GIANO Y-band spectroscopy of dwarf stars: Phosphorus, Sulphur, and Strontium abundances
E. Caffau, S. Andrievsky, S. Korotin, L. Origlia, E. Oliva, N. Sanna, H.-G. Ludwig, P. Bonifacio
aa r X i v : . [ a s t r o - ph . S R ] O c t c (cid:13) ESO 2018
Astronomy & Astrophysics
GIANO Y-band spectroscopy of dwarf stars: Phosphorus, Sulphur,and Strontium abundances ⋆ E. Ca ff au , , S. Andrievsky , , S. Korotin , L. Origlia , E. Oliva , N. Sanna , H.-G. Ludwig , P. Bonifacio GEPI, Observatoire de Paris, PSL Resarch University, CNRS, Universit´e Paris Diderot, Sorbonne Paris Cit´e, Place Jules Janssen,92195 Meudon, France Zentrum f¨ur Astronomie der Universit¨at Heidelberg, Landessternwarte, K¨onigstuhl 12, 69117 Heidelberg, Germany Department of Astronomy and Astronomical Observatory, Odessa National University, Isaac Newton Institute of Chile, OdessaBranch, Shevchenko Park, 65014, Odessa, Ukrain INAF, Osservatorio Astronomico di Bologna, Viale Ranzani 1, 40127 Bologna, Italy INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, ItalyReceived ...; Accepted ...
ABSTRACT
Context.
In recent years a number of poorly studied chemical elements, such as phosphorus, sulphur, and strontium, have receivedspecial attention as important tracers of the Galactic chemical evolution.
Aims.
By exploiting the capabilities of the infrared echelle spectrograph GIANO mounted at the Telescopio Nazionale Galileo, weacquired high resolution spectra of four Galactic dwarf stars spanning the metallicity range between about one-third and twice thesolar value. We performed a detailed feasibility study about the e ff ectiveness of the P, S, and Sr line diagnostics in the Y band between1.03 and 1.10 µ m. Methods.
Accurate chemical abundances have been derived using one-dimensional model atmospheres computed in local thermo-dynamic equilibrium (LTE). We computed the line formation assuming LTE for P, while we performed non-LTE analysis to derive Sand Sr abundances.
Results.
We were able to derive phosphorus abundance for three stars and an upper limit for one star, while we obtained the abun-dance of sulphur and strontium for all of the stars. We find [P / Fe] and [S / Fe] abundance ratios consistent with solar-scaled or slightlydepleted values, while the [Sr / Fe] abundance ratios are more scattered (by ± Conclusions.
We verified that high-resolution, Y-band spectroscopy as provided by GIANO is a powerful tool to study the chemicalevolution of P, S, and Sr in dwarf stars.
Key words.
Stars: abundances – Stars: atmospheres – Line: formation – Galaxy: evolution – Galaxy: disk – radiative transfer
1. Introduction
The infrared (IR) spectrograph GIANO (Oliva et al. 2012) ismounted at the Nasmyth focus of the 4 m Telescopio NazionaleGalileo (TNG) in La Palma. It observes in the range 950-2450 nm at a resolving power of 50000. We exploit the capa-bilities of this new facility to perform a feasibility study aimedat verifying the e ff ectiveness of high-resolution spectroscopy inthe Y band to derive reliable abundances of phosphorus and twoother elements, namely S and Sr, in Galactic dwarf stars.Phosphorus, with an atomic number of 15, is a light ele-ment, which is abundant in the Universe and essential for thelife as we know it on Earth. The systematic analysis of phos-phorus in Galactic stars is somewhat recent (see Ca ff au et al.2011; Roederer et al. 2014). In fact, the presence of P in the stel-lar atmospheres of F-G stars can be revealed by P i near infrared(IR) lines at about 1050 nm or by ultra-violet (UV) lines at about213 nm (Roederer et al. 2014), which have to be observed fromspace. An exhaustive review of the chemical evolution of phos-phorus and of its investigations in peculiar stars can be found inthe recent paper of Roederer et al. (2014). Send o ff print requests to : E. Ca ff au ⋆ Based on observations obtained with GIANO
Sulphur is an α -element that is e ff ectively produced in mas-sive stars at the final stage of their evolution (SNe of type II).Determination of its abundance in F-G-K stars relies on a lim-ited number of S i lines in the visual and near-IR spectral range.The situation with the lines available for measurements in thespectra of metal-poor stars becomes more complicated. Only thestrongest IR S i lines of the first (at about 920 nm) and third mul-tiplets are observable in the spectra of stars with metallicities[M / H] < − .
