Abundances and Depletions of Neutron-capture Elements in the Interstellar Medium

Abstract

We present an extensive analysis of the gas-phase abundances and depletion behaviors of neutron-capture elements in the interstellar medium (ISM). Column densities (or upper limits to the column densities) of Ga ii, Ge ii, As ii, Kr i, Cd ii, Sn ii, and Pb ii are determined for a sample of 69 sight lines with high-and/or mediumresolution archival spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. An additional 59 sight lines with column density measurements reported in the literature are included in our analysis. Parameters that characterize the depletion trends of the elements are derived according to the methodology developed by Jenkins. (In an appendix, we present similar depletion results for the light element B.) The depletion patterns exhibited by Ga and Ge comport with expectations based on the depletion results obtained for many other elements. Arsenic exhibits much less depletion than expected, and its abundance in low-depletion sight lines may even be supersolar. We confirm a previous finding by Jenkins that the depletion of Kr increases as the overall depletion level increases from one sight line to another. Cadmium shows no such evidence of increasing depletion. We find a significant amount of scatter in the gas-phase abundances of Sn and Pb. For Sn, at least, the scatter may be evidence of real intrinsic abundance variations due to s-process enrichment combined with inefficient mixing in the ISM.The primary aim of our extensive search of the HST /STIS archive was the identification of sight lines showing significant absorption from As ii λ1263, Cd ii λ2145, Sn ii λ1400, and Pb ii λ1433. While these lines are the principal transitions used to study these species in diffuse interstellar clouds, they are only rarely seen in UV spectra (although Sn ii λ1400 is somewhat more common than the others). To construct our sample, we started by examining the sight lines analyzed by R11 in their survey of B ii absorption in the diffuse ISM. The absorption features sought in the present survey are expected to have similar strengths compared to B ii λ1362. Moreover, for each of the directions studied by R11, we have detailed knowledge of the line-ofsight component structure from moderately strong absorption lines of dominant ions such as O i λ1355. This facilitates the search for weak features from other dominant ions that are expected to exhibit similar absorption profiles. We also expanded our search by examining the available STIS data for all other sight lines in the HST archive with the necessary wavelength coverage, considering observations acquired using either the high-resolution gratings (E140H and E230H) or the medium-resolution gratings (E140M and E230M) of the STIS FUV and NUV Multi-Anode Microchannel Array (MAMA) detectors. We noted any sight lines that showed apparent absorption from one or more of the four principal lines of interest at a velocity similar to that of O i λ1355. This process resulted in 14 sight lines being added to our initial list of 55...

A Closer Look at Some Gas-phase Depletions in the ISM: Trends for O, Ge, and Kr versus , f(H2), and Starlight Intensity*

“…The result for A 3 {Kr, F * } is not as definitive as the determination of A Kr in the study by Ritchey et al (2018), who found that this coefficient differed from zero at the 2.8σ significance level. However, the preferred value of A 3 {Kr, F * } is identical to the value of A Kr listed in J09 and not far from that of Ritchey et al (2018). Our general finding for the strength of the depletions, as indicated by B 3 {Kr}, is less than all previous determinations.…”

“…The measurable detections of Kr absorption have a median outcome log(Kr/H tot ) = −8.85. This abundance level is higher than the general levels of around −9.0 reported by earlier investigators (Cardelli & Meyer 1997 ;Cartledge et al 2003Cartledge et al , 2008Ritchey et al 2018), and this disparity probably arises from a recognition in the present study of additional, but not easily apparent, absorptions spanned by the wider velocity intervals defined by the limits of the strongest line of Mg II, as discussed at the beginning of Section 3.1 and highlighted in Figure 2. On average, the earlier investigations may have missed about 30% of the total Kr, which may apply especially for lines of sight HD207308 HD165246 HD75309 HD191877 HD72648 HD147888 HD201345 HD137595 HD208440 HD104705 HD170740 HD115071 HD114886 HD144965 HDE303308 HD94493 HD203374 HD108639 HD99890 HD206267 HD25443 HD101190 HD69106 HD177989 HD209339 HD220057 HD122879 HD152590 HD207198 HD53975 HD219188 HD185418 HD152424 HD151805 HD109399 HD99857 HD152249 HD206773 HD198781 HD40893 HD52266 HD90087 HD91824 HD1383 HD124314 HD103779 HD15137 HD202347 HDE308813 HD97175 HD195455 HD195965 HD93129 HD91983 HD210839 CPD-59 4552 HD92554 HD93843 HD108 HD13268 HD148937 HD198478 HD210809 BD+53 2820 HD192035 HD207538 HDE232522 HD46223 HD99953 CPD-59 2603 HD66788 HD124979 HD93222 HD116781 HD224151 HD218915 HD35914 HD148422 HD41161 HD93205 HD116852 BD+35 4258 HD178487 HD100199 CPD-69 1743 HD168941 HD14818 HD3827 HD63005 HD88115 HD111934 HD13745 HD12323 HD179407 HD167402 HD115455 HD13841 HD89137 HD163522 spanning large distances across other spiral arms in the Galaxy.…”

“…The joint probability distributions are shown in 2-dimensional form in Figures 4 and 5, along with the marginalized outcomes shown as simple graphs. While the slope coefficients A 2 {O, F * } and A 2 {Kr, F * } are negative, as expected from earlier studies (J09, Ritchey et al 2018), their significance levels are poor. However, the negative trend for [Ge gas /H] against F * is clearly established.…”

“…The negative trend for [Ge gas /H] against F * is still significant, although its magnitude has decreased slightly from that expressed by the A 2 coefficient, and it seems much less steep than the relationships A Ge = −0.615 ± 0.083 and −0.371 ± 0.081 found by J09 and Ritchey et al (2018), respectively. It is probable that the diminished response to F * compared to the findings of the previous studies arises from the effects discussed above for Kr.…”

“…On average, the earlier investigations may have missed about 30% of the total Kr, which may apply especially for lines of sight HD207308 HD165246 HD75309 HD191877 HD72648 HD147888 HD201345 HD137595 HD208440 HD104705 HD170740 HD115071 HD114886 HD144965 HDE303308 HD94493 HD203374 HD108639 HD99890 HD206267 HD25443 HD101190 HD69106 HD177989 HD209339 HD220057 HD122879 HD152590 HD207198 HD53975 HD219188 HD185418 HD152424 HD151805 HD109399 HD99857 HD152249 HD206773 HD198781 HD40893 HD52266 HD90087 HD91824 HD1383 HD124314 HD103779 HD15137 HD202347 HDE308813 HD97175 HD195455 HD195965 HD93129 HD91983 HD210839 CPD-59 4552 HD92554 HD93843 HD108 HD13268 HD148937 HD198478 HD210809 BD+53 2820 HD192035 HD207538 HDE232522 HD46223 HD99953 CPD-59 2603 HD66788 HD124979 HD93222 HD116781 HD224151 HD218915 HD35914 HD148422 HD41161 HD93205 HD116852 BD+35 4258 HD178487 HD100199 CPD-69 1743 HD168941 HD14818 HD3827 HD63005 HD88115 HD111934 HD13745 HD12323 HD179407 HD167402 HD115455 HD13841 HD89137 HD163522 spanning large distances across other spiral arms in the Galaxy. Ritchey et al (2018) recognized the importance of using a strong absorption feature (in their case, the 1355Å line of O I) to define the velocity limits and gave specific examples where their results gave larger values of N (Kr I) than previous determinations that were based purely on the appearance of the weak line being measured.…”