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arXiv:1105.6123v1 [astro-ph.SR] 30 May 2011 ltdtene o eprsre ie orahfitrmag fainter reach to aimed survey deeper sti a o BAO for results at need The conducted the survey up. ulated spectroscopic follow first of the kind by (BAO) this tained Observatory for examples Astrophysical typical Byurakan are the at conducted lumino . the are sequence having main halo object a close the of a in sign of 2001, a giants indicative has motion distant fraction proper al. negligible way non et this a Christlieb but In discovered, 1998, usually 2004). al. al. et et Totten Downes (eg. bands fea obje Swan absorption galactic C2 characteristic the as h showing identified spectra as their then ”flagged” by and initially candidates contaminants QSO of m redshift FHLCs fraction the QSO, small of search a to those aimed are to surveys similar in QSOs; very numerous are more colours their spe surveys. that the spectroscopic interest seldom or are photometric dedicated they of reason target this for therein); references h an ihlttd abnsas(HC R , (FHLCs carbon latitude high faint The Introduction 1. aedniyls hn00 deg 0.06 than less density indica distribution face spatial observed The objects. rare are bevtr n .3 eecp fteAig Observatory. Asiago the of telescope 1.83m and Observatory eie lsicto fteSScro trcniae inc candidates star carbon SBS the of classification Revised coe ,2018 8, October ⋆ Astrophysics & Astronomy h icvre frdfitsasfo h rs surveys prism the from stars faint red of discoveries The ae nosrain aea h .2 eecp fteBolo the of telescope 1.52m the at made observations on Based in ye h orhcniae B 1310 SBS candidate, fourth The type. giant msinlnsi t pia pcrm eetmtdabsolu estimated We spectrum. optical its in lines emission 1 2 3 e words. Key iciiat o CH for discriminants Conclusions. Results. Methods. wit objects nearby howev of halo; Aims. percentage Galactic increasing the an of revealed structure has the probe to used often Context. eevdFbur 8 01;accepted ; 2011 28, February Received tr n ofimdtecro aueo h eann orst four remaining the of nature carbon the confirmed and stars eateto hsc,Uiest aSpez,Paz A.Mo Piazza Sapienza, e-mail: La University Physics, of Department e-mail: Armenia, province, Aragatzotn 0213, Observatory(BA Astrophysical Byurakan Ambartsumian A. V. reinSaePdggclUivriyAtrK.Aoyna Abovyan Kh. e-mail: After Uniiversity Pedagogical State Armenian h icvr fanweiso iedafcro star carbon dwarf line emission new a of discovery the ocaiytentr ftetncro trcniae prese candidates star carbon ten the of nature the clarify To evrfidta w tr r N are stars two that verified We [email protected] an ihlttd abnsasaerr bet omnyt commonly objects rare are stars carbon latitude high Faint eaaye e pia pcr n htmtyadue astr used and photometry and spectra optical new analyzed We [email protected] mar uvy tr:cro tr:eiso ie–sas indiv stars: – line emission stars: – carbon stars: – surveys u iie apecnre h nraigeiec htsp that evidence increasing the confirmed sample limited Our [email