High-resolution CRIRES spectra of Terzan1: a metal-poor globular cluster toward the inner bulge
aa r X i v : . [ a s t r o - ph . S R ] D ec Astronomy&Astrophysicsmanuscript no. evalenti˙final c (cid:13)
ESO 2018May 20, 2018
High-resolution CRIRES spectra of Terzan 1: a metal-poor globularcluster toward the inner bulge ⋆ . E. Valenti , L. Origlia , A. Mucciarelli , and R.M. Rich European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Muenchen, Germany INAF - Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy e-mail: [email protected] University of Bologna, Physics & Astronomy Dept., Viale Berti Pichat 6-2, I-40127 Bologna, Italy Physics and Astronomy Bldg, 430 Portola Plaza Box 951547 Department of Physics and Astronomy, University of California atLos Angeles, Los Angeles, CA 90095-1547, USReceived .... ; accepted ...
ABSTRACT
Aims.
Containing the oldest stars in the Galaxy, globular clusters toward the bulge can be used to trace its dynamical and chemicalevolution. In the bulge direction, there are ∼
50 clusters, but only about 20% have been subject of high-resolution spectroscopicinvestigations. So far, the sample observed at high resolution spans a moderate-to-high metallicity regime. In this sample, however,very few are located in the innermost region ( R GC ≤ | l , b | ≤ ◦ ). To constrain the chemical evolution enrichment of theinnermost region of Galaxy, accurate abundances and abundance patterns of key elements based on high-resolution spectroscopy arenecessary. Here we present the results we obtained for Terzan 1, a metal-poor cluster located in the innermost bulge region. Methods.
Using the near-infrared spectrograph CRIRES at ESO / VLT, we obtained high-resolution (R ≈ r ≤ / or contamination by telluric lines, allowed accurate chemical abundancesand radial velocities to be derived. Results.
Fifteen out of 16 observed stars are likely cluster members, with an average heliocentric radial velocity of + ± / s andmean iron abundance of [Fe / H] = –1.26 ± α / Fe] abundance ratios, finding average valuesof [O / Fe] =+ ± / Fe] =+ ± / Fe] =+ ± / Fe] =+ ± Conclusions.
The α enhancement ( ≈ + . Key words.
Techniques: spectroscopic – stars: Population II – stars: abundances – Galaxy: globular clusters: individual: Terzan 1infrared: stars
1. Introduction
Observational and theoretical studies have long shown thatGalactic globular clusters (GCs) contain the oldest stars in theGalaxy. They represent an important tracer of the underlyingpopulation, therefore deriving accurate abundances and abun-dance patterns in GCs represents a crucial test for tracing theearly dynamical and chemical evolution of the Galaxy. The ele-mental abundance distributions and the abundance ratio of cer-tain critical elements, such as Fe-peak, CNO, and α -elements(i.e., those synthesized from α particles, such as O, Ne, Mg, Si,Ti, Ca and S) are particularly suitable for this purpose. Indeed,these elements are synthesized in stars of di ff erent masses, hencereleased into the interstellar medium on di ff erent timescales. Aparticularly useful abundance ratio is [ α / Fe]. Thanks to the timedelay in the bulk of Fe production relative to α -elements, the[ α / Fe] abundance ratio can be e ffi ciently used as a cosmic clock(see, e.g., McWilliam, 1997).An overall [ α / Fe] enhancement with respect to the solarvalue has been well established in the Galactic halo GCs pop-ulation for several years (see Gratton et al., 2004, for a generalreview), indicating a major enrichment by type II SNe on a shorttimescale. On the other hand, accurate abundance patterns are ⋆ Based on data taken at the ESO / VLT Telescope, within the observ-ing program 093.D-0179(A) only available for some GCs in the bulge direction. Within atotal population of about 50 GCs located in the bulge (Harris,1996, 2010 compilation), only about 20% have been subjectto spectroscopic investigation at moderately high spectral res-olution (R ≥ α / Fe] ratios are enhanced by a factor between2 and 3 over the whole range of metallicity spanned by the ob-served clusters. All these measurements indicate that the bulkof the bulge GC population probably formed from a gas that ismainly enriched by type II SNe and on a short timescale, beforesubstantial contribution of type Ia SNe took place.With the only exception of Terzan 4 (Origlia & Rich, 2004),which has a metallicity [Fe / H] = –1.6 dex, all the bulge GCs Fig. 1.
