ORFEUS II Echelle Observations of Molecular Hydrogen in the ...

4 downloads 1219 Views 260KB Size Report
dissociation rates for the clouds toward HD 219188 and HD 94473. The results. H. 2 ... sight to HD 94473, for which published CO emission data are available ...
THE ASTROPHYSICAL JOURNAL, 529 : 251È258, 2000 January 20 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ORFEUS II ECHELLE OBSERVATIONS OF MOLECULAR HYDROGEN IN THE GALACTIC HALO KWANG SUN RYU,1 W. VAN DYKE DIXON,2 MARK HURWITZ,2 KWANG IL SEON,3 KYOUNG WOOK MIN,1 AND JERRY EDELSTEIN2 Received 1999 April 19 ; accepted 1999 September 13

ABSTRACT The far-ultraviolet spectra of HD 219188, HD 94473, and HD 18100, three early-type stars in the Galactic halo, were obtained with the echelle spectrometer on the ORFEUS II telescope in 1996 November. We derive H column densities for the rotational levels up to J@@ \ 6 and estimate excitation tem2 peratures and H dissociation rates for the clouds toward HD 219188 and HD 94473. The results 2 indicate that strong radiation Ðelds might exist near these clouds, but these radiation Ðelds do not seem to act as major heating sources. No signiÐcant H absorption is seen toward HD 18100. The correlation 2 between our H column densities and published extinction maps conÐrms that the molecular clouds are 2 associated with dust clouds that lie between the observed stars and the Sun. We estimate the conversion factor between N(H ) and W (12CO) to be X \ 4 ^ 2 ] 1019 cm~2 (K km s~1)~1 for the clouds along 2 CO the line of sight to HD 94473, consistent with previous estimates to within a factor of a few. Subject headings : dust, extinction È galaxies : halos È Galaxy : halo È ISM : abundances È ISM : clouds È ISM : lines and bands È ISM : structure È ultraviolet : ISM 1.

INTRODUCTION

GHz emission of the 1È0 rotational transition of CO has been used as the standard tracer of molecular clouds. Recently, Magnani et al. (1998) studied the variation of the CO-to-H conversion factor X . The ratio CO N(H )/W 2(12CO), especially in high-latitude molecular 2 has heretofore been measured only by indirect clouds, methods and has displayed considerable scatter, ranging from 0.3 to 7 ] 1020 (K km s~1)~1 (Magnani & Onello 1995). In this paper, we analyze the molecular hydrogen absorption lines in the spectra of three early-type stars in the Galactic halo, HD 219188, HD 94473, and HD 18100, using the echelle spectrometer on board the ORFEUS II telescope (KraŽmer et al. 1990 ; Barnstedt et al. 1999) and directly derive the conversion factor X along the line of sight to HD 94473, for which publishedCOCO emission data are available (Keto & Myers 1986).

Accounting for 10%È20% of the mass in the inner disk of our Galaxy, interstellar clouds play a major role in the cooling, heating, and evolution of interstellar matter (ISM). Since Eddington (1937) Ðrst suggested the possible existence of molecular hydrogen in some portions of interstellar clouds, theoretical studies (Gould & Salpeter 1963 ; Hollenbach & Salpeter 1971) have predicted that H may consti2 tute a large fraction of their mass. First detection of the interstellar hydrogen molecule was made by Carruthers (1970) using the electronic absorption spectra toward g Persei with a rocket-borne ultraviolet spectrometer. Since then, observations from Copernicus and other spacecraftborne instruments have revealed that the bulk of interstellar H lies in clouds with densities 10È1000 cm~3, diameters 2 than a few tens of parsecs, and column densities larger less than 1020 cm~2, which allow for the rapid formation of H on dust grains and provide self-shielding against disso-2 ciating photons. UV absorption studies are usually limited to unreddened clouds that lie within 2È3 kpc of the Sun (Wilson, Je†erts, & Penzias 1970). Nevertheless, they provide useful information and reveal interesting structures in the local interstellar medium. H emission lines from the v \ 1È0 vibration2 in the infrared can be used to probe the rotation band global energetics and morphology of the molecular clouds (Luhman & Ja†e 1995). These emission lines, Ðrst detected by Gautier et al. (1976), are ascribed to UV excitation by nearby O and B stars or shock heating of the molecular gas. Unfortunately, the H emission is faint and difficult to detect outside of the2 energetic cloud cores, often lying beyond the sensitivity of current instruments. Hence, 115

2.