protected] − yecro tr . stars carbon type aucitn.Carbon˙astroph no. manuscript .Rossi C. − 2 e.Dwe ta.20 and 2004 al. et Downes (eg. 1 .S Gigoyan S. K. , − yegat led ofimdb te uvy.W on that found We surveys. other by confirmed already giants type + 6 dnie ihahg rprmto tr sarr yeof type rare a is star, motion proper high a with identified 561 > Rsac Note) (Research 13, | b e sur- a tes | ue of tures > ti of is It ificant 2 emgiue n itne otedafcro n h three the and carbon dwarf the to distances and magnitudes te uioiiso ansqec stars. sequence main of h .G Avtandilyan G. M. , ABSTRACT 30 cific rmr cuaeivsiaino htmti n spectros and photometric of investigation accurate more er uch sity gna igh m- cts r;tecaatrsiso he fte r ossetwit consistent are them of three of characteristics the ars; b- ◦ ) - o015 oa Italy Roma, 00185, ro dIacNwo nttt fCie reinBac,Armeni Branch, Armenian Chile, of Institute Newton Isaac nd )n sa etnIsiueo hl,Amna rnh Byu Branch, Armenian Chile, of Institute Newton Isaac O)and ti h eea aao fteScn yrknSre (SBS) Survey Byurakan Second the of Catalog General the in nt A ewe 94ad19 ihte1 cmd telescope Schmidt 7 1m range the the the in at with prisms obtained objective 1991 were three and and SBS the 1974 of between plates exce BAO UV The with lines. objects emission extragalactic and for searching in nitudes + nsm ae olsaswr losrniiosydiscover 1517 SBS serendipitously being also important were most stars subdwarfs. the sdB cool and cases (WD) some dwarf ob In white stellar mainly of found, number a also by-product were a resolu as better and, after with soon programs started spectroscopic up follow 1990); otitrsigojc fti sample. this of bein object therefore interesting spectrum, most the in lines emission showing (dC) ornwcnre -yesaswt atclratninto attention particular with stars characteristic C-type the 1310 SBS discuss confirmed will new We four candidates. star carbon wer ten only stars; type stars. M carbon possible probably as si are indicated them stars of red Most 41. as Table in classified is inclu listed also (2005) candidates, it star Stepanian The but type late by objects 1994). extragalactic published al. to hot et SBS dedicated quite (Liebert the mainly a star of by carbon Catalog composed dwarf system General a binary and of dwarf type white rare t a revealed is spectra object resolution Higher distribution. energy 49 ogtt edsat uiosgat.Frti esnte a they reason this For giants. luminous distant, be to hought da B 1310 SBS idual ntepeetpprw lrf h aueo hs ten these of nature the clarify we paper present the In ◦ nmcldtbssaalbeo h web. the on available databases onomical < δ < crsoyo ooraoeaentcnlsv luminosity conclusive not are alone colour or ectroscopy + 6 hc edsoee ob wr abnstar carbon dwarf a be to discovered we which 561 + 61 3 n .Sclavi. S. and , ◦ + Mrain&Seain 93 tpna tal. et Stepanian 1983, Stepanian, & (Markarian 561 1 orcniae r type M are candidates four + wr abnshowing carbon dwarf 07 hwn peculiar a showing 5017, h 40 m neryCH early an h < α < oi surveys copic Hstars. CH