5. The lines of the first multiplet are very strongand also detectable in extremely metal-poor stars (Spite et al.2011), but they are often contaminated by telluric absorption.The lines of the third multiplet at 1045 nm are not so strong, butclear from telluric absorption and also very useful for abundancedetermination (Nissen et al. 2007; Ca ff au et al. 2007a) at metal-poor regimes. As shown by Korotin (2009), the NLTE e ff ectshave di ff erent influences on di ff erent S i lines.As for phosphorus, the systematic analysis of the chemicalevolution of sulphur is recent, but in the latest ten years this re-search has grown. For an updated vision on the status of S chem-ical investigations, see Matrozis et al. (2013).Strontium is an astrophysically interesting element, since itsabundance is often used as a measurement of the e ffi ciency of Here and elsewhere in the paper we adopt the multiplet numberingof Moore (1945)a ff au et al.: Phosphorus, Sulphur, and Strontium abundances from GIANO spectra the slow neutron capture process in the intermediate mass stars.Together with Y and Zr, strontium belongs to the peak of lights-process elements. In the visual part of the spectrum there areonly a few lines of Sr ii . Among them there are two resonant linesat 407.7 nm and 421.5 nm, and a subordinate line at 416.1 nm.All the lines in the visual part are blended to some extent.For instance, the wing of the resonant line at 407.7 nm is dis-torted by the La ii line at 407.73 nm, Cr ii line at 407.75 nm, andDy ii line at 407.79 nm. Another resonant line at 421.5 nm hasan even more distorted profile due to blending with a strongFe i line at 421.543 nm and molecular CN band. The subordinateline at 416.1 nm is situated in the red wings of the two strongFe i ii ii lines for the abun-dance determination is their weak sensibility to the abundancechange in stars of the solar metallicity (they are strong). In con-trast, IR Sr ii lines are free of these problems, they are practi-cally unblended. Nevertheless, as it was shown by Andrievskyet al. (2011) they are strongly a ff ected by the NLTE e ff ects.Depending upon the atmosphere parameters and metallicity, theNLTE corrections can achieve up to − . ff erent metallicities can be derived from theirnear-IR lines with the help of the NLTE analysis only (Spite etal. 2011; Andrievsky et al. 2011).We present the chemical analysis of phosphorus, sulphur,and strontium in four unevolved stars observed with GIANO.Two of these stars also have CRIRES spectra and we find agood agreement with the P abundance derived by Ca ff au et al.(2011). For sulphur, we compared the S abundance we derivedfrom the GIANO spectra to observations taken with SOPHIEand to previous analysis based on multiplets eight and six. Wefind a good agreement with the SOPHIE spectra, but not alwayswith previous analysis. For the analysis on Sr, we compared theresults based on the GIANO observations with ultraviolet linesin SOPHIE spectra and find a good agreement.