OBSERVATIONS AND DATA REDUCTION

The echelle spectrometer, sharing the 1 m diameter ORFEUS II telescope with the Berkeley EUV/FUV spectrometer, was integrated aboard the freeÑier platform ASTRO-SPAS (Grewing et al. 1991) for the 1996 November/December mission of space shuttle Columbia. Covering the range 900È1400 AŽ , the echelle spectrometer provides a spectral resolution of 0.1 AŽ FWHM with an e†ective area of about 1.3 cm2. These speciÐcations make it ideally suited for studying absorption lines of bright far-UV sources. We have analyzed the spectra of three early-type stars observed with this instrument to derive column densities of molecular hydrogen along their lines of sight. Characteristics of these stars are shown in Table 1. The procedures for data extraction and Ñux calibration are described in Barnstedt et al. (1999). Determination of the stellar continuum is critical in analyzing the interstellar absorption features because of the complicated absorption and/or emission features originating in the star itself. As can be seen in Figure 1 of HD 219188, the stellar continuum is complex and cannot be modeled with a simple linear function of the wavelength. Spectra of stars with the same spectral type, which can be

1 Postal address : Department of Physics, Korea Advanced Institute of Science and Technology, 373-1 Gusong-dong, Yusong-gu, Taejon 305-701, Korea ; ksryu=space.kaist.ac.kr, kwmin=space.kaist.ac.kr. 2 Postal address : Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 94720-7450 ; vand=ssl.berkeley.edu, markh=ssl.berkeley.edu, jerrye=ssl.berkeley.edu. 3 Postal address : Satellite Technology Research Center, Korea Advanced Institute of Science and Technology, 373-1 Gusong-dong, Yusong-gu, Taejon 305-701, Korea ; kiseon=satrec.kaist.ac.kr.

251

252

RYU ET AL. TABLE 1 STAR CHARACTERISTICS

Characteristics

HD 219188

HD 94473

HD 18100

Spectral type . . . . . . . . . . V ....................... E(B[V ) . . . . . . . . . . . . . . . E(B[V )a . . . . . . . . . . . . . . R.A.(J2000) . . . . . . . . . . . . Decl.(J2000) . . . . . . . . . . . l ........................ b ....................... d (kpc) . . . . . . . . . . . . . . . . . z (kpc) . . . . . . . . . . . . . . . . . v sin i (km s~1)c . . . . . . V (km s~1)d . . . . . . . . . rot T (K)d . . . . . . . . . . . . . . . eff log (g/g )d . . . . . . . . . . . . _ Integration time (s) . . . S/N . . . . . . . . . . . . . . . . . . . .

B0.5 III 6.93 0.08 0.075 ^ 0.002 23h14m00s. 56 ]04¡59@49A. 52 83.03 [50.17 2.38 [1.83 304 120 25600 [1.1 1512 31

B5 III 7.3 0.14 0.09 ^ 0.006 10h53m58s. 58 [26¡44@45A. 80 272.83 ]29.17 0.66(0.28)b ]0.32(0.14)b ... 130 15000 [0.95 611 10

B1 V 8.46 0.02 0.018 ^ 0.0004 02h53m40s. 84 [26¡09@21A. 07 217.93 [62.73 3.1 [2.8 265 160 25400 [0.5 2852 39

NOTE.ÈBasic stellar parameters are from Fruscione et al. 1994 for HD 219188 and HD 18100 and from Penprase 1992 for HD 94473. a E(B[V ) from the extinction maps of Schlegel et al. 1998. Values shown are the mean and standard deviation of a 3 ] 3 pixel window centered at the location of the star. b Distance values are derived from trigonometric parallax measurements of Hipparcos (ESA 1997). c Data from Sembach, Danks, & Savage 1993. d Stellar data from Lang 1991.

