, we infer that the molecular hydrogen column densities are negligible in comparison to neutral hydrogen column densities toward these targets (Jura & York 1978). b GHRS H I data are the weighted means of Bohlin et al. 1978 and Diplas & Savage 1994 results. GHRS H data are from Savage et al. 1977, in 2 which the low molecular hydrogen column density sight lines are listed without error estimates.

NGC 2023 ; HD 147888 (o Oph D) lies near the o Oph dark cloud ; HD 207198 is a member of the Cep OB2 association. The four sight lines toward HD 27778, HD 37021, HD 147888, and HD 207198 have nearly identical ratios of O/Kr. Their weighted mean is log [N(O)/N(Kr)] \ 5.45 gasAmong ^ 0.03, 2.5 p below the GHRS value of 5.56 ^ 0.04. the translucent cloud sight lines, there is a trend toward lower O/Kr ratios with increasing reddening. This tendency resembles trends that have been predicted by models of translucent clouds (e.g., Spaans 1996) in which oxygen depletion onto grains is enhanced with A . Assuming that V the low O/Kr abundance ratio is due to enhanced oxygen depletion, one Ðnds that the average oxygen depletion for this group of four sight lines is 40% greater than the mean contribution to local ISM dust, using the mean gas-phase -8.5 STIS Targets GHRS Targets

-8.6 -8.7 log(N(Kr)/N(H))

Solar ratio

-8.8 -8.9 -9

GHRS mean

-9.1 -9.2 -9.3 -9.4 -9.5 20

20.5

21

21.5

22

log N(H) FIG. 3.ÈGas-phase Kr/H abundance ratios for STIS (this work) and GHRS (Cardelli & Meyer 1997) sight lines. The STIS sight lines each agree with the GHRS mean gas-phase Kr/H abundance ratio, represented by the dashed line, although each of HD 27778, HD 37021, HD 147888, and HD 207198 fall below the GHRS mean for both gas-phase O/Kr and O/H abundance ratios. This suggests that krypton remains a reliable measure of the total hydrogen column density for translucent ISM sight lines. The dotted line indicates the solar Kr/H value (Anders & Grevesse 1989).

abundance of Meyer et al. (1998) and assuming that the solar value represents the total ISM O/H abundance ratio. This trend makes no distinction dependent on implied grain size. Based on the ratio of total-to-selective extinction (R ) V derived from E(B[V ) and visual extinction measurements, dust grains toward HD 37021 (R [ 5) and HD 147888 (R B 4) are systematically larger Vthan in the low-density V Galactic ISM (R B 3.1), whereas grains toward HD 27778 and HD 207198V(R \ 3) are systematically smaller. The enhanced depletionsVin these sight lines suggest that oxygen has no preferred depletion channel with respect to grain size. This is not surprising, given that Cardelli, Clayton, & Mathis (1989) suggest that larger size distributions are primarily due to coagulation of grains rather than to accretion. However, since our STIS sample is small, more observations are needed to clarify the nature and signiÐcance of these results. The sight line whose O/Kr ratio lies farthest below the GHRS mean is toward HD 152590, a member of open cluster NGC 6231, the nucleus of the Sco OB1 association (Perry, Hill, & Christodoulou 1991). For the Ðeld with radius 10¡ around the cluster, Raboud, Cramer, & Bernasconi (1997) report that mean visual absorption is strong (D3.5 mag kpc~1) from 100 to 300 pc and 1000 to 1200 pc, with approximate transparency from 300 to 1000 pc and between 1200 pc and the cluster. These intervals are roughly consistent with local gas and the estimated position of the Carina Arm (Georgelin & Georgelin 1976). We have Ðtted both the O I j1356 and Kr I j1236 proÐles with a threecomponent model, with resulting O/Kr logarithmic abundance ratios 5.30`0.12 and 5.39`0.09 for the stronger two ~0.16 and redward, ~0.11 respectively. The true components, blueward absorption proÐle is undoubtedly more complex than the assumed three-component model, and the ambiguity of the distanceÈtoÈheliocentric-velocity relation in this direction (Clemens 1985) implies that neither value exclusively reÑects the bulk properties of the Carina or Orion Spur Arms. Rather, each is a measure of a blend of gas from both regions. The low O/Kr ratios of these two components, however, may indicate that the composition of the Carina

CARTLEDGE ET AL.