Left panel: SofI K-band image of the core region of Ter 1. The field of view shown in the map is ∼ ′ × ′ , north is up andeast on the left. The observed stars are numbered (cf. Table 1). Right panel: Observed K, (J-K) CMD, and mean RGB ridgeline ofTer 1 in the central ∼ / H] ≥ –1.0 dex. In this sample observed at high-resolution,there are very few clusters located in the very innermost bulgeregion (i.e., R GC ≤ | l , b | ≤ ◦ ).With the aim of understanding the chemical composition ofthe innermost stellar populations in the Galactic bulge, we usedCRIRES at the VLT (K¨aufl et al. , 2004) to measure the chemicalcomposition of a few GCs located toward the center of the bulge.Here we present the results for Terzan 1.Terzan 1 is a GC located close to the plane, with somewhatcontroversial reddening and distance estimates. An optical pho-tometric study with HST-WFPC2 by Ortolani et al. (1999) sug-gested E(B–V) = = = = ff erent ∆ E(B–V) = ∼ / H] = -1.1 dex (Valenti, Ferraro & Origlia, 2010). Low-resolution opti-cal spectroscopy of 11 giants that are likely members of Terzan 1which was obtained by Idiart et al. (2002, hereafter I02), sug-gests a [Fe / H] ≈ –1.3 and some [Mg / Fe] enhancement with re-spect to the solar ratio.
2. Observations and data reduction
To minimize the risk of contamination from bulge field stars, weselected 16 bright giants (8 ≤ K ≤ .
5) using the near-IR CMDby Valenti, Ferraro & Origlia (2010) in the innermost region (atr ≤ ≈
1” . We used CRIRES in non-AO mode and selected a slit widthof 0.4” providing a spectral resolution R ≈ / order / order ff ecting the four detectors, only portionsof the spectra in the ranges: 1.547-1.553 µ m , 1.557-1.635 µ m ,1.575-1.581 µ m , 1.585-1.590 µ m , and 1.595-1.601 µ m were ac-tually usable.The integration time, on source and per setting, was 20 min.The data was reduced by using the CRIRES pipeline , whichperforms sky subtraction using the pair of spectra obtained whenthe object is nodded along the slit and flat-fielding correction.Wavelength calibration was performed using a ThAr lamp anda second-order polynomial solution. Because of the gaps withinthe four CRIRES detectors and the relatively narrow wavelengthrange covered by each setting, the derived wavelength solutionusing the ThAr lamp strongly depends upon the number and dis-tribution of available lines within a given detector. Therefore,to check and refine the pipeline wavelength solutions, we cross--correlated (using the IRAF task fxcor ) the spectra of an O starobserved for telluric correction before or after the targets, with ahigh-esolution spectrum of the Earth’s telluric features retrievedfrom the ESO web page . The cross-correlation between thetemplate and the O-type star spectra was performed over a re-gion around 1.577 µ m where several telluric features are present. http: // / sci / software / pipelines http: // / sci / facilities / paranal / decommissioned / isaac / tools / spectroscopic standards.html2alenti et al.: CRIRES spectra of Terzan 1 Fig. 2.
CRIRES spectra centered on some lines of interest of two giant stars ( ff erent temepratures. The coolest giant ( e f f = e f f =
3. Chemical abundance analysis
To measure the chemical abundances of the RGB stars inTerzan 1 from the CRIRES spectra, we performed the sameanalysis as in our previous works on bulge GC and field gi-ants (see Valenti, Origlia & Rich, 2011; Rich, Origlia & Valenti,2012, and references therein).We made use of full spectral synthesis techniques and theequivalent width measurements of selected lines, which weresu ffi ciently isolated, free of significant blending and / or contami- nation by telluric absorption. Telluric absorptions were carefullychecked on an almost featureless O-star spectrum.To compute suitable synthetic spectra and modelthe observed giant stars, we used an updated version(Origlia, Rich & Castro, 2002) of the code first describedin Origlia, Moorwood & Oliva (1993). The code uses the LTEapproximation, which is based on the molecular blanketedmodel atmospheres of Johnson, Bernat & Krupp (1980) at tem-peratures ≤ Table 1.
