Detection of Pb II in the Ultraviolet Spectra of Three Metal-Poor Stars
Ian U. Roederer, James E. Lawler, Erika M. Holmbeck, Timothy C. Beers, Rana Ezzeddine, Anna Frebel, Terese T. Hansen, Inese I. Ivans, Amanda I. Karakas, Vinicius M. Placco, Charli M. Sakari
DDraft version October 1, 2020
Typeset using L A TEX twocolumn style in AASTeX63
Detection of Pb II in the Ultraviolet Spectra of Three Metal-Poor Stars ∗ Ian U. Roederer,
1, 2
James E. Lawler, Erika M. Holmbeck,
4, 2
Timothy C. Beers,
5, 2
Rana Ezzeddine,
6, 2
Anna Frebel,
7, 2
Terese T. Hansen,
8, 9
Inese I. Ivans, Amanda I. Karakas,
11, 12
Vinicius M. Placco,
13, 2 andCharli M. Sakari Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE), USA Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623, USA Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA02139, USA George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX77843, USA Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA School of Physics and Astronomy, Monash University, VIC 3800, Australia ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) NSF’s Optical-Infrared Astronomy Research Laboratory, Tucson, AZ 85719, USA Department of Physics and Astronomy, San Francisco State University, San Francisco, CA 94132, USA (Accepted for publication in the Astrophysical Journal Letters)
ABSTRACTWe report the first detection of the Pb ii line at 2203.534 ˚A in three metal-poor stars, using ultravioletspectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope .We perform a standard abundance analysis assuming local thermodynamic equilibrium (LTE) to derivelead (Pb, Z = 82) abundances. The Pb ii line yields a higher abundance than Pb i lines by +0.36 ± ± i lines had beendetected previously. The Pb ii line is likely formed in LTE, and these offsets affirm previous calculationsshowing that Pb i lines commonly used as abundance indicators underestimate the Pb abundancein LTE. Pb is enhanced in the s -process-enriched stars HD 94028 ([Pb/Fe] = +0.95 ± ± Pb is the dominant Pb isotope in thesetwo stars. The log ε (Pb/Eu) ratio in the r -process-enhanced star HD 222925 is 0.76 ± r -process ratio and indicates that the Solar System r -process residuals forPb are, in aggregate, correct. The Th/Pb chronometer in HD 222925 yields an age of 8.2 ± r -process-enhanced stars. Keywords:
Nucleosynthesis (1131); R-process (1324); S-process (1419); Stellar abundances (1577);Ultraviolet astronomy (1736)
Email: [email protected] ∗ Based on observations made with the NASA/ESA
Hubble SpaceTelescope , obtained at the Space Telescope Science Institute(STScI), which is operated by the Association of Universitiesfor Research in Astronomy, Inc. (AURA) under NASA contractNAS 5-26555. These observations are associated with programsGO-14161, GO-14765, and GO-15657. This paper includes datataken at The McDonald Observatory of The University of Texasat Austin. INTRODUCTIONThe element lead (Pb, Z = 82) has fascinating nucle-osynthetic origins. Three of the four stable Pb isotopes( Pb,
Pb, and
Pb) and the one stable bismuth(Bi, Z = 83) isotope ( Bi) serve as both the high-mass termination point of the slow n -capture process( s -process) and the low-mass termination point of ac- a r X i v : . [ a s t r o - ph . S R ] S e p Roederer et al. tinide α -decay chains of radioactive isotopes producedin the rapid n -capture process ( r -process).Pb and Bi accumulate during the s -process as n -capture and α -decay reactions cycle indefinitely (e.g.,Burbidge et al. 1957). Clayton & Rassbach (1967) rec-ognized that most Pb in the Solar System could notform through the r -process or a “smooth extension ofthe [ s -process] circumstances attending the synthesis for A <
Pb, which sits atboth the Z = 82 proton shell closure and the N = 126neutron shell closure, and its n -capture cross section isnearly an order of magnitude smaller than that of neigh-boring nuclei (cf. Ratzel et al. 2004). Observations con-firm enhanced Pb and Bi abundances, relative to lighter s -process elements, in many s -process-enriched metal-poor stars (e.g., Van Eck et al. 2001; Aoki et al. 2002;Ivans et al. 2005).Pb is also a remarkable element in r -process nucle-osynthesis because it is the final decay product for mostisotopes heavier than Bi, including the three long-lived isotopes of the actinides thorium (Th, Z = 90)and uranium (U, Z = 92): Th,
