Herzberg Institute of Astrophysics, National Research Council, 5071 W. Saanich Road, Victoria, BC, V8X 4M6 Canada. AND. D. G. YORK. University of Chicago ...
THE ASTROPHYSICAL JOURNAL, 449 : L135–L138, 1995 August 20 q 1995. The American Astronomical Society. All rights reserved. Printed in U.S.A.
INTERSTELLAR LEAD 1 D. E. WELTY University of Chicago, Astronomy and Astrophysics Center, 5640 S. Ellis Avenue, Chicago, IL 60637
L. M. HOBBS University of Chicago, Yerkes Observatory, Williams Bay, WI 53191-0258
J. T. LAUROESCH University of Chicago, Astronomy and Astrophysics Center, 5640 S. Ellis Avenue, Chicago, IL 60637
D. C. MORTON Herzberg Institute of Astrophysics, National Research Council, 5071 W. Saanich Road, Victoria, BC, V8X 4M6 Canada AND
D. G. YORK University of Chicago, Astronomy and Astrophysics Center, 5640 S. Ellis Avenue, Chicago, IL 60637 Received 1995 April 25; accepted 1995 June 14
ABSTRACT The spectrum of 1 Sco has been recorded in the region of the 1434 Å line of Pb II at S/N 5 220, using the ECH-A mode of the HST/GHRS. Absorption by interstellar Pb II is seen with an equivalent width W l 5 0.3 H 0.2 (2 s) mÅ, at a radial velocity which agrees satisfactorily with that of the strongest component of the interstellar Na I D 1 line seen in high-resolution optical spectra. If a theoretical oscillator strength f 5 0.865 is adopted, the resulting logarithmic depletion is D 5 20.97 (10.22, 20.48) with respect to the meteoritic abundance of Pb. This depletion, which is consistent with Cardelli’s result toward z Oph, is stronger than that expected from the low condensation temperature, T c 5 496 K, of Pb, in light of the general correlation between the depletions and condensation temperatures of 29 other elements. Some previous ideas about the formation and the evolution of interstellar grains are briefly considered. Subject headings: ISM: abundances — ISM: atoms — ISM: clouds — dust, extinction — nuclear reactions, nucleosynthesis, abundances density of interstellar hydrogen is about N(H) 5 1.6 3 10 21 cm 22 , the Pb II line would be expected to attain an equivalent width near W l 5 2.8 mÅ if lead were undepleted. The relatively low condensation temperature of lead, T c 5 496 K (Wasson 1985), suggests that only modest depletion of this element may occur. A positive detection of the 1434 Å line, or even a sufficiently stringent upper limit on its strength, would usefully extend the study of gaseous interstellar abundances to the sixth row of the periodic table. In particular, the depletions of all of the stable elements in a single column (C, Si, Ge, Sn, and Pb) of the periodic table, which should show crudely similar chemical behavior, could be intercompared for the first time. Such comparisons may eventually provide critical insights into the processes which determine the various elemental depletions from the interstellar gas (Barlow 1978). In cycle 2 we were awarded a small amount of Director’s discretionary time, to search for Pb II toward 1 Sco, one of the stars in whose spectra we had identified interstellar Sn II absorption. The Pb II program was not actually implemented until the latter part of cycle 4; the purpose of this Letter is to report those results. Meanwhile, Cardelli (1994) has successfully measured the interstellar 1434 Å line in the spectrum of z Oph at a strength W l 5 0.44 H 0.30 (2 s) mÅ, a value substantially weaker than that for the case of negligible depletion, as indicated above. (We will cite 2 s errors throughout this paper.) Our observations of 1 Sco allow a comparison of the abundances of gaseous lead along these two distinct light paths through the interstellar gas.