2. Observed spectra
The GIANO spectra of the four Galactic dwarfs were ac-quired during two technical nights on October 22 (HD 1355 andHD 22484) and 23 (HD 146 and HD 22484), 2013. GIANO is in-terfaced to the telescope with a couple of fibers mounted on thesame connector at a fixed projected distance on sky of 3 arcsec.We observed the science targets by nodding on fiber, i.e. targetand sky were taken in pairs and alternatively acquired on fiberA and B, respectively, for an optimal subtraction of the detec-tor noise and background. HD 1461 was observed by setting twopairs of AB exposures for a total on-source integration time of20 min, while for the other three stars (brighter) only one pairof AB spectra for a total exposure time of 10 min have been ac-quired. We also observed the hot (O6.5V) dwarf star HIP 029216as a telluric standard.From each pair of exposures, an (A-B) 2D-spectrum hasbeen computed. As a result of the image slicer, each 2D framecontains four tracks per order (two per fiber). To extract andwavelength-calibrate the echelle orders from the 2D GIANOspectra, we used the ECHELLE package in IRAF and some new,ad hoc scripts grouped in a package named GIANO TOOLS,which can be retrieved at the TNG WEB page . We used 2Dspectra of a tungsten calibration lamp taken in the daytime to http: // / instruments / giano / giano tools v1.2.0.tar.gz Table 1.
Infrared lines analysed in this work.
Element λ [nm] E low log g f [nm] vacuum / air [eV]P i / + . i / + . i / + . i / − . i / + . ii / − . ii / − . ii / − . map the geometry of the four spectra in each order and for flat-field purposes. A U-Ne lamp spectrum was used for wavelengthcalibration. We extracted the positive (A) and negative (B) spec-tra of the target stars and summed them together to get a 1Dwavelength-calibrated spectrum with the best possible signal-to-noise ratio (SNR).For the scientific purpose of this paper aimed at deriving theabundances of phosphorus, sulphur, and strontium, we focusedour analysis on the GIANO spectral orders between µ m. This band israther clean from telluric contamination and contains a numberof suitable transitions for the chemical species we are interestedin. In order to make a comparison of S and Sr abundancesderived from IR and optical spectra, we retrieved from theSOPHIE archive ( http://atlas.obs-hp.fr/sophie/ ) spec-tra observed at Observatoire de Haute Provence for HD 13555and HD 22484. For HD 13555, we retrieved 30 spectra observedin the high resolution (HR) mode of SOPHIE ( R ≈ / N ≈
700 at 550 nm. For HD 2483,we only retrieved two HR SOPHIE spectra observed on October23 and 26 2013 with exposure times of 200 s and 150 s, respec-tively. The S / N of the summed spectrum is about 300 at 550 nm.SOPHIE (Bouchy & Sophie Team 2006; Perruchot et al. 2008,2011) is an echelle spectrograph fiber-fed from the Cassegrainfocus of the 1.93 m telescope at Observatoire de Haute-Provence(OHP). It can work at high e ffi ciency (HE) or HR, correspondingto a resolving power of about 40 000 and 80 000, respectively.SOPHIE spectra have a wavelength range from 387.2 nm to694.3 nm. SOPHIE spectra are reduced by the Geneva pipeline,which also provides radial velocity.
3. Chemical abundance analysis
We analysed two P i lines of Mult. 1 at 1052 nm and 1058 nm,the three S i lines of Mult. 3, located at 1045 nm, and three linesof Sr ii located at 1003, 1032, and 1091 nm. The atomic data weused are shown in Table 1.For our four stars, we adopted the stellar parameters(T e ff / log g [Fe / H]) listed in Table 2 from Chen et al. (2002);Takada-Hidai et al. (2002); Gonz´alez Hern´andez et al. (2010).For each star, we computed a 1D-LTE model atmosphere withthe code ATLAS 12 in its Linux version (Kurucz 2005; Sbordoneet al. 2004; Sbordone 2005).Our sample of stars includes four unevolved stars. The starHD 10453 is an astrometric binary, the companion is 1.3 mag ff au et al.: Phosphorus, Sulphur, and Strontium abundances from GIANO spectra fainter and has moved from a distance of about 4” in 1820 to 50mas in 2011. This should not a ff ect the spectra. The broadeningin the spectrum seems much larger than that expected from theresolving power of 50 000 of the spectrograph. By investigatingthe P i and S i lines, we presume the star has a rotational velocityof at least 15km s − . According to Ammler-von Ei ff & Reiners(2012), the stellar rotation is definitely lower ( V sin( i ) = . ± . − ).We analysed the SOPHIE spectra of the stars HD 22484and HD 13555, which we used to confirm the S and Sr abun-dances we derived from GIANO spectra. For both stars, wefound a very good agreement with previous analysis. The de-rived stellar parameters ( T e ff / log g / [Fe / H], microturbulence) are5933 / / − .