U, and
U, whichdecay to
Pb,
Pb, and
Pb, respectively. Theactinide elements can only be produced by r -process nu-cleosynthesis, yet our understanding of actinide produc-tion remains incomplete. Pb abundances in metal-poorstars can bridge this gap in our understanding. Morethan 85% of Pb in old, metal-poor stars enhanced in r -process elements is formed through the decay of ra-dioactive nuclei with A >
209 (Cowan et al. 1999), soPb abundances provide an important constraint on theproduction of these isotopes (e.g., Schatz et al. 2002;Wanajo et al. 2002; Eichler et al. 2019).A few studies (e.g., Plez et al. 2004) have attemptedto constrain model predictions by assessing the r -processcontribution to Pb abundances, but that work has beenlimited by observational uncertainties. Only a few linesof Pb i are detectable in the optical spectrum. The mostcommonly-used line, at 4057.807 ˚A, is often weak andblended. Furthermore, Pb is mostly ( (cid:38) i lines (Mashon-kina et al. 2012). These observational challenges canbe overcome by detecting Pb in its dominant ionizationstate, singly-ionized Pb.In this Letter, we examine new and archival ultravi-olet (UV) spectra of three metal-poor stars taken withthe Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope . These spectra showthe Pb ii line at 2203.534 ˚A, which is the only Pb ii tran-sition accessible in near-UV, optical, or near-infraredspectra of late-type stars. This line has been observed previously in the spectra of a few chemically-peculiar A-type stars (e.g., Faraggiana 1989; Cowley et al. 2016).Previous attempts (Roederer et al. 2014b) to detect thePb ii line in STIS E230M spectra ( R ≡ λ/ ∆ λ = 30,000)of metal-poor stars have been unsuccessful. Here, wepresent the first detection of this Pb ii line in metal-poor stars. OBSERVATIONS AND STELLAR SAMPLESTIS spectra of only three metal-poor stars yield com-pelling detections of the Pb ii line at 2203.534 ˚A. Thesestars were selected for observations over the years be-cause they are bright, metal-poor, and in two cases showextreme enhancements of n -capture elements. UV spec-tra obtained with STIS (Woodgate et al. 1998) cover thePb ii line at 2203 ˚A with the high resolving power ( R = 114,000) of the E230H grating, as summarized in Ta-ble 1. Archival observations of the star HD 94028 weredownloaded from the Mikulski Archive for Space Tele-scopes (MAST). Two new sets of observations of thestars HD 196944 and HD 222925 were also downloadedthrough the MAST and processed automatically by theCALSTIS software package. All spectra were shifted toa common velocity, co-added, and continuum normal-ized using IRAF. The signal-to-noise (S/N) ratios perpixel in the co-added spectra are listed in Table 1. Thesemodest S/N ratios are sufficient to detect the lines of in-terest because of the high resolving power.HD 94028 shows moderate levels of enhancement ofboth r -process and s -process elements (Roederer et al.2016; Peterson et al. 2020; see Table 1). Roederer et al.also found evidence that an intermediate n -capture pro-cess ( i -process) may contribute to some of the Z < i line at 2833.054 ˚A and concludedthat the s -process dominates the origin of the Pb inHD 94028. There is no evidence from radial velocity(RV) measurements that HD 94028 is in a binary sys-tem.HD 196944 is one of the original “Pb stars” iden-tified by Van Eck et al. (2001). Numerous studiesover the years have confirmed that HD 196944 is acarbon-enhanced metal-poor star enhanced in elementsproduced by the s -process (CEMP- s star; e.g., Zaˇcset al. 1998; Placco et al. 2015). The Pb abundance inHD 196944 has been derived previously from Pb i linesat 2833 and 4057 ˚A. HD 196944 exhibits RV variations,and Placco et al. calculated an orbital period of 1325 ±
12 d.HD 222925 is a member of the class of highly r -process-enhanced, or r -II, stars (as defined in Holm-beck et al. 2020). All heavy elements in HD 222925were produced via r -process nucleosynthesis (Roedereret al. 2018a). Pb had not been detected previouslyin HD 222925, but Roederer et al. placed a tight up-per limit on the Pb abundance from the Pb i line at etection of Pb ii in Metal-Poor Stars Table 1.