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Table 1. Revised data for the ten C star candidates of the SBS Table 2. CCD optical photometry of our confirmed carbon stars. General Catalog. SBS GSC2.3 BVR E(B − V) SBS Coordinates (GSC2.3) sp. SIMBAD ref number number mag mag mag mag number α δ class association 0759+533 N8T4000463 14.15 13.23 12.50 0.034 0748+540 07 52 04.0 +53 56 54 N CGCS 1889 [1] 1310+561 N4BP000199 16.6 14.76 14.00 0.015 0759+533 08 03 12.4 +53 11 34 CH 1543+555 N4G7000252 16.42 14.83 14.20 0.011 0832+534 08 36 27.6 +53 17 52 N CGCS 2244 [1] 1701+555 N4ID000249 16.24 15.19 14.12 0.016 0837+503 08 40 49.5 +50 08 12 M X UMa [2] 0854+530 08 57 44.3 +52 47 28 M9 GSC03805-0109 [3] 1310+561 13 12 42.6 +55 55 54 dC 3. Recent observations 1444+503 14 46 36.4 +50 11 28 M4V 1537+571 15 38 39.3 +57 01 33 M AY Dra [2] We observed the four C star candidates on 18 and 19 1543+555 15 45 22.4 +55 21 33 CH January 2010 with the 1.52m Cassini telescope of the Bologna 1701+555 17 02 05.6 +55 27 13 CH Astronomical Observatory equipped with the Bologna Faint Object Spectrometer and Camera BFOSC, and a EEV P129915 Notes. Description of the columns: 1) SBS number according to CCD detector in the spectroscopic and photometric (B, V, R Stepanian (2005); 2) coordinates in the Guide Star Catalogue (Lasker Johnson bands) modes. We obtained moderate resolution spec- et al.2008) of the identified objects; 3) revised spectral classification; 4) other association according to SIMBAD database; 5) References: [1] tra for all the stars in the range 3700−8500Å(grism #4, Alksnis et al. (2001); [2] Samus et al. (2010); [3] Maciejevski et al. dispersion 3.9Å/pixel). We also obtained spectra in the (2004). range 4000−6600Å(grism #7, dispersion 1.8Å/pixel) for SBS 0759+533 and SBS1310+561. We reduced all the data by means of standard IRAF procedures. In Table 2 we present the averages of our photometry (3 values for each band); the corresponding ”rms” from the individual values range be- tween 0.05 and 0.09 mag, quite good in spite of the fact 2. SIMBAD association that the nights were not excellent for photometric observa- tions. In the last column we report the Galactic colour ex- The coordinates of the ten candidates published in cess in the direction of the stars according to the the SBS General Catalogue refer to the equinox 1950, with un- maps by Schlegel et al. (1998). We finally obtained a spec- certainties of the order of 1−2 arcmin, making practically im- trum of our M candidate SBS0854+530 with the same equip- possible the direct identification of the objects on the Digital ment with grism #4 on 6 February 2011 and a spectrum of Sky Survey maps. We precessed the coordinates then, to check SBS 1444+503 on 9 February 2011 with the 1.82m telescope of their carbon rich nature, we started looking for these stars the Asiago Astronomical Observatory equipped with the Asiago in the Hamburg Quasar Survey low resolution spectroscopic Faint Object Spectrometer and Camera AFOSC and a TK1024 database (HQS, Hagen et al. 1995) in a field of 5 arcmin ra- CCD and grism #4, with a dispersion 4.8Å/pixel in the range dius around the positions of each SBS object. From a closer in- 3900−8500Å. spection we verified that two of them, namely SBS0748+540 and SBS0832+534, were also candidates from other surveys with different names and were already confirmed as N type 4. Data analysis giants on the basis of spectroscopic or photometric follow up 4.1. Spectral classification (Knapik et al. 1999). These two stars are already included in the General Catalogue of Galactic Carbon Stars (GCGCS, Alksnis The spectra of SBS0854+530 and SBS1444+503 confirm their et al. 2001). Four of the remaining eight stars are in fact M type nature of M type stars. SBS0854+530 presents features typical stars with erroneous classification as C stars in the SBS General for very late giant, very similar to those of the M7.5 giant RX Catalog. Two of them could be identified with known M type Boo; the most prominent absorptions belong to the TiO bands variables, already included in the General Catalogue of Variable at 4761, 4954, 5167, 5448, 5862, 6159, 6700, 7055, 7600Å and Stars (XUma is a Mira and AYDra is a Semi Regular Variable the VO bands of the red system with several bandheads in the star, Samus et al. 2010). We classified the other two objects asM range 7334−7472Å, and 7851−7973Å, seen only in late type stars with our recent observations described below. The position stars. SBS 1444+503 can be classified as a M4 dwarf similar to of SBS0854+530 corresponds to a with no previ- GJ402 and LHS486; its most prominentfeaturesare: the TiO ab- ously published spectral classification (Maciejevski et al. 2004). sorption bands at 4954, 5167, 5448, 6700, 7100, 7600Å; the Mg The last M star, SBS 1444+503 is a faint object not associated b triplet 5167,5173,5184Å;very strong NaD doublet; the CaOH with known sources. diffuse bands centered at 5550 and 6230Å. the MgH bands at The remaining four carbon star candidates were not included 4780, 5211Å; the CaH at 6382, 6908, 6946Å. We will study in in GCGCS; although their HQS spectra showed strong absorp- more details these stars in a forthcoming paper dedicated to the tion bands, the resolution was not sufficient to derive more other 29 late type star candidates of the General Catalog of the detailed information. We better clarified their carbon nature SBS. and kinematical properties by the use of modern astronomical Concerning the four carbon candidates, the raw spectral clas- databases and by our recent dedicated observations described in sification from HQS was confirmedby our new spectra which al- the following sections. lowed the determination of the carbon type class, finding in one In Table 1 we give the revised data for all the ten SBS car- case very interesting results. All the six low resolution spectra bon candidates, including the four M and the two known carbon in the range 3850-7600Å, are presented in Figure 1, including stars. those of the two M stars. The spectra of the four carbon stars