Stars toward Terzan 1 observed with CRIRES and se-lected from Valenti, Ferraro & Origlia (2010) catalog.
ID J H K RA (J2000) DEC (J2000)1 11.947 10.564 10.140 263.9564516 -30.48296332 11.796 10.392 9.963 263.9568131 -30.48268903 11.648 10.250 9.838 263.9574730 -30.48033504 10.542 9.088 8.585 263.9430320 -30.48808505 10.478 - 8.575 263.9457667 -30.47337976 11.618 10.237 9.792 263.9413450 -30.47920407 12.018 10.618 10.179 263.9416433 -30.48073818 10.783 9.294 8.826 263.9394140 -30.47805409 11.565 10.113 9.638 263.9401400 -30.476868010 11.450 10.028 9.593 263.9473110 -30.474285011 11.498 10.123 9.715 263.9451520 -30.475838012 10.398 8.960 8.444 263.9504970 -30.480883013 10.485 9.020 8.504 263.9495090 -30.483232014 10.356 8.913 8.398 263.9460670 -30.478106015 10.191 8.688 8.245 263.9480840 -30.478563016 11.466 10.045 9.492 263.9573380 -30.4793950 sitions from the Kurucz database , from Biemont & Grevesse(1973), and from Melendez & Barbuy (1999). Moleculardata are from our compilations (Origlia, Moorwood & Oliva,1993, and subsequent updates) and from B. Plez (privatecommunication). The reference Solar abundances are fromGrevesse & Sauval (1998).An initial guess of the temperature and gravity of the ob-served stars has been derived from their near-IR photometry,while for the microturbulence velocity we adopted an averagevalue of 2.0 km / s (see also Origlia et al., 1997). These photo-metric estimates of the stellar parameters were used as inputto produce a grid of model spectra that span a wide range interms of abundances and abundance patterns, while keeping thestellar parameters (temparature and gravity) around the photo-metric values. The model that reproduces the overall observedspectrum and the equivalent widths of selected lines better waschosen as the best fit model. Equivalent widths were computedby Gaussian fitting the line profiles, with an overall uncertainty ≤ CO bandhead by means of fullspectral synthesis. A few, quite faint CN lines are also present inthe spectra of the observed stars but the signal-to-noise is notadequate to get reliable nitrogen abundances.For each observed star, Table 2 lists the final adopted pho-tospheric parameters, the measured radial velocity, and chemi-cal abundances. In particular, the tabulated star temperature andgravity values are photometric, but spectroscopically fine-tunedto get simultaneous spectral fitting of the CO bandhead, the OHmolecular lines, and the atomic lines observed in the CRIRESspectra. The impact of using slightly di ff erent assumptions forthe stellar parameters on the derived abundances is discussed inSect. 3.1. However, it is worth mentioning that since the CO andOH molecular line profiles are very sensitive to e ff ective tem-perature, gravity, and microturbulence variations, they reliablyconstrain the values of the phtospheric parameters, therefore sig-nificantly reducing their initial range of variation and ensuring http: // / amp / ampdata / kurucz23 / sekur.html a good self-consistency of the overall spectral synthesis proce-dure (Origlia, Rich & Castro, 2002; Origlia & Rich, 2004). Asa clear example, Fig. 2 shows the CRIRES spectra centered onsome lines of interest of two giant stars ( ff erent temperatures. The coolestgiant ( e f f = e f f = We can quantify random and systematic errors in the measure-ment of the equivalent widths and in the derived chemical abun-dances as follows. The typical random error of the measured lineequivalent widths is 10%, arising mostly from a ±
2% uncertaintyin the placement of the pseudo-continuum, as estimated by over-lapping the synthetic and the observed spectra. Such random un-certainties in the line equivalent width measurements correspondto abundance variations ranging from a few hundredths to 1 tenthof a dex. This ≤ σ scatter in the derived abundances from di ff erent lines, whichnormally ranges between 0.1 and 0.2 dex. The errors quoted inTable 2 for the final abundances are obtained by dividing these1 σ errors by the squared root of the number of used lines. Whenonly one line is available, a 0.1 dex value has been adopted.Most of the systematics arise from varying the adopted stel-lar parameters. In order to properly quantify them, we gener-ated a grid of test models with varying the stellar parameterswithin ±