1. INTRODUCTION
The combination of the Hubble Space Telescope (HST) and the Goddard High-Resolution Spectrograph (GHRS) has allowed the discovery of a number of elements heavier than zinc (Z . 30) in the interstellar gas. Generally very weak interstellar absorption lines of gallium, germanium, krypton, and tin, which reflect the low cosmic abundances of these heavy elements with atomic numbers Z 5 31, 32, 36, and 50, respectively, have been measured in the spectra of up to six stars (see, e.g., Cardelli, Savage, & Ebbets 1991; Hobbs et al. 1993, hereafter Paper I). The resulting estimates of the depletions of these heavy elements from the interstellar gas into the dust grains have provided valuable new information about how and where the depletion occurs (Paper I; Cardelli 1994; York 1994). At Z 5 82, lead has the highest cosmic abundance among the elements heavier than barium (Z . 56) (Anders & Grevesse 1989). Pb II has an ionization potential of 15.0 eV and should be the principal ionization stage of gaseous lead in cold, neutral interstellar clouds; this first ion also has an intrinsically strong resonance line at 1433.905 Å (Wood et al. 1974), with a theoretical oscillator strength f 5 0.865 (Migdalek 1976; Cardelli et al. 1993). In the spectra of bright stars such as z Oph and 1 Sco, toward which the total column 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
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FIG. 2.—Spectrum of 1 Sco near 1434 Å, after removal of the broad stellar photospheric lines. The abscissae give heliocentric radial velocities for the Pb 21 II line; the detected Pb II feature is at 25.4 km s .
FIG. 1.—Average of four ECH-A subexposures (lower), after small relative shifts have been applied to the respective wavelength zero-points and after each subexposure has first been divided by the properly shifted, fixed instrumental pattern (upper), as described in the text. The resulting data were smoothed numerically to achieve proper sampling. The vertical tick mark (near pixel 471) locates the deepest absorption feature, which is identified with Pb II. 2. OBSERVATIONS AND DATA REDUCTION
Our previous GHRS observations of eight weak interstellar lines of Cu II, Ga II, Ge II, Kr I, or Sn II in the spectrum of 1 Sco were displayed and analyzed in Paper I. The Pb II observations, identified as HST program 5725, were carried out with the GHRS in the ECH-A mode and through the large science aperture (LSA) for a total exposure time of 24 minutes. The exposure was divided into a number of subexposures in order to achieve proper sampling of the data, to reduce the effects of the spatial sensitivity variations over the photocathode of the Digicon detector, and to correct partially for the sensitivity variations among the 512 diodes of the detector. Thus, the instrumental parameters STEP-PATT 5 7, FP-SPLIT 5 DSFOUR, and COMB 5 4 were selected (Soderblom 1993). Each of the subexposures obtained at the four slightly different grating tilts dictated by the FP-SPLIT parameter covered a spectral interval of about 7.5 Å. A resolution near Dl 5 0.021 Å (FWHM), which corresponds to l/Dl 5 6.8 3 104 or a velocity resolution Dv 5 4.4 km s21 , was delivered by the echelle and the LSA at 1434 Å. The primary steps in the data reduction were carried out using both the POFFSETS, DOPOFF, and SPECALIGN tasks in the STSDAS package and similar programs which we have independently developed. The subexposures obtained at the four different grating tilts were summed, after small corrections (=half a resolution element) were made to the zero-points of three of the respective default wavelength scales in order to achieve the best mutual alignments of the various stellar lines present. In order to improve the final S/N ratio, a simultaneous, iterative solution for both the stellar spectrum and the ‘‘fixed’’ instrumental pattern (or flat-field pattern) was determined, by intercomparing the four subexposures. In practice, each subexposure was divided by the derived fixed pattern before being added to the other subexposures. Owing to the magnetic deflections of the individual subexposures along the diode array which are used to correct for the Doppler shifts caused by the telescope’s orbital motion, shifts of the fixed pattern by a few pixels from one subexposure to another also had to be determined during this process. The resulting fixed pattern (granularity) and the final average of the four corrected subexposures in the region of the Pb II line are shown in Figure 1, with approximately proper sampling of the data.