2 Please give a shorter version with: \authorrunning and/or \titilerunning prior to \maketitle in the wavelength region 3900−4960Å normalized to the con- and references therein), but in our case there are at least two in- tinuum are shown in Figure 2. To be consistent with the data dications for it being a object: one is the high of SBS1543+555 and SBS1701+555 for which we only have µα = −118 mas/y and µδ =+28 mas/y (PPMXL grism#4 spectra,and to show the regionbelow4000Å thelower catalog, Roesner et al. 2010); the other three stars have negli- resolution spectra of SBS1310+561 and of SBS0759+533 have gible proper motions in both directions. Another promising low been plotted in this figure. The better resolved spectra of these good indicator for CH stars is the strength of the two two stars are shown in Figure 3. We classified our stars on the CaH bands at λ 6382 and λ 6750−6950Å (Margon et al. 2002). basis of a number of spectroscopic and photometric character- From a statistically significant sample of FHLCs those authors istics. At a first look the spectra could indicate either C−R or found these features strong only in the cool stars having detected CH type stars. Although our spectral resolution does not allow proper motion. We measured an indicative value of the heliocen- us to derive isotopic ratios, in any case it is sufficient to detect tric of the star by using the hydrogen emission and measure the prominent spectral features defining the carbon lines of the higher resolution spectra; from Gaussian fits of H α type by comparing our targets with those of known standards. and H β we found -50 and -30 km/s with uncertainties of about In Figure 2 important lines and molecular bands are indicated 20 km/s. with vertical bars. Three stars, SBS0759+533, SBS1543+555 and SBS1701+555 are characterized by a number of absorption features which can be summarized as follows: 4.2. Optical and infrared photometry Atomic lines : Ca ii K and H; Fe i 4271Å; Ba ii 4935, 4554, 6497Å. The last two absorptions could have a contribution from The optical colours are typical for CH-type stars with the dwarf close CN lines, but the features are well visible also in the spec- being the reddest. We can assume that these colours are very trum of SBS1701+555 where the CN bands are faint and the close to the intrinsic values because at the Galactic latitudes of strength of the isolated 4935Å line is similar in the three stars our stars the interstellar reddening is negligible (see Table 2). All the data from our new photometry agree quite well with with an equivalent width ≃1.5 Å with respect to the local con- i those of GSC2.3 within the errors, indicating that no major tinuum. the Ca at 4226Å is not detectable (marginally visible changes occurred between the different epochs of observations. in the grism #7 spectrum of SBS0759+533). For SBS0759+533, SBS1310+561 and SBS1543+555 there Molecular bands: strong G band of CH, with prominent sec- are also photometric data obtained during the Sloan Digital Sky 12 12 ondary P branch at 4342Å; strong C C bands at 4737, 5165, Survey. The first star is saturated; for the other two stars the orig- 5635, 6122, 6192Å; 12C13C bands, if present are faint and not inal photometry, converted to the Johnson bands with the trans- resolved. Anyhow 12C13C band head at 4744Å is marginally formation equations by West et al. (2005), yields the following detected in the grism #7 spectrum of SBS0759+533. The CN magnitudes: for SBS1310+561: B = 16.8, V = 15.0, R = 13.8 bandsat 4215,5264,5746,6206,6360Å are present in the spec- and for SBS1543,+555: B = 16.6, V = 15.2, R = 14.2. Taking tra of SBS0759+533 and SBS1543+555. In SBS1701+555 into account our