Log of Observations, Model Atmosphere Parameters, Metal-licities, Ba, Eu, and Pb Abundances
Quantity HD 94028 HD 196944 HD 222925Prog. ID GO-14161 GO-14765 GO-15657PI Peterson Roederer RoedererData Sets OCTKB0010-6030 OD5A01010-14010 ODX901010-60030 V mag T eff (K) 6087 ±
84 (1) 5170 ±
100 (2) 5636 ±
103 (3)log g [cgs] 4.37 ± ± ± v t (km s − ) 1.10 ± ± ± V t (km s − ) 1.6 ± ± ± − ± − ± − ± − ± − ± − ± ε (Ba) 1.06 ± ± ± ε (Eu) − ± − ± ± ε (Pb) 1.34 ± ± ± ± ± ± ± − ± ± ± ± ± Note —We have rederived the barium (Ba, Z = 56; 3 optical Ba ii lines), europium(Eu, Z = 63; 2 optical Eu ii lines), and Pb (1 UV Pb i line) abundances inHD 94028 using our adopted model atmosphere and a high-resolution opticalspectrum obtained using the Tull Coud´e spectrograph on the 2.7 m Harlan J.Smith Telescope at McDonald Observatory (see Roederer et al. 2014a for details).These model parameters and the Ba and Eu log ε abundances are in agreementwith those derived by Peterson et al. (2020). References are indicated by thenumbers in parentheses: (1) Roederer et al. (2018b); (2) Placco et al. (2015); (3)Roederer et al. (2018a); (4) This study; (5) Roederer et al. (2008); (6) Roedereret al. (in preparation). ANALYSIS3.1.
Atomic Data
There are four stable isotopes of Pb:
Pb (1.4% inSolar System material),
Pb (24.1%),
Pb (22.1%),and
Pb (52.4%). Of these isotopes,
Pb has nonzeronuclear spin, I = 1 /
2, and thus has hyperfine splitting(HFS) structure. The field shift, which results from thevolume difference between nuclei with the same num-ber of protons but different numbers of neutrons, alsocreates isotope shifts (IS). We adopt the ground leveland excited level HFS A values and the IS measure-ments from Bouazza et al. (1986) for the Pb ii lineat 2203 ˚A. We adopt the atomic transition probabil-ity from Quinet et al. (2007), log( gf ) = − ± gf ) = − Model Atmospheres
We adopt the model parameters (effective tempera-ture, T eff ; log of the surface gravity, log g ; microturbu-lent velocity, v t ; model metallicity, [M/H]) derived previ-ously for these three stars, for consistency. HD 94028 isa main sequence dwarf, while HD 196944 and HD 222925are red horizontal branch stars. We interpolate modelatmospheres from the 1D, α -enhanced ATLAS9 grid ofmodels (Castelli & Kurucz 2004). Our synthetic spectraalso include a macroturbulent velocity ( V t ) component,which improves the fits to the high-resolution E230Hspectra. We derive V t by fitting the observed profilesof isolated lines of Fe-group elements. These values arelisted in Table 1. 3.3. Pb Abundances
Figure 1 illustrates the Pb ii line in the spectrum ofeach of the three stars. Continuum regions around thisline are easily identified, and they are matched by thesynthetic spectra. We are confident that the absorptionat 2203.534 ˚A is due to Pb ii for several reasons. The linestrength varies with the expected heavy-element abun-dances in these stars, not the abundances of iron-groupelements that are responsible for most UV absorptionlines. This line is—by many orders of magnitude—thestrongest Pb ii line with λ > ii lines could be detectable.Furthermore, no other plausible species are found atthis wavelength in the NIST ASD or the Kurucz (2011)line lists. Unidentified lines at 2203.427 and 2203.645 ˚Acould be explained by Co ii and V i transitions, respec-tively, only if the log( gf ) values recommended by theNIST ASD or the Kurucz (2011) linelists are underesti-mated by several dex. We treat their strengths as freeparameters in our analysis, and this choice does not in-fluence the derived Pb abundances.The Pb ii line is on the linear part of the curve-of-growth in HD 94028 and HD 222925, but it is saturatedin HD 196944. We derive abundances using the 2017version of the LTE line analysis software MOOG (Sne-den 1973; Sobeck et al. 