Arm ISM is markedly di†erent from the local ISM in terms of the fraction of oxygen contained in either grains or molecules. The reliability of these results depends critically on krypton as a Ðducial for total hydrogen column density. Cardelli & Meyer (1997) and Meyer et al. (1998) demonstrated the validity of using krypton over a broad range of f (H ), yet the Ñatness of the Kr/H abundance ratio has not 2 previously been extended to translucent ISM sight lines. 4.

GAS-PHASE Kr/H ABUNDANCE RATIO

Far Ultraviolet Spectroscopic Explorer (FUSE) observations have been made of HD 27778, HD 75309, and HD 185418, all of which are now publicly available. We have measured H I and H column densities from the STIS and 2 FUSE data, respectively, to determine total hydrogen column densities for each of these sight lines. HD 37021 and HD 37061 have little or no Cl I (ionization potential 12.97 eV) absorption, implying that chlorine is predominantly ionized ; thus f (H ) should be small (Jura & York 1978). We have measured H2 I toward HD 37021 using the Lya line present in our STIS data, and have adopted the Diplas & Savage (1994) value for HD 37061. For HD 207198, Diplas & Savage (1994) list an H I column density, but with a large error, apparently due to strong contamination by a stellar wind and uncertainty in the background subtraction applied to International Ultraviolet Explorer (IUE) data for this target. We have measured the neutral hydrogen column density from the Lyb line and combined the weighted mean of these values with the H column density reported by 2 since HD 147888 (o Oph D) Rachford et al. (2000). Finally, is closely related to o Oph, we have adopted the weighted mean of o Oph H I column densities from Copernicus and IUE data (Diplas & Savage 1994) and a Copernicus H measurement (Savage et al. 1977) as representative of HD2 147888, for the purposes of comparing Kr/H and O/H values. The hydrogen column densities, measured and adopted, are presented in Table 3. In general, H I and H column density measurements were made using the2 continuum-reconstruction procedure originally described by Bohlin (1975) and adopted by Savage et al. (1977), Bohlin, Savage, & Drake (1978), and Diplas & Savage (1994). The molecular hydrogen b-values were determined from the weaker lines of higher rotational levels, although the continuum reconstructions were relatively insensitive to small variations. A grid of model hydrogen proÐles for H I and the two lowest rotational levels of H were divided into the calibrated spectra, and the column 2density was determined as the shape of the reconstructed spectrum became reasonable. Independently measured total hydrogen column densities have been derived for HD 75309 and HD 185418 (Andre et al. 2001) that agree within error with our measurements. Gas-phase Kr/H abundance ratios for these seven STIS sight lines provide the Ðrst evidence that use of Kr I j1236 as a measure of total hydrogen column density can be extended to translucent ISM sight lines. Figure 3 compares the new STIS sight lines with previous GHRS results (Cardelli & Meyer 1997), showing that each data point agrees with the GHRS logarithmic mean of [9.02 ^ 0.02. As noted by Cardelli & Meyer (1997), this value represents about 60% of the solar value of [8.77 ^ 0.07 (Anders & Grevesse 1989), although the solar abundance is somewhat uncertain because of the series of approximations by which

Vol. 562

it is derived from the 84Kr meteoric abundance. The agreement of the STIS data with the GHRS mean reinforces the concept of a standard Kr/H abundance ratio for the local ISM, regardless of its agreement with the solar value. This agreement further indicates, noting that HD 27778, HD 37021, HD 147888, and HD 207198 are included, that the lower gas-phase O/Kr abundance ratios along these sight lines are due to enhanced oxygen depletion. 5.