200 K in temperature (T e f f ), ± ± − in microturbulence velocity ( ξ ), andthe abundances by ± -0.1-0.2 dex accordingly, to reproduce theline depths. As a figure of merit of the statistical test we adoptthe di ff erence between the model and the observed spectrum. Toquantify systematic discrepancies, this parameter is more power-ful than the classical χ test, which is instead equally sensitive torandom and systematic scatters (see Origlia & Rich, 2004, formore discussion and references therein).All these alternative solutions turn out to be somewhat lessstatistically significant (typically at 1 ≤ σ ≤ > ff erent sensitivity, rela-tive abundances are less dependent on stellar parameter assump-tions, reducing the systematic uncertainty to <
4. Results and discussion
For all of the 16 stars observed with CRIRES, Table 2 lists themeasurement of the radial velocities and abundances. The typ-ical random error in the radial velocity estimate is ≈ / s.Fifteen out of the 16 stars show heliocentric radial veloci-ties consistent with being clustered around an average valueof +
57 km / s with 1 σ dispersion of 7 km / s. The 15 starswith similar radial velocities also show very similar iron abun-dances, with an average [Fe / H] = -1.26 ± σ dis-persion of 0.10 ± α / Fe] abundance ratios, finding average valuesof [O / Fe] =+ ± σ = ± / Fe] =+ ± σ = ± / Fe] = + ± σ = ± / Fe] = + ± σ = ± α / Fe] abundanceratios as a function of [Fe / H]. These values of [ α / Fe] are fullyconsistent with those normally observed in the bulge stars: anenhancement of a factor of ∼ − Fig. 3. [ α / Fe] abundance ratios as a function of [Fe / H] for the 15 stars likely cluster members. The big dots indicate the averagevalues and 1 σ dispersions.up to about solar metallicity. We also measured carbon abun-dances, finding an average [C / Fe] = -0.18 ± σ dis-perison of 0.08 ± / Fe] abun-dance ratio with respect to the solar value is normally observedin low-mass giant stars that are brighter than the RGB bump.As shown in Fig.4, there is no evidence for any specific trendbetween the measured radial velocity and metallicity or distancefrom the cluster center, making all the 15 stars reasonable clus-ter member candidates. Only one star ( ff erent (by -104 km / s) from theaverage value of +
57 km / s shown by the 15 other stars, thus mak- ing it a candidate field star. Moreover, this star also has a metalcontent that is higher by about a factor of 3.Eight stars in our sample (see Tab 2) have previously beenobserved by I02 at low resolution (R ≤ λ − λ − ×
2’ image taken with the LNA 1.6 m telescope. Themembership of the eight stars in common based on derived ra-dial velocities is consistent between the two studies with the onlyexception of star
Fig. 4.
Heliocentric radial velocities for the 15 stars likely cluster members as a function of metallicity (left panel) and distance formthe cluster center (right panel). The big dot in the left panel indicates the average value and 1 σ dispersion.field star. However, in this case there might be a mismatch inthe cross-identification of the observed star owing the presenceof a nearby second source, which is clearly resolved in our SofIimage (see Fig. 1), even better in the CRIRES acquisition cam-era, which has a resolution of 0.047” / pixel, but not as evidentin the optical image shown in Fig. 1 of I02. Finally, because theposition of star ff er-ent stars. In contrast to what can be seen in our near-IR map, inthe optical image of I02 star + ±
14 km / s, which is ∼
60 km / shigher than our estimate based on a comparable sample. Whenwe consider all the stars in common, but ff erence of 69 km / s in the derived velocities and 1 σ disper-sion of 19 km / s. Our result is more consistent with a previousestimate based on integrated spectroscopy (Armandro ff & Zinn,1988), which reported a mean velocity of +
35 km / s although, ad-mittedly, the integrated spectrum could have been heavily con-taminated by the presence of bright field sources in the center.After carefully checking the derived wavelength calibrationof CRIRES spectra (see §
2) we can rule out any systematic shiftof ∼
70 km / s in radial velocity. On the other hand, it could bethat at much lower resolution, a possible mismatch of spectralfeatures produces a substantial shift in the radial velocity mea-surements.Our result is instead in good agreement with Vasquez et al.(in prep., private communication), who derive a mean radial ve-locity of +
65 km / s based on data obtained with FORS2 in multi-slit mode (R ≈ ff erence in terms of spectral resolutionbetween this work and I02, the metallicity estimates for thestars in common are reasonably consistent, within ∼ ≤ ff erence.