2011). We adopt an s -processmix of Pb isotopes (Sneden et al. 2008) for HD 94028and HD 196944 and an r -process mix for HD 222925.We generate the line list based on Kurucz (2011), Pe-terson et al. (2017), and the NIST ASD. We match thesynthetic spectra to the observed spectra following thegeneral methods described by Roederer et al. (2012).Table 1 lists the derived abundances. The Pb abun-dance is defined as log ε (Pb) ≡ log ( N Pb /N H )+12.0.The abundance ratio of elements Pb and Fe rel-ative to the Solar ratio is defined as [Pb/Fe] ≡ log ( N Pb /N Fe ) − log ( N Pb /N Fe ) (cid:12) , where log ε (Pb) (cid:12) =2.04 and log ε (Fe) (cid:12) = 7.50. Following Roederer et al.(2018a), we compute 1 σ uncertainties by drawing 10 resamples of the stellar parameters, log( gf ) values, andequivalent widths approximated from the abundance Roederer et al.
Table 2.
Hyperfine Structure and Isotope Shifts for the Pb ii Line at 2203 ˚AWavenumber λ air F upper F lower Component Position Component Position Strength Isotope(cm − ) (˚A) (cm − ) (˚A)45367.486 2203.5342 0.5 1.5 − − − − − − Note —Energy levels from the NIST ASD and the index of air are used to compute the center-of-gravity wavenum-bers and air wavelengths, λ air , for a Solar System isotopic composition (Meija et al. 2016). Line componentpositions are given relative to those values. The strengths of each component are easily adjustable using Table 2because a Solar System abundance pattern has not been assumed, and strengths are normalized to sum to 1for each isotope. For example, the log( gf ) value of the Pb component with F upper = F lower = 1.0 in a SolarSystem mix with f = 0.221 would be log (0 . × . × − . ) = − derived via synthesis using a reverse curve-of-growthmethod. 3.4. The Pb Isotope Mix
The Pb isotope mix has not been assessed previouslyin any metal-poor star. The HFS of the
Pb isotope,particularly the upper level of the line at 2203 ˚A, andthe IS of the four Pb isotopes are wide compared tothe width of the stellar line profiles shown in Figure 2.As the isotope mix shifts from the r -process, where the Pb isotope (35.9%) and wide HFS of the
Pb iso-tope (45.1%) are expected to dominate, to an s -processmix, where the Pb isotope is expected to dominate(69.5%), the absorption line profile narrows and shiftsto shorter wavelengths.The observed Pb ii line profiles in HD 94028 andHD 196944 both favor a narrower profile, suggestingthat an even- A isotope dominates. We evaluate whichisotope it might be by comparing the line centroid to theisotope wavelengths. We set the local wavelength zero-point of the observed spectrum relative to the syntheticspectra using three relatively unblended Fe ii lines, withwavelengths known to better than 0.0011 ˚A (Nave &Johansson 2013), located in the same echelle order thatcontributes most of the signal to the Pb ii line. We alsoaccount for uncertainty in the center-of-gravity wave-length of the Pb ii line (0.0007 ˚A; Wood et al. 1974)and measurement uncertainties in the Fe ii and Pb ii line centroids. As shown in Figure 2, the centroid of theobserved Pb ii line in HD 94028 and HD 196944 favorsabsorption by the isotopes situated farthest to the blue, Pb and
Pb. Our method of setting the wavelengthzeropoint using Fe ii lines appears to have produced asmall mismatch between the observed and synthetic line profiles in HD 94028 and HD 222925. Manual adjust-ment ( ≤ σ ) of the observed spectrum in each case sothat it matches the blue side of the line, which is rela-tively insensitive to the Pb isotope mix, still favors the Pb and
Pb isotopes.We conclude from these two tests—the narrow lineprofiles and the positions of the line centroids—that the
Pb isotope is dominant in HD 94028 and HD 196944.The S/N is too low to draw any conclusions aboutHD 222925. DISCUSSION4.1.