GAS-PHASE O/H ABUNDANCE RATIO

Gas-phase oxygen to total-hydrogen abundance ratios are plotted in Figure 4, comparing ratios from GHRS data (Cardelli & Meyer 1997) and current STIS data for HD 27778, HD 37021, HD 37061, HD 75309, HD 147888, HD 185418, and HD 207198. The STIS sight lines that lie below the GHRS mean, log [N(O I)/N(H)] \ [3.46 ^ 0.02, are HD 27778, HD 37021, HD 37061, HD 147888, and HD 207198, while HD 75309 and HD 185418 agree with or lie slightly above the mean. This grouping mimics the distribution in Figure 2 : those sight lines with low gas-phase O/Kr abundance ratios are also low in gas-phase O/H abundance ratios. HD 37061 is unique in that it agrees with the GHRS O/Kr abundance ratio and sits lower in Figure 4 than the GHRS mean O/H ratio, yet it should be noted that its O/Kr agreement is marginal, and this is the sight line that has the largest oxygen column density. The GHRS mean gas-phase O/H ratio is approximately 2/3 the new solar O/H ratio (Holweger 2001). As discussed by SoÐa & Meyer (2001), this revised solar abundance indicates that solar abundances can reasonably be used as a standard for interstellar abundances. In particular, current dust models bring the GHRS mean gas-phase O/H ratio into agreement with the solar value. Hence, deviations from this mean, accounting for reasonable uncertainties, should indicate variations in the depletion of oxygen into dust and molecules. Enhanced oxygen depletion implies incorporation of oxygen with other species in some combination of dust grains and molecules. Further study of these sight lines will involve measurements of abundances for elements such as -3 -3.1 -3.2 log(N(O)/N(H))

398

STIS Targets GHRS Targets Solar ratio

-3.3 -3.4

GHRS mean

-3.5 -3.6 -3.7 -3.8 -3.9 -4 20

20.5

21

21.5

22

log N(H) FIG. 4.ÈGas-phase O/H abundance ratios for STIS (this work) and GHRS (Meyer et al. 1998) sight lines. HD 27778, HD 37021, HD 147888, and HD 207198 fall below the GHRS mean for both gas-phase O/Kr and O/H abundance ratios, yet agree with the GHRS mean Kr/H abundance ratio. The dashed line denotes the GHRS mean O/H abundance ratio, and the dotted line is the solar O/H value (Holweger 2001).

No. 1, 2001


magnesium and nickel, which are signiÐcantly depleted and whose gas-phase abundance relative to hydrogen may be noticeably a†ected by incorporation with oxygen into dust or molecules. With the amount of interstellar hydrogen available, another possibility is that signiÐcant fractions of oxygen are frozen into water ices. Among the sight lines in our STIS data, the 3.07 km water ice feature is reported toward only HD 147888 (Martin et al. 1992), although there is a suggestion of this feature in the spectrum for NGC 2023, which is illuminated by HD 37903. HD 27778 lies within the Taurus dark cloud complex, yet A toward this star lies V below the Taurus threshold (3.3 mag) for observation of the water ice feature (Whittet et al. 1988). Thus, the current evidence is consistent with the low gas-phase O/Kr abundance ratio sight lines providing the Ðrst indications of

399

enhanced oxygen depletion in di†use and translucent ISM clouds. Several of the remaining STIS sight lines have been reserved for current and future FUSE observing programs, although they have not yet been scheduled for observation. Once these data have been acquired, they will be key to determining any pattern of enhanced oxygen depletion as measured by deviations in gas-phase O/Kr and O/H ISM abundance ratios. Support for this work was provided by the Space Telescope Science Institute through grants to Northwestern University and Whitman College. This research has made use of the SIMBAD database, operated at the Centre de Donnees astronomiques de Strasbourg, Strasbourg, France.