17, 1.32 km / s for HD 22484 and 6470 / / − . / s for HD 13555. The di ff erence in [Fe / H] of about0.1 dex with respect to the previous analysis for the latter star canbe explained by the lower microturbulence of 1.47 km / s we de-rived, to be compared to 2.4 km / s by Takada-Hidai et al. (2002).HD 13555 shows a rotational velocity of about 10 km / s in itsSOPHIE spectrum, which is also confirmed by the GIANO spec-trum. We measured the equivalent width (EW) of the P i lines, by usingthe iraf task splot with a Gaussian profile for the line profilefitting. The two P i lines we could detect (at 1052 and 1058 nm)are blended with a Ni i and Si i line, respectively. According tothe strength of the lines and the stellar V sin i , we fitted the P i line alone, or took into account the close-by line at the sametime, using the deblending option of splot . The two P i linesare present in both order 72 and 73. We analysed both spectraand took as EW the average value. We used the code WIDTH(Kurucz 1993, 2005; Castelli 2005) to derive the P abundancefrom the EW values. The results are listed in Table 2.For HD 1461, we have previous observations with CRIRESand we find a good agreement between the abundances derivedfrom the CRIRES and GIANO observations (see Fig. 1).For star HD 13555, we did not use the order 72 to derive theEW of the line at 1053 nm because of the low SNR. The agree-ment with the results from the CRIRES spectra is not perfect(about 0.1 dex in P abundance) but is still good.The only P i line detectable in the spectrum of HD 10453 isthat at 1058 nm, but it is blended with a Si i line and no P abun-dance can be derived because of the relatively high rotation ofthe star. However, we can provide an upper-limit on A(P).The spectrum of HD 22484 is the best quality spectrum. Thetwo lines of P i give abundances in good agreement, A(P) = . ± .
04, but we decided to exclude the line at 1052 nm fromthe order 72 owing to the low SNR.As the uncertainty in A(P) we quadratically added the obser-vational uncertainty (the scatter of the abundance derived fromthe two lines added to the uncertainty in the EW measurement)to the systematic uncertainty related to uncertainty in the stellarparameters. We are aware of no NLTE study on phosphorus, norof the existence of any model atom. Ca ff au et al. (2007b) inves-tigated the granulation e ff ects in the case of the Sun and derivedvery tiny e ff ects on these weak lines. Image Reduction and Analysis Facility, written and supportedby the IRAF programming group at the National Optical AstronomyObservatories (NOAO) in Tucson, Arizona. http: // iraf.noao.edu / Fig. 1.
The two P i lines are shown (solid black line thin-ner and thicker for order 72 and 73, respectively) in the caseof HD 1461, in comparison with a synthetic spectrum withA(P) = We derive the sulphur abundance by fitting the S i lines of Mult. 3located at 1045 nm with NLTE profiles based on the ATLAS 12models. The S i lines of Mult. 3 are clearly detected in all fourstars (see Fig. 2) and we derived the S abundances reported inTable 2. The three S i lines have been fitted simultaneously sothat we cannot derive a line-to-line scatter. An uncertainty of0.13 dex takes the random (0.05 dex) and systematic (0.12 dex)uncertainties into account.We compared a synthetic spectrum with the S abundance de-rived from the third multiplet to the S i lines of multiplet sixin the SOPHIE spectra of each star and derived a very goodcorrespondence for all four stars. We also find a general goodagreement in the A(S) with previous determination based onMult. 6 and 8 for our stars except for the star HD 13555. For thisstar, the S abundance we derive from the third multiplet in theGIANO spectra is about 0.3 dex lower than the previous analy-sis by Takada-Hidai et al. (2002) based on Mult. 8. We use thesame stellar parameters, except for microturbulence, where theyderive 2.4 km / s and we adopt 1.5 km / s, which cannot explain thedi ff erence because a larger microturbulence would cause an evenlarger disagreement.Ca ff au et al. (2007a) investigated the granulation e ff ects onthe A(S) determination from Mult. 3, in the solar case. The 3Dcorrections happen to be of the order of 0.1 dex in the solar spec-trum. In the case of hotter stars, the 3D corrections are larger;e.g. for Procyon they became more than twice the solar case,while at lower temperature they are smaller, e.g. for a 5000 K ff au et al.: Phosphorus, Sulphur, and Strontium abundances from GIANO spectra Table 2.