Finally, the relatively broad stellar lines defining the local ‘‘continuum’’ near the Pb II line were divided by polynomial fitting functions to yield the normalized spectrum shown in Figure 2. The measured S/N ratio is near 220. This value is in satisfactory agreement with the scatter expected from photon noise alone and indicates that the effects of the fixed instrumental pattern have been effectively removed. The zero point adopted for the absolute wavelength scale is the default one. 3. RESULTS
No narrow absorption line as strong as W l 5 1 mÅ is seen anywhere in this echelle exposure. The deepest such feature shows W l 5 0.3 H 0.2 (2 s) mÅ and lies at 1433.879 Å, or a heliocentric radial velocity of 25.4 km s 21 , if the feature is identified with Pb II (Fig. 2). The 2 s uncertainty in the equivalent width takes into account both the photon noise (Jenkins et al. 1973) and the uncertainty in the continuum placement (Sembach, Savage, & Massa 1991; Savage, Cardelli, & Sofia 1992), added in quadrature. Given typical uncertainties in the default GHRS wavelength scale of about one resolution element, the measured radial velocity agrees satisfactorily with that of the strongest Na I line component at v r 5 28.6 km s 21 seen in ground-based spectra of 1 Sco obtained at a much higher instrumental resolution of FWHM 5 0.5 km s 21 (Welty, Hobbs, & Kulkarni 1994). The apparent Pb II line has a narrow width of only about 2 pixels, which is indistinguishable from the instrumental resolution. Thermal broadening of a Pb II line amounts to only FWHM 5 0.13 km s 21 at 80 K, while the high-resolution Na I profile yielded an upper limit b , 0.81 km s 21 , or FWHM , 1.35 km s 21 , upon the turbulent broadening of the strongest Na I component seen toward 1 Sco. If this limit which obtains for a minor stage of ionization applies also to Pb II, the measured line width should indeed be strongly dominated by the instrumental broadening of FWHM 5 4.4 km s 21 , as is observed. The corresponding column density N(Pb II) 5 (1.9 H 1.3) 3 1010 cm22 yields a fractional gaseous abundance N(Pb)/N(H) 5 (1.2 H 0.8) 3 10211 and a logarithmic depletion D 5 20.97 (10.22, 20.48), where D 5 log {[N(Pb)/N(H)]/[N(Pb)/N(H)]m}. The column density N(H) 5 1.58 3 1021 cm22 toward 1 Sco was taken from Bohlin, Savage, & Drake (1978), and the comparison abundance adopted is the meteoritic value, [N(Pb)/N(H)]m 5 1.12 3 10210 (Anders & Grevesse 1989). The depletion of lead along the light path to 1 Sco agrees, within the mutual uncertainties, with Cardelli’s (1994) comparably precise result for z Oph, D 5 20.71 (10.24, 20.50). An unweighted average of the Pb depletions toward 1 Sco and z Oph is plotted in Figure 3, which shows the variation of
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FIG. 3.—Variation of interstellar depletion with condenstion temperature for 30 elements, in an average dense cloud as defined by Jenkins (1987). Except for the new results for Pb and for As, Se, Tl, and V (Cardelli 1994) and for B and Co (Federman et al. 1993), the sources of the data were enumerated in Paper I. Circles show average depletions derived from observations of at least three different stars; triangles, those from fewer than three stars. Squares identify depletions inferred indirectly, from observations of minor stages of ionization.