measurement uncertainties and the fact that the only the band at 4215Å is strong, other CN bands are barely SDSS conversions are not calibrated to carbon stars, the agree- visible or absent. ment is very good. Considering the global appearance of the spectra one can see From the infrared magnitudes published in the Two Micron that with marginal differences in the CN bands SBS1701+555 All Sky Survey (2MASS) catalogue (Skrustkie et al. 2006), matches quite well the spectrum of HD5223, used as CH stan- we could check the position of our stars in the infrared dard by Goswami et al. (2010) and classified as C−H3, C2 4.5, colour−colourdiagrams. In Table 3 we present the 2MASS mag- CH5 in Barnbaum et al. (1996). The spectra of SBS0759+533 nitudes and R − J colour. In the last two columns we report and SBS1543+555are similar to that of the CH star VAri, clas- the colours J − H, H − K in the SAAO photometric system sified as C−H3.5, C2 5.5, CH4.5 in Barnbaum et al. (1996), but obtained by transforming the 2MASS magnitudes according to also to that of the late−R star RV Sct (Goswami et al. (2010) and the formulae by Koen et al. (2007). In the infrared two−colour references therein). The close inspection of the spectral lines and diagram shown in Figure4 SBS0759+533, SBS1543+555 and the supplementary diagnostics of the infrared colors (see below) SBS 1701+555 are placed inside the region occupied by the have been fundamental in discarding this second hypothesis. great majority of CH−type stars studied by Totten et al 2000, In summary, the spectroscopic and photometric characteris- colors confirming the spectral classification. For these stars the tics of SBS0759+533, SBS1543+555 and SBS1701+555 led need for a spectroscopic classification originates from the fact us to classify these stars as belonging to the CH class, with the that there is no obvious distinction in the colors of CH and early differencesin the strength of CN bandsmost probablydepending C−R stars. We will better discuss this argument in section 5. on the chemical composition of the atmospheres of individual SBS 1310+561 lies on the ill defined boundary between stars (see Wallerstein & Kanpp 1998, Lloyd Evans 2010). CH and dwarf zone. Border line colours are not unique in + The most interesting object is SBS1310 561 which shows the small family of the emission line dC. To our knowledge ff a number of di erences with respect to the other stars: the NaD there are only three confirmed dC stars previously reported i doublet is very strong (EW = 15.0±0.5Å). Ca at λ 4226Å and showing emission lines: CLS29, identified by Totten et al. the molecular band at λ 5211Å of MgH are present with equiv- (2000), which also lies on the boundary region between CH alent width of 3.5 and 4.1±0.2Å, respectively. Noticeably all the and dwarfs; PG0824+289 placed in the CH domain, has a com- Balmer lines are in emission; the Ca ii doublet at λ 3934, 3968Å posite spectrum (Heber et al. 1993); in this case most proba- is also in emission with EW= 7.0±0.4Å and 6.0±0.5Å. These bly the optically visible companion affects the in- values must be taken with caution given that owing to the low frared emission of the system. The opposite case is the third star, S/N ratio at the very blue end of the spectrum and the spec- SDSS J085853.3+012243which is the reddest object of the sam- tral resolution we could not evaluate the contribution from the ple studied by Downes et al. (2004). The situation is less clear absorption components. While not common, emissions are not for SBS0759+533, SBS1543+555 and SBS1701+555 by us- unexpected in carbon giants (see for example Totten et al. 2000 ing directly the 2MASS colours and the more general diagrams