REFERENCES Anders, E., & Grevesse, N. 1989, Geochim. Cosmochim. Acta, 53, 197 Martin, P. G., Adamson, A. J., Whittet, D. C. B., Hough, J. H., Bailey, J. A., Andre, M., et al. 2001, BAAS, 198, 4110 Kim, S.-H., Sato, S., Tamure, M., & Yamashita, T. 1992, ApJ, 392, 691 Biemont, E., & Zeippen, C. J. 1992, A&A, 265, 850 Mason, N. J. 1990, Meas. Sci. Tech., 1, 596 Bohlin, R. C. 1975, ApJ, 200, 402 Meyer, D. M., Jura, M., & Cardelli, J. A. 1998, ApJ, 493, 222 Bohlin, R. C., Savage, B. D., & Drake, J. F. 1978, ApJ, 224, 132 Meyer, D. M., Jura, M., Hawkins, I., & Cardelli, J. A. 1994, ApJ, 437, L59 Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245 Mihalas, D., & Binney, J. 1981, Galactic Astronomy : Structure and KineCardelli, J. A., & Meyer, D. M. 1997, ApJ, 477, L57 matics (2d ed. ; San Francisco : Freeman) Cardelli, J. A., Meyer, D. M., Jura, M., & Savage, B. D. 1996, ApJ, 467, 334 Nowak, G., Borst, W. L., & Fricke, J. 1978, Phys. Rev. A, 17, 1921 Chan, W. F., Cooper, G., Guo, X., Burton, G. R., & Brion, C. E. 1992, Phys. Perry, C. L., Hill, G., & Christodoulou, D. M. 1991, A&AS, 90, 195 Rev. A, 46, 149 Raboud, D., Cramer, N., & Bernasconi, P. A. 1997, A&A, 325, 167 Clemens, D. P. 1985, ApJ, 295, 422 Rachford, B. L., et al. 2000, BAAS, 32, 1403 Diplas, A., & Savage, B. D. 1994, ApJS, 93, 211 Roy, J.-R., & Kunth, D. 1995, A&A, 294, 432 Georgelin, Y. M., & Georgelin, Y. P. 1976, A&A, 49, 57 Savage, B. D., Bohlin, R. C., Drake, J. F., & Budich, W. 1977, ApJ, 216, 291 Gri†en, P. M., & Hutchenson, J. W. 1969, J. Opt. Soc. Am., 59, 1607 Savage, B. D., & Sembach, K. R. 1991, ApJ, 379, 245 Holweger, H. 2001, Solar and Galactic Composition, ed. R. F. WimmerSembach, K. R., & Savage, B. D. 1992, ApJS, 83, 147 Schweingruber (Berlin : Springer), in press SoÐa, U. J., & Meyer, D. M. 2001, ApJ, 554, L221 Howk, J. C., & Sembach, K. R. 2000, AJ, 119, 2481 Spaans, M. 1996, A&A, 307, 271 Jenkins, E. B., Drake, J. F., Morton, D. C., Rogerson, J. B., Spitzer, L., & Wells, W. C., & Zipf, E. C. 1974, Phys. Rev. A, 9, 568 York, D. G. 1973, ApJ, 181, L122 Welty, D. E., Hobbs, L. M., Lauroesch, J. T., Morton, D. C., Spitzer, L., & Johnson, C. E. 1972, Phys. Rev. A, 5, 2688 York, D. G. 1999, ApJS, 124, 465 Jura, M., & York, D. G. 1978, ApJ, 219, 861 Welty, D. E., Hobbs, L. M., & York, D. G. 1991, ApJS, 75, 425 Lang, D., Roth, H., Lang, K., & Schmoranzer, H. 1999, in Proc. 6th Intl. Whittet, D. C. B., Bode, M. F., Longmore, A. J., Adamson, A. J., Colloq. on Atomic Spectra and Oscillator Strengths, ed. W. L. Weise, & McFadzean, A. D., Aitken, D. K., & Roche, P. F. 1988, MNRAS, 233, D. C. Morton (Stockholm : Royal Swedish Aacademy of Sciences) 321 Mar, D. P., & Bailey, G. 1995, Proc. Astron. Soc. Australia, 12, 239 Zeippen, C. J., Seaton, M. J., & Morton, D. C. 1977, MNRAS, 181, 527