obtained from published catalogs such as the Copernicus spectral atlas (Snow & Jenkins 1977), might be used as models of the stellar continuum, but they are often contaminated by strong interstellar absorption lines. Hence, we adopt the LTE model atmospheres of Kurucz (1992). We select a model atmosphere corresponding to a given starÏs temperature, surface gravity, and metallicity. From this model, we generate a high-resolution synthetic stellar spectrum using the synthetic spectral codes of Hubeny (1988), using the techniques and results of Brown, Ferguson, &

Davidsen (1996). The model stellar spectrum shown in Figure 1 as a dashed line is generated using the stellar parameters listed in Table 1, convolved with a stellar rotation proÐle and the instrumental point-spread function (a Gaussian Ðt to the narrowest absorption line in the spectrum), then scaled to match the observed continuum. Once the continuum is determined, we model the interstellar absorption features using an ISM line-Ðtting package (Dixon, Hurwitz, & Bowyer 1998) written by Hurwitz and Saba. We concentrate on the wavelength region between 1045 and 1060 AŽ , which contains the v \ 0È4 vibrational band in the Lyman series with minimal line blending and complicating stellar features. Using a database of H and atomic (Morton 1991) lines, the 2 program computes a Voigt proÐle for each line, given the column density, Doppler broadening parameter, and relative velocity of the species and convolves the result with a Gaussian proÐle to the instrument resolution. The process is iterated with each set of parameters, and the set minimizing s2 is chosen for the Ðnal result. Optically thick lines are Ðtted Ðrst to minimize the e†ect of Lorenzian wings on other lines. We assume in the present analysis a single absorption component with zero velocity relative to the observer and an e†ective Doppler parameter b \ 3 km s~1 for all three targets, in view of the Copernicus results (b \ 3È5 km s~1 ; van Dishoeck & Black 1986) obtained from the J@@ \ 2È4 lines of H . To determine the sensitivity of our results to the 2 assumed value of b, we repeat the procedure, changing the Doppler parameter for each line. For the optically thick J@@ \ 0, 1 lines, the derived column densities are insensitive to changes in b for b \ 10 km s~1 but fall by D0.2 dex for b \ 10 km s~1 and by D1.5 dex when b \ 15 km s~1. The J@@ \ 2, 3 absorption lines show greater sensitivity to b, with column densities decreasing by 0.5 dex when b is raised even to 5 km s~1. The J@@ º 4 lines show little sensitivity to b value. Jenkins et al. (1989) have found evidence for multiple clouds along the line of sight to n Scorpii, with individual

10

o

Flux (photons cm-2 s-1 A-1 )

8

6

4

2

0 1045

1050

1055 o Wavelength ( A )

1060

1065

FIG. 1.ÈEchelle spectrum of HD 219188 (solid line) overlaid with noise (dotted line) and KuruczÏs model of stellar atmosphere (dashed line). The spectrum has been background-subtracted and Ñux-calibrated and is plotted in photon units at full resolution.

12

o

Flux (photons cm-2 s-1 A-1 )

10

8

6

4

2

(a) 0 1046

1048

1050

1052 1054 o Wavelength ( A )

1056

1058

1060

1048

1050

1052 1054 o Wavelength ( A )

1056

1058

1060

2.0

o

Flux (photons cm-2 s-1 A-1 )

1.5

1.0

0.5

(b) 0.0 1046

FIG. 2.È(a) Echelle spectrum of HD 219188 (solid line) overlaid with the best-Ðtting model (dot-dashed line) and KuruczÏs model of stellar atmosphere (dashed line). The data have been smoothed with a 5 pixel boxcar. The positions of atomic and molecular lines are also marked in the plot. (b) and (c) are the spectra for HD 94473 and HD 18100, respectively.

254

RYU ET AL.