Stellar parameters and phosphorus abundances of our programme stars and comparisons, when available, with the analysison the CRIRES spectra from (Ca ff au et al. 2011). Target T e ff log g [Fe / H] [S / H] Ref EW [pm] A(P) EW [pm] A(P)Crires Giano Crires Giano Crires Giano Crires GianoK 1053.2 1053.2 1058.4 1058.4HD 1461 5765 4.38 + . − .
05 G10 2.00 2.10 5.59 5.62 2.70 2.70 5.59 5.59HD 10453 6368 3.96 − . − .
29 C02 < . < . − . − .
25 T02 1.80 2.34 5.14 5.28 2.90 3.13 5.22 5.27HD 22484 5960 4.02 − . − .
28 T02 1.90 5.33 3.00 5.41The column “Ref” refers to the reference for the stellar parameters and [S / H], and corresponds to G10: Gonz´alez Hern´andez et al. (2010); T02:Takada-Hidai et al. (2002); C02: Chen et al. (2002).
Fig. 2.
The three S i lines of Mult. 3 are shown (solid black) in comparison with the best fit (dashed red). For display purposes, thespectra are vertically displaced.dwarf star model they became negligible (see Ca ff au et al. 2007afor details). We derived the Sr abundances by line profile fitting using NLTEsynthetic profiles, and the Sr abundances we derived are pre-sented in Table 2. In Fig. 3 we show the Sr ii lines for HD 13555.The Sr abundances derived from the best fit on the Sr ii linesin the GIANO spectra have been used to synthesise profiles tocompare to the Sr ii lines in the SOPHIE spectra, at 407.7, 416.1, 421.5, and 430.5 nm. The agreement is generally very good. Twoof our stars show a [Sr / Fe] of about + − .
1. This large scatter in [Sr / Fe] around solar metallicity is notunknown and consistent with what is found by Mashonkina &Gehren (2001).The uncertainty in the A(Sr) determination is of 0.12 dex. Itis related to the uncertainty in the line profile fitting (0.1 dex,mainly due to continuum determination) and systematic uncer-tainty related to uncertainty in the stellar parameters (0.05 dex).We investigated granulation e ff ects for these Sr ii linesby analysing three hydrodynamical models (solar model, ff au et al.: Phosphorus, Sulphur, and Strontium abundances from GIANO spectra Table 3.
Stellar parameters, sulphur, and strontium abundances of our programme stars.
Target T e ff log g [Fe / H] A(P) [P / Fe] A(S)
NLTE [S / Fe] A(Sr) [Sr / Fe]KHD 1461 5765 4.38 + .
19 5.60 + .
05 7.26 − .
09 3.07 + . − . < . < + .
28 6.81 + .
06 2.67 + . − .
27 5.28 − .
09 6.67 − .
22 2.82 + . − .
10 5.37 + .
01 7.01 − .
05 2.72 − . Fig. 3.