5. Conclusions
We have obtained high-resolution infrared spectroscopy of 16giants in the heavily obscured Galactic GC Terzan 1. Based onthe derived radial velocity estimates, 15 out of 16 observed gi-ants turned out to be candidate cluster members. We measureda cluster mean heliocentric radial velocity of +
57 km / s with 1 σ dispersion of 7 km / s. This value is significantly less than whatis reported in the Harris (1996) catalog based on the I02 study,but it is in good agreement with the most recent low-resolutionoptical estimate by Vasquez et al. (in prep.)From our analysis of the candidate cluster members wefind average [Fe / H] = -1.26 ± / Fe] = + ± / Fe] = + ± / Fe] = + ± / Fe] = + ± α / Fe] enhancement measured in Terzan 1 is fully con-sistent with those measured in other bulge clusters observedin near-IR by our group and in optical by other teams (seeValenti, Origlia & Rich, 2011, and references therein), as well aswith the estimate found in field giants located in the innermostbulge fields (Rich, Origlia & Valenti, 2012).The derived iron content measured on Terzan 1 giants placesthis cluster on the metal-poor tail of the observed metallic-ity distribution function of the bulge field population (seeZoccali et al., 2008; Gonzalez et al., 2011; Johnson et al, 2011,2013; Ness et al., 2013, and reference therein). Additionally, theobserved α -enhancement makes Terzan 1 chemically similar tothe cluster and field bulge metal-poor (i.e., [Fe / H] ∼ Acknowledgements.
EV warmly thanks the telescope operators and night as-tronomers of Paranal working on UT1, for their competence and dedicationwhile doing observations for this program.
Table 2.
Chemical abundances of the stars toward Terzan 1 observed with CRIRES.
ID T e ff lg g V r [Fe / H] [C / Fe] [O / Fe] [Mg / Fe] [Si / Fe] [Ti / Fe] ID-I02 a +
60 -1.45 ± ± + ± + ± + ± +
61 -1.45 ± ± + ± + ± + ± +
51 -1.07 ± ± + ± + ± + ± +
67 -1.30 ± ± + ± + ± + ± + ± +
53 -1.34 ± ± + ± + ± + ± + ± +
65 -1.26 ± ± + ± + ± + ± + ± +
56 -1.25 ± ± + ± + ± + ± +
73 -1.25 ± ± + ± + ± + ± + ± ± ± + ± + ± + ± +
61 -1.29 ± ± + ± + ± + ± + ± +
50 -1.24 ± ± + ± + ± + ± + ± +
53 -1.26 ± ± + ± + ± + ± + ± +
50 -1.21 ± ± + ± + ± +
47 -1.18 ± ± + ± + ± + ± +
58 -1.22 ± ± + ± + ± + ± + ± +
54 -1.20 ± ± + ± + ± + ± + ± a Stars in common with I02 sample. The star reference identification is taken from their Table 3.
RMR acknowledges support from grants AST-1212095, AST-1413755from the National Science Foundation.The authors thank Ivo Saviane and Sergio Velasquez for sharing informationon their results on Terzan 1 radial velocity measurements that will be presentedin a forthcoming paper (Vasquez et al., in preparation).NSO / Kitt Peak FTS data used here were produced by NSF / NOAO.
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