Vol. 529

12

o

Flux (photons cm-2 s-1 A-1 )

10

8

6

4

2

(c) 0 1046

1048

1050

1052 1054 o Wavelength ( A )

1056

1058

1060

FIG. 2.ÈContinued

components as narrow as b D 0.8 km s~1, consistent with pure thermal Doppler broadening. We Ðnd that assuming a single velocity component for each absorption feature leads to a systematic underestimate of the column density. The e†ect is greater for lines with larger transition probabilities (i.e., lower values of JA). Column densities derived for the J@@ \ 0È6 rotational levels of H are presented in Table 2. Figure 2 shows the 2

observed spectra of HD 219188, HD 94473, and HD 18100, smoothed with a 5 pixel boxcar, and the best-Ðt models. Interstellar atomic and molecular lines are identiÐed. Low signal-to-noise ratios complicate the Ðtting of highly excited rotational lines (J@@ º 4), especially in HD 94473, as can be seen in the Ðgure. We estimate that our results are correct to within 0.2 dex in J@@ \ 0, 1 ; 0.5 dex in J@@ \ 2, 3 ; and 0.3 dex in J@@ \ 4È6. 3.

TABLE 2 MOLECULAR HYDROGEN COLUMN DENSITIES Column Density

HD 219188

HD 94473

HD 18100

N(H I)a . . . . . . . . . . . . . . N(H ) . . . . . . . . . . . . . . . 2 N(0) . . . . . . . . . . . . . . . . . N(1) . . . . . . . . . . . . . . . . . N(2) . . . . . . . . . . . . . . . . . N(3) . . . . . . . . . . . . . . . . . N(4) . . . . . . . . . . . . . . . . . N(5) . . . . . . . . . . . . . . . . . N(6) . . . . . . . . . . . . . . . . . N(7) . . . . . . . . . . . . . . . . . N(8) . . . . . . . . . . . . . . . . . N(HD, 0) . . . . . . . . . . . N(HD, 1) . . . . . . . . . . . N(HD, 2) . . . . . . . . . . . N(4)/N(0) . . . . . . . . . . . . N(5)/N(1) . . . . . . . . . . . . N(HD, 0)/N(0) . . . . . . T (K) . . . . . . . . . . . . . . 01

20.85 19.20(19.34)b 18.70(18.90)b 19.00(19.15)b 18.00 17.00 14.56 14.75 14.00 \14.00 \14.00 14.50 \14.40 \14.00 [4.2 [4.4 [4.25 110(103)b

20.90 19.06 18.44 18.94 17.31 16.25 15.25 \14.50 \13.80 \14.00 \14.00 \14.00 ... ... [3.2 \[4.4 \[4.4 160

20.14 \14.8 \14.5 \14.5 ... ... ... ... ... ... ... ... ... ... ... ... ... ...

NOTE.ÈAll column densities are given as logarithms (cm~2). a H I column densities are from Savage et al. 1977 and Fruscione et al. 1994. b Data from Copernicus observations (Savage et al. 1977).

ANALYSIS

The strongest UV lines arise from the J@@ \ 0 and J@@ \ 1 rotational levels. As their populations are established by thermal proton exchange collisions, the mean excitation temperatures of the clouds along each line of sight can be derived from the column densities N(0) and N(1) using the relation N(1) g [E [170 K 01 \ 9 exp \ 1 exp , (1) N(0) g kT T 0 01 01 where g and g are the statistical weights of J@@ \ 0 and 0 1 J@@ \ 1, respectively (Shull & Beckwith 1982). The column densities and the temperatures derived from ORFEUS II observations are shown in Table 2, with results from the Copernicus observation of HD 219188 given in parentheses. Column densities from the two observations are consistent within the measurement uncertainties. No signiÐcant molecular hydrogen is found toward HD 18100. Our derived cloud temperatures are slightly above those obtained with Copernicus by Savage et al. (1977), who found that T ranges from 45 to 128 K with a mean of 77 ^ 17 01 They are also larger than the values for the clouds (rms) K. in the Galactic disk observed with the Berkeley spectrometer on ORFEUS I (Dixon et al. 1998).

No. 1, 2000

ORFEUS II HYDROGEN OBSERVATIONS

255

-40

(a)

HD 219188

-45

-50

-55

-60 100

90

80

70

FIG. 3.ÈSlices of the extinction map of Schlegel, Finkbeiner, & Davis (1998) centered on (a) HD 219188 and (b) HD 94473. Color excess values are plotted on a logarithmic scale so that E(B[V ) \ 0.04 for black and E(B[V ) \ 0.13 for white pixels. Galactic coordinates are overplotted, and the position of each star is marked at the center by an asterisk.