the interstellar depletions of 30 elements with condensation temperature T c . The depletions plotted are those found in an ‘‘average’’ dense cloud line of sight with average density ^n(H)& 5 N(H)/d 5 3 cm 23 (Jenkins 1987), similar to ^n(H)& for both 1 Sco and z Oph, where d is the distance to the star in question. Except for the new results for Pb and also for As, Se, Tl, and V (Cardelli 1994) and B and Co (Federman et al. 1993), the figure was presented and discussed in some detail in Paper I. The observed depletion of Pb is more severe than is expected from the general trend seen in the figure, although several other elements show even larger departures from this loose correlation. The principal conclusion reached here, that Pb is more depleted along the light paths to both 1 Sco and z Oph than is expected from its condensation temperature, is unchanged if our 3 s detection of the Pb II line is alternatively regarded as the upper limit W l (1434) , 0.3 mÅ. Our conclusion depends on the reliability of the f -value adopted for the observed Pb II transition, for which there is no experimental check. A decrease of 0.2 dex in the oscillator strength would bring the upper error bar in Figure 3 into agreement with the mean values of the elements on either side, while a decrease of 0.4 dex would bring the mean Pb II value itself into line. Although the two independent calculations for the 2 P 1/ 2 – 2 D 3/ 2 transition agree closely, an error as large as 0.2– 0.3 dex is conceivable. Here we shall assume that the depletion of lead remains greater than expected from its condensation temperature. 4. DISCUSSION
Recent observational and theoretical studies of interstellar gas and dust seem consistent with a picture of the formation and evolution of dust grains in which (1) grain cores consisting of graphite, silicate, and perhaps metallic oxide are formed in stellar outflows; (2) in moderately dense interstellar clouds, the cores can acquire mantles, which consist of various elements; (3) in regions of lower density, shocks can remove the mantles, primarily via sputtering, and vaporize some cores; and (4) there is continuous cycling of gas and dust between regions of higher and lower density (see, e.g., Draine 1990; Jones et al. 1994; Sofia, Cardelli, & Savage 1994). The relatively high spectral resolution and S/N available in UV
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FIG. 4.—Depletions of 18 elements along the light paths to 1 Sco and z Oph. For both stars, the results shown refer to the total column densities, summed over many clouds (.6) having different properties. For a few elements (Zn; perhaps P and Si) additional possible systematic errors associated with line saturation are not included in the 2 s error bars.
spectra obtained with the GHRS are now yielding more accurate abundances for the main grain constituents (Sofia et al. 1994); abundances for additional, less abundant elements (see, e.g., Paper I; Federman et al. 1993; Cardelli et al. 1993); and, together with even higher resolution optical spectra, abundances and physical conditions, such as temperatures, densities, and radiation fields, for individual interstellar clouds in various environments (see, e.g., Welty et al. 1996a). These new data can provide detailed tests for the various proposed processes which may affect the evolution of interstellar dust. Barlow (1978) investigated reactions between hydrogen and various other elements which might occur on the surfaces of existing graphite and silicate grain cores. For example, he suggested that the formation of saturated hydrides XH n , where X is some metal and n 5 1– 4, on grain surfaces in moderately dense clouds could prevent depletion of X if the energy released in molecule formation were greater than the total adsorption energy of the constituents, equal to E ads (X) 1 nE ads (H). Given the estimated adsorption energies, only Ca, Ti, and Fe should stick to silicate cores; all other elements should desorb as monohydrides, which then photodissociate in the gas phase. Fewer elements would avoid depletion onto graphite cores, however. Zn and the alkali elements Na, K, Rb, and Cs could escape via the reaction XH 1 H 3 X 1 H 2 , while B could desorb after the formation of BH 3 . The elements Si, Ge, Sn, and Pb, all in the same column of the periodic table, should form tetrahydrides, with net energy release of 3.0, 2.3, 1.3, and 0.3 eV, respectively, on graphite cores. Barlow thus suggested that Si, Ge, and Sn should not be incorporated into grain mantles (onto either silicate or graphite cores), while Pb, heavier and with smaller net energy release, might be somewhat depleted onto graphite grains. Abundances of Si, Ge, Sn, and Pb, as well as many other elements, are shown in Figure 4 for the lines of sight to z Oph and 1 Sco (Welty et al. 1996b). These two lines of sight have similar total hydrogen column densities and depletions which generally agree within the mutual 2 s errors for the lightly depleted elements with D . 21.0. The line of sight to z Oph, however, shows much higher abundances of various molecular and trace neutral atomic species, as well as more severe depletions for several elements with D , 21.5, results which are perhaps indicative of denser gas and/or a lower ambient UV radiation field. In both cases, Si is less depleted than most