Neutral Pb and Non-LTE Effects
The abundances derived from the Pb ii line are higherthan the abundances or upper limits derived from thePb i lines in all three stars: [Pb ii /Pb i ] = +0.36 ± ± > +0.04 in HD 94028, HD 196944,and HD 222925, respectively. Mashonkina et al. (2012)computed non-LTE corrections to the LTE abundancesfor several metal-poor atmospheres. That study foundthat the lower levels of the Pb i lines at 2833 and 4057 ˚Aexperience similar deviations from LTE. The groundstate of singly-ionized Pb is well-described by LTE intheir calculations. We assume that the excited 6 s p level that gives rise to the Pb ii line at 2203 ˚A is alsowell-described by LTE. The Mashonkina et al. non-LTEcorrections for red giants (dwarfs) range from +0.26 to+0.62 dex (+0.22 to +0.32 dex) when using a Drawinscaling factor, S H , of 0.1, which relates to the strength ofinelastic collisions with neutral hydrogen. No models intheir grid exactly match the stars in our sample, but theclosest models predict non-LTE corrections ≈ +0.27 and+0.52 dex for HD 94028 and HD 196944, respectively,which match the offsets we derive in LTE. We support etection of Pb ii in Metal-Poor Stars N o r m a li z e d F l u x Pb II
Cr IICo II Co II Fe I Co II(?) Cr II Fe II
HD 94028 N o r m a li z e d F l u x Pb II
Cr IICo II Co II Fe I Co II(?) V I(?) Cr II Fe II
HD 196944 N o r m a li z e d F l u x Pb II
Cr IICo II Co II Fe I Co II(?) V I(?) Cr II Fe II
HD 222925
Figure 1.
Sections of the STIS/E230H spectra ofHD 94028, HD 196944, and HD 222925 around the Pb ii line at 2203 ˚A. The filled dots represent the observed spec-trum. The solid lines represent a synthetic spectrum with thebest-fit abundance in each star, and the gray bands repre-sent a change in this abundance by a factor of ± the conclusion of Mashonkina et al. that departures fromLTE impact Pb abundances derived from Pb i lines.Pb is often used to constrain the s -process or i -processmodels used to explain nucleosynthesis patterns in stars(e.g., Hampel et al. 2019). A change in [Pb/Fe] by+0.4 dex is significant and could affect the final neutronexposure inferred from models, which sets, for example,the estimated timescale for an i -process event. Futurework should incorporate non-LTE corrections to abun-dances derived from Pb i lines or derive Pb abundancesin LTE directly from the UV Pb ii line.4.2. Pb in the s-process
Our LTE results confirm the enhanced Pb abun-dances in the s -process-enhanced stars HD 94028 andHD 196944. We derive [Pb/Ba] = +1.05 ± s -process model of Bisterzo et al. (2010) discussed atlength in Placco et al. (2015) (see also Abate et al. 2015).We derive [Pb/Ba] = +0.42 ± N o r m a li z e d F l u x ( r/s )HD 94028 s -processSolar r -process N o r m a li z e d F l u x HD 196944(CEMP- s ) s -processSolar r -process N o r m a li z e d F l u x HD 222925( r -II) s -processSolar r -process Figure 2.