An excitation temperature T can similarly be derived ex from the J@@ \ 2 and 3 levels (Spitzer & Cochran 1973). The T D 300 K for both HD 219188 and HD 94473, suggesting ex high rotational states are populated nonthermally. the According to Jura (1975), the levels J@@ \ 4 and J@@ \ 5 are populated both by direct formation into these levels of newly created molecules and by photon pumping from J@@ \ 0 and J@@ \ 1, respectively. For densities less than 104 cm~3, the levels J@@ \ 4 and J@@ \ 5 are depopulated by spontaneous emission. From the column densities presented in Table 2, we derive b , the total excitation rate of 0 Lyman- and Werner-band photons at the exterior of the cloud. The total excitation rate reÑects the radiation Ðeld in which the cloud is immersed. To include the e†ect of attenu-

ation within the clouds, we follow Jura (1974) in relating the observational parameters to the external radiation Ðeld (see eq. [A8] of Jura 1974). We assume that H is formed on grains with rate R. For 2 10~15 s~1, consistent with the value HD 219188, Rn is 5 ] 5 ] 10~16s~1 \ Rn \ 3 ] 10~14 s~1 from the Copernicus observations (Jura 1975). If we adopt R \ 3 ] 10~17 cm3 s~1 (Jura 1975), we Ðnd n to be 160 ^ 80 cm~3 and the diameter of the cloud roughly 1.5 pc in the direction along the line of sight. The excitation rate at the surface of the cloud is b B 16 ] 10~10 s~1, 3 times the value of 0 measured for the solar neighborhood. In the 5 ] 10~10 s~1 case of HD 94473, Rn \ 2.5 ] 10~14 s~1, n \ 800 ^ 400 cm~3, the diameter B0.3 pc, and b B 76 ] 10~10 s~1. The 0

256

RYU ET AL.

20

Vol. 529

(b)

HD 94473

25

30

35

40

265

270

275

280

2

FIG. 3.ÈContinued

large b values for these targets suggest that the clouds are located0near strong UV sources. 4.

DISCUSSION

Desert, Bazell, & Boulanger (1988) used the correlation between the infrared brightness and the total gaseous (H and H ) content of the ISM to derive a map of the molecular 2component of the ISM by subtracting the infrared brightness expected from the measured 21 cm H I emission data from the observed 100 km IR emission. Two molecular clouds in their catalog, DBB 105 and DBB 405, lie along the lines of sight to HD 219188 and HD 94473, respectively. Recently, Schlegel, Finkbeiner, & Davis (1998) have presented a full-sky extinction map based on a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps. Figure 3 shows images from this extinction map

along the lines of sight to our program stars. As can be seen in the Ðgure, there are dust clouds along the lines of sight to HD 219188 and HD 94473. We have omitted the map of HD 18100 in which dust features do not appear. Our identiÐcation of the optically thick lines as molecular hydrogen absorption is consistent with the DBB assumption that the molecular clouds are spatially related to the observed dust and that the dust clouds lie between the observed stars and the Sun. Toward HD 94473, the reddening derived from optical colors is signiÐcantly greater than that derived from the di†use infrared emission (see Table 1). If the di†erence does not reÑect errors in the observational data or the interpretation thereof, it may indicate the presence of an unresolved interstellar cloud directly along the sight line, or circumstellar dust, with a reddening of about 0.05 mag. The high H 2