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other elements of comparable condensation temperature T c , such as Cr, Fe, and Ni. The depletions of Ge and Sn are more similar to those of other elements of comparable T c , including B, F, S, and Zn, but Pb seems to be somewhat more depleted, despite its even lower T c . These observed depletions may be consistent with the picture advanced by Barlow, in that the depletions of Si, Ge, and Sn may reflect the presence of silicate grain cores, while the more severe depletions of such elements as Cr, Fe, Ni, and Pb may indicate that some mantling has occurred. 2 If this picture is correct, then Pb should show less depletion where Cr, Fe, and Ni are less depleted. When comparing depletions of different elements and attempting to evaluate possible grain processing models, it is necessary to have accurate and appropriate reference abundances for each element. The reference abundances generally adopted are those measured for the solar photosphere and/or for C1 chondritic meteorites (see, e.g., Anders & Grevesse 1989). While the most well determined photospheric and meteoritic abundances are usually in quite good agreement, there are a few elements for which the differences are greater than the estimated errors. In addition, as more accurate abundance determinations become available for stars other than the Sun, some differences from solar abundances may be noted (see, e.g., Gies & Lambert 1992), perhaps indicating that a single ‘‘cosmic’’ abundance standard may not exist for all elements. Sofia et al. (1994) have suggested that the B star abundances of Gies and Lambert may provide a better reference for N and O than the solar abundances, but that the solar abundance may be more appropriate for C. [See related comments by Federman et al. 1993 for B and by Cardelli 1994 for Kr.] For Pb, Anders and Grevesse give a photospheric abundance of 1.85 H 0.05 and a meteoritic abundance of 2.05 H 0.03, on the usual logarithmic scale for which the hydrogen abundance is 12.00. While Grevesse & Meyer (1985) found no obvious explanation for this apparently significant difference, the photospheric abundance rests essentially on a single Pb I line. Adoption of the photospheric Pb abundance would not itself be sufficient to bring the observed Pb depletion into agreement with that of other elements of comparable Tc , however. The main isotopes of Pb are thought to be produced primarily via the s-process, with about 30% due to the main s-process and the remaining 70% due to the so-called strong s-process, which has been invoked to account for the observed 2 We note that Sofia et al. (1994) have attributed the Si depletion to a combination of silicate grain cores and mantling of Si onto an additional population of Fe and/or metallic oxide cores.
solar/meteoritic abundances of nuclei with atomic weights between 204 and 209 (see, e.g., Cameron 1982; Kappeler et al. 1982). Since the abundances of Kr, which is produced primarily by the weak s-process, and Sn, due primarily to the main s-process, are both within a factor of 2 of solar toward both z Oph and 1 Sco (Fig. 4; Paper I), elements produced by the weak and main components of the s-process are not likely to have subsolar total abundances in those directions. If products of the strong s-process were somehow less abundant in the interstellar clouds toward z Oph and 1 Sco, however, then Pb would actually be less depleted into dust than we have inferred, and the Pb depletions determined for other lines of sight with different strong s-process enrichment would show a lack of correlation with the depletions of other elements. Since the site of the strong s-process is thought to be the same as that for the main s-process, this scenario seems unlikely, however. 5. SUMMARY
Along the light path to 1 Sco, we find that Pb is more depleted than is expected from its condensation temperature, in agreement with Cardelli’s (1994) conclusion for z Oph. Additional observations of Pb II absorption, especially in lines of sight with smaller overall depletion, may distinguish among three possibilities: (1) A close correlation between Pb depletion and the depletions of typically heavily depleted elements such as Fe, Ti, Cr, and Ni would be consistent with the incorporation of those elements in grain mantles. (2) A uniform, apparently substantial depletion of Pb similar to that inferred toward 1 Sco and z Oph could indicate neglible actual depletion, coupled with an error in either the oscillator strength of the 1434 Å line or the reference abundance adopted for Pb. Experimental confirmation of the adopted oscillator strength and determination of additional stellar Pb abundances would be valuable. (3) Significant variations in Pb depletion which are uncorrelated with the depletions of other elements could indicate incomplete mixing of different nucleosynthetic products in the local interstellar medium. It is a pleasure to acknowledge several valuable discussions with Jim Truran about s-process nucleosynthesis. Support for this work was provided by NASA through grant number GO-2251.01-87A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS526555.
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