The three Sr ii lines for HD 13555 are shown (solid black)in comparison with the best fit (dashed red). For display pur-poses, the spectra are vertically displaced.5900 / / / / CO BOLD (Freytag et al. 2012).The 3D e ff ects are small for all three Sr ii lines in the solar model,lower than 0.08 dex. These e ff ects becomes larger (about 0.1 dexfor the line at 1003 nm and about 0.25 dex for the other two lines)for a model with parameters 5900 / / / / ii lines, in the case ofthe 5900 / / τ . In the case of the stronger 1032 nm line, thecontribution function shows a double peak. This also happensin the case of the Li i doublet at 670.7 nm as shown in Fig. 1 bySte ff en et al. (2010), where the authors state that in the case oflithium that NLTE e ff ects are large and this is evident by compar-ing the contribution function for the 3D-LTE and the 3D-NLTEsynthesis. We know that 1D-NLTE e ff ects for the Sr ii IR lines
Fig. 4.
The contribution function for the EW for two Sr ii lines inthe case of the CO BOLD model with parameters 5900 / /
4. Discussion and conclusions
In this work, we analysed the GIANO spectra in the Y band offour unevolved Galactic stars, spanning a metallicity range be-tween about one-third and twice the solar value, with the pur-pose of measuring accurate abundances of P, S, and Sr. Forthree out of four stars, we could derive P abundance, and forthe fourth an upper limit. For the two stars for which CRIRESspectra are available we find a concordance in the derived abun-dances. Phosphorus abundances from the GIANO spectra alsofit perfectly into the [P / Fe] versus [Fe / H], derived by Ca ff au etal. (2011) and plotted in Fig. 5, where [P / Fe] smoothly decreaseswith increasing stellar metallicity with solar-scaled values withinone σ at [Fe / H] ≥ α -elementand, as discussed in Cescutti et al. (2012), such a Galactic evo-lution of phosphorus can be explained with P mainly producedin core-collapse supernovae with a minor contribution from su-pernovae type Ia. However, the yields have to be increased by afactor of about three to fit the observed abundances. In Fig 5, theabundances derived by Roederer et al. (2014) from UV lines arealso shown (blue solid diamonds) for comparison. At its high-est metallicities, these measurements show a similar behaviourin [P / Fe] vs. [Fe / H] as ours, although the increase in [P / Fe] withdecreasing [Fe / H] has a steeper slope. At [Fe / H] < − .
5, [P / Fe]show a constant value around the solar-scaled value. Jacobsonet al. (2014) explain this behaviour as a buildup of P with in-creasing Fe (see their Fig. 2). From the sample of Roederer et al.(2014) and according to Jacobson et al. (2014), it is not so clear ff au et al.: Phosphorus, Sulphur, and Strontium abundances from GIANO spectra which is the enhancing factor for the yields needed to reproducethe observed trend.The GIANO Y-band S i lines of the third multiplet are par-ticularly useful for deriving S abundance in metal-poor stars be-cause they are strong, but not contaminated by strong telluricabsorption as are the stronger lines of the first multiplet. We de-rived S abundance from third multiplet for our stars with metal-licities in the − . < [Fe / H] < + .
19 range, and we find agood agreement when comparing synthetic profiles with the de-rived A(S) with weaker S i lines, e.g. those of the sixth multipletat optical wavelengths. We find [S / Fe] abundance ratios consis-tent with solar-scaled or slightly depleted values.Three mostly clean Sr ii lines are also present in the GIANOY-band spectra of dwarf stars. We derived Sr abundance fromthese IR lines and compared synthetic spectra computed with ourSr abundances to optical Sr ii lines in SOPHIE spectra, finding agood agreement. We find [Sr / Fe] abundance ratios scattered by ± ff erent diagnostic lines at optical wavelengths. Acknowledgements.
The project was funded by FONDATION MERAC. We ac-knowledge support from the Programme Nationale de Physique Stellaire (PNPS)of the Institut Nationale de Sciences de l’Universe of CNRS. SMA and SAK ac-knowledge the SCOPES grant No. IZ73Z0-152485 for financial support.
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