No. 1, 2000

ORFEUS II HYDROGEN OBSERVATIONS

dissociation rate observed toward this star demonstrates that at least some of the molecular gas is illuminated by a bright UV radiation Ðeld. Other stars could in principle provide this radiation, but HD 94473 itself is the most likely suspect. If HD 94473 is the source of the UV radiation, much of the H must be D150 pc from the Galactic plane 2 (adopting the distance measurement of Hipparcos), perhaps in a circumstellar shell. In ° 3, we estimated a density of 800 cm~3 and a path length of about 0.3 pc for the molecular hydrogen toward HD 94473, assuming it lies in a single cloud. At the Hipparcos distance of HD 94473 (D280 pc), a spherical cloud would have an angular diameter of only about [email protected]. The spatial structure of the extinction map is complex (Fig. 3b), but it appears that cirrus clouds with column densities comparable to that seen toward HD 94473 extend over spatial scales much larger than [email protected]. Although uncertainties remain in the cloud properties derived from the H absorption lines 2 and in the cloudÏs distance, the disparity between the cloudÏs extent along and perpendicular to the line of sight suggests that the cirrus morphology is better characterized as sheets or shells than as spherical cloudlets. Our data provide direct measurements of the column density of H through high-latitude cirrus clouds. This 2 important quantity is more commonly determined through indirect methods, such as by assuming a value for the ratio of the total gas column to the infrared emission, then subtracting the measured N(H I) (Reach, Koo, & Heiles 1994), or through measurement of CH as a surrogate molecular species (Magnani et al. 1998). Although substantial column densities of H toward HD 219188 and HD 94473 are not 2 the D0.08È0.09 mag of di†use reddening surprising, given inferred from the infrared maps (Schlegel et al. 1998), it is interesting to note that the sight lines pass through at most the ““ outskirts ÏÏ of the cirrus in the vicinity (see Fig. 3). By contrast, the sight line toward the bright active galactic nucleus ESO 141-G055 passes more closely to what appears to be a well-deÐned infrared ““ clump ÏÏ (Sembach, Savage, & Hurwitz 1999), with a reddening of 0.12 ^ 0.008 mag (derived from the Schlegel et al. maps). Despite the somewhat higher reddening and the more well-deÐned appearance of that cloud, the H column density toward E141 is lower than that toward 2HD 94473 or HD 219188 by a factor of D3 (Sembach et al. 1999). These results suggest that the cirrus structure seen in the infrared maps is generally correlated with the presence of molecular gas, but that the detailed relationship between N(H ) and the cirrus mor2 phology is nontrivial. Keto & Myers (1986) observed the 2.6 mm J@@ \ 1È0 rotational transition of CO in the region of HD 94473. The

257

cloud, centered at (l, b) \ (272¡.9,]29¡.3) and has a velocityintegrated antenna temperature W (12CO) of 0.6 K km s~1 at the position of peak CO emission. Because the peak CO emission does not coincide exactly with the line of sight to HD 94473, we can set only a lower limit of X º 2 ] 1019 CO cm~2 (K km s~1)~1 on the conversion factor between N(H ) and W (12CO). Considering the position of HD 94473 2 in a contour map in Figure 1 of Keto & Myers (1986), we estimate the conversion factor of 4 ^ 2 ] 1019 cm~2 (K km s~1)~1 along the line of sight to the star. The uncertainty in this estimate comes from the coarse grid of the CO map ; a more precise measurement of W (12CO) toward this star would allow a better estimate of X . The conversion CO factor, especially in high-latitude molecular clouds, has heretofore been measured only by indirect methods and shows considerable scatter, ranging from 0.3 to 7 ] 1020 (Magnani & Onello 1995). Our value is an order of magnitude lower than those determined by Magnani & Onello for the Galactic molecular cloud ensemble (2È4 ] 1020). 5.

SUMMARY

We have observed H molecular absorption along the 2 HD 219188, HD 94473, and HD lines of sight to halo stars 18100 with the FUV echelle spectrometer on the ORFEUSII mission. We conÐrm the existence of molecular clouds toward HD 219188 and HD 94473, but the H column 2 density toward HD 18100 is very low. The excitation temperatures of the clouds derived from N(0) and N(1) toward HD 219188 and HD 94473 are slightly higher than those obtained for disk clouds by Copernicus and ORFEUS I. Column densities for rotational levels up to J@@ \ 6 are used to estimate the density and size of the clouds as well as the radiation Ðelds at their surfaces. There seem to exist strong UV radiation sources near these clouds. The correlation between our H column densities and the E(B[V ) values 2 map of Schlegel et al. (1998) suggest that from the extinction the molecular clouds lie between the observed stars and the Sun. Using the CO survey of Keto & Myers (1986), we estimate the conversion factor between N(H ) and 2 W (12CO) to be X \ 4 ^ 2 ] 1019 cm~2 (K km s~1)~1 for CO the clouds in the line of sight to HD 94473. This research has made use of the NASA ADS Abstract Service and the Catalogue Service of the CDS, Strasbourg, France. We thank J. Black for providing H transition data 2 the premature in electronic format. We note with sadness passing of our colleague and friend Gerhard KraŽmer, a longtime participant in the ORFEUS project.