Tight zoom around the Pb ii line. ThePb abundance is held fixed and the isotope mixes are var-ied. The blue solid line represents an s -process isotope mix( f /f /f /f = 0.025/0.143/0.137/0.695) from Sne-den et al. (2008), the yellow dashed line represents the Solarisotope mix (0.014/0.241/0.221/0.524) (Meija et al. 2016),and the red studded line represents an r -process isotope mix(0.000/0.359/0.451/0.190) from Sneden et al. The filled dotsrepresent the observed spectrum. The shaded gray box ineach panel represents the ± σ line centroid of the Pb ii linein the observed spectrum. The isotope mixes inferred by ouranalysis (Section 3.4) are insensitive to the initial isotopemixes assumed for each star (Section 3.3) once the abun-dances have been fixed. supports the interpretation of Roederer et al. (2016)that the Ba and Pb in HD 94028 originated mainlyvia the s -process. Our isotopic analysis reaffirms the-oretical predictions that the large Pb overabundancesin low-metallicity s -process environments are dominatedby Pb. 4.3.
Pb in the r-process
The log ε (Pb/Eu) ratio in HD 222925, 0.76 ± r -process ratio, log ε (Pb/Eu)= 0.76 ± r -process residuals for the Pb isotopes are, in aggregate,correct when the effects of low-metallicity AGB stars(i.e., the so-called “strong component”) are included. Roederer et al.
Clayton & Rassbach (1967) argued that the dominant r -process isotopes of Pb must be Pb and
Pb. ThePb ii line centroid in HD 222925 is not in conflict withthis reasoning, although the S/N is too low in our spec-trum to support a more definitive statement.The close coupling between Th and Pb enables theuse of Th/Pb as a chronometer pair that is relativelyinsensitive to the details of the r -process model used tocalculate the initial production ratio. HD 222925 doesnot exhibit a prominent actinide boost (Roederer et al.2018a), and its log ε (Th/Pb) ratio is − ± ε (Th/Pb) ratio in HD 222925 corresponds to an ageof 8.2 ± ii line in future ob-servations would improve the age precision. Pb linesare easier to detect in metal-poor stars than the U ii line at 3859 ˚A, and the Th/Pb chronometer offers analternative model-insensitive age indicator to the U/Thchronometer in r -process-enhanced stars.ACKNOWLEDGMENTSWe thank E.A. Den Hartog for useful discussions andthe referee for a quick and helpful report. I.U.R., J.E.L.,T.C.B., A.F., and V.M.P. acknowledge support pro- vided by NASA through grants GO-14765 and GO-15657 from STScI, which is operated by the AURAunder NASA contract NAS5-26555. I.U.R., T.C.B.,R.E., A.F., E.M.H., and V.M.P. acknowledge finan-cial support from grant PHY 14-30152 (Physics Fron-tier Center/JINA-CEE) awarded by the U.S. NationalScience Foundation (NSF). We acknowledge additionalsupport from NSF grants AST-1716251 (A.F.) and AST-1815403 (I.U.R.). T.T.H. acknowledges generous sup-port from the George P. and Cynthia Woods Institutefor Fundamental Physics and Astronomy at Texas A&MUniversity. Parts of this research were supported by theAustralian Research Council Discovery Project scheme(DP170100521) and Centre of Excellence for All SkyAstrophysics in 3 Dimensions (ASTRO 3D), throughproject number CE170100013. This research has madeuse of NASA’s Astrophysics Data System BibliographicServices; the arXiv pre-print server operated by CornellUniversity; the SIMBAD and VizieR databases hostedby the Strasbourg Astronomical Data Center; the ASDhosted by NIST; the MAST at STScI; and the ImageReduction and Analysis Facility (IRAF) software pack-ages. Facility:
HST (STIS), Smith (Tull Coud´e)
Software:
IRAF (Tody 1993), matplotlib (Hunter2007), MOOG (Sneden 1973), numpy (van der Walt et al.2011), R (R Core Team 2013)REFERENCES
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