REFERENCES Barnstedt, J., et al. 1999, A&AS, 134, 561 Hubeny, I. 1988, Comput. Phys. Commun., 52, 103 Brown, T. M., Ferguson, H. C., & Davidsen, A. F. 1996, ApJ, 472, 327 Jenkins, E. B., Lees, J. F., van Dishoeck, E. F., & Wilcots, E. M. 1989, ApJ, Carruthers, G. 1970, ApJ, 161, L81 343, 785 Desert, F. X., Bazell, D., & Boulanger, F. 1988, ApJ, 334, 815 Jura, M. 1974, ApJ, 191, 375 Dixon, W. V., Hurwitz, M., & Bowyer, S. 1998, ApJ, 492, 569 ÈÈÈ. 1975, ApJ, 197, 581 Eddington, A. S. 1937, Observatory, 60, 99 Keto, E. R., & Myers, P. C. 1986, ApJ, 304, 466 ESA. 1997, The Hipparcos and Tycho Catalogues (ESA SP-1200 ; NoordKraŽmer, G., et al. 1990, in Observatories in Earth Orbit and Beyond, ed. wijk : ESA) Y. Kondo (Dordrecht : Kluwer), 177 Fruscione, A., Hawkins, I., Jelinsky, P., & Wiercigroch, A. 1994, ApJS, 94, Kurucz, R. L. 1992, in IAU Symp. 149, The Stellar Populations of Gal127 axies, ed. B. Barbuy & A. Renzini (Dordrecht : Kluwer), 225 Gautier, T. N., Fink, U., Tre†ers, R. R., & Larson, H. P. 1976, ApJ, 207, Lang, K. R. 1991, Astrophysical Data : Planets and Stars (New York : L129 Springer) Gould, R. J., & Salpeter, E. E. 1963, ApJ, 138, 393 Luhman, M. L., & Ja†e, D. T. 1995, Rev. Mexicana Astron. AstroÐs., 3, 109 Grewing, M., et al. 1991, in Extreme Ultraviolet Astronomy, ed. R. F. Magnani, L., & Onello, J. S. 1995, ApJ, 443, 169 Malina & S. Bowyer (New York : Pergamon), 437 Magnani, L., Onello, J. S., Adams, N. G., Hartmann, D., & Thaddeus, P. Hollenbach, D. J., & Salpeter, E. E. 1971, ApJ, 163, 155 1998, ApJ, 504, 290

258

RYU ET AL.

Morton, D. C. 1991, ApJS, 77, 119 Penprase, B. E. 1992, ApJS, 83, 273 Reach, W. T., Koo, B.-C., & Heiles, C. 1994, ApJ, 429, 672 Savage, B. D., Bohlin, R. C., Drake, J. F., & Budich, W. 1977, ApJ, 216, 291 Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 Sembach, K. R., Danks, A. C., & Savage, B. D. 1993, A&AS, 100, 107

Sembach, K. R., Savage, D., & Hurwitz, M. 1999, ApJ, 524, 98 Shull, J. M., & Beckwith, S. 1982, ARA&A, 20, 163 Snow, T. P., & Jenkins, E. B., 1977, ApJS, 33, 269 Spitzer, L., & Cochran, W. D. 1973, ApJ, 186, L23 van Dishoeck, E. F., & Black, J. H. 1986, ApJS, 62, 109 Wilson, R. W., Je†erts, K. P., & Penzias, A. A. 1970, ApJ, 161, L43