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CaI 3

2

1

FeI Ba II Ba II Ba II

0 Sr I CaII CN CH CN

4000 4200 4400 4600 4800

Fig. 2. Spectra of the four carbon stars in the wavelength region 3900−4960Å, normalized to the continuum and vertically shifted. The most prominent features are indicated.

Table 3. Infrared photometry and colours of the four carbon stars. The last two columns are in the SAAO system.

SBS 2MASS J H Ks R − J J − H H − K number identification mag mag mag mag mag mag 0759+533 J08031240+5311340 10.84 10.27 10.05 1.66 0.66 0.16 1310+561 J13124251+5555546 12.06 11.22 10.80 1.94 0.92 0.36 1543+555 J15452240+5521327 12.57 11.85 11.62 1.63 0.80 0.19 1701+555 J17020556+5527134 12.87 12.34 12.19 1.25 0.60 0.12

of figures 2 (R − J vs J − Ks) and 3 (J − H vs H − Ks) of the values are measured from an unfiltered CCD detector, there- Lowrance et al. (2003). But these authors remark that the tracks fore the zero strongly depends on the stellar colours; for giants and dwarfs overplotted in their figure 3 were obtained for the considered stellar type the NSVS magnitudes are fainter by interpolating the data of figure 5 of Bessel & Brett (1988), of about 0.3-0.5 magnitudes with respect to the Johnson filter. based on colours of non carbon stars. Anyhow in these plots SBS 1310+561 falls well inside the region defined by known dC stars. 5. Discussion The photometric behavior of all the four stars has been mon- It is not necessary to repeat here the detailed discussion made by itored by the Northern Sky Variability Survey (NSVS, Wozniak Margon et al. (2002) and Downes et al. (2004) on the reliability et al. 2004) between March 1999 and May 2000. SBS0759+533 of the spectroscopic and photometric luminosity discriminants is constant with very little scatter from the mean magnitude. The for CH-type stars. It can be sufficient to remember their final other three stars show fluctuations of about ±0.4 mag amplitude comment that at low dispersion ’spectra and colours of the dis- as shown in Figure 5. It is worth to rememberthat the NSVS sys- parate classes are frustratingly similar’. The same consideration tem has an overall response similar to the Red Johnson band but is valid when the various types of carbon stars are considered.

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5.1. Looking for a companion for SBS1310+561

1.2

N-type It is important to remember that, since according to the evolu- tionary models no carbon is produced by main sequence hydro-

1 gen burning stars, the most reasonable, widely accepted explana- tion is that dCs belong to evolved binary systems; the presently CH-type observed carbon stars are thought to be formed through of carbon enriched material while the initial primary 0.8 component was ascending the as a C giant (Green 2000 and references therein). deKool and Green (1995) constructed several models of binary system evolution

0.6 leading to dC ; their predictions have been con- firmed by radial velocity and chemical abundances studies of dwarfs several targets (Lucatello et al. 2005, Behara et al. 2010). The characteristics of the individual spectra depend on the distance 0.4 between the components of the binary system and the nature of the collapsed companion:

0 0.2 0.4 0.6 0.8 the prototype G77−61 is a single line spectroscopic binary not H-K showing emission lines; the companion, invisible in the opti- cal tange is a WD with Teff < 6000K (Dearborn et al. 1986). Fig. 4. Near infrared two−colour diagram in the SAAO pho- In SBS1517+5017 no emissions are visible but the hot white tometric system; the zones limiting the loci of the different dwarf contributes to the continuum and to the absorption lines carbon classes are those defined in the Figure 3 by Totten et already in the blue spectral region. CLS29 only shows Hα in al. (2000). Open triangles: the three SBS−CH giants; filled emission ( figure 5 of Totten and Irwin 1998) For PG 0824+289 triangle: SBS1310+561; filled square: CLS29; filled circle: the Balmer emission lines have been attributed to the heating of PG0824+289; star: SDSS J085853.3+012243. the dC atmosphere by the visible WD. Except for the emission lines, SBS 1310+561does not show a composite spectrum. Most probably the visible star has an optically invisible companion, possibly a close WD. This can be suggested by the emission of the Ca ii H and K lines which are normally indicators of strong While infrared colors of N and late−R giants are well separated chromospheric activity. For a single this should not from CH and early−R stars , there is no clear distinction be- be the case, but in a binary system a white dwarf companion in tween CH and early−R, as we noted in section 4.2, nor between close orbit can spin up the M dwarf via tidal locking and thus N and late−R types. This problemhas been discussed in detail by trigger chromospheric activity on the cool star. In this respect Zamora et al 2009 who made an accurate photometric and spec- we remember that mass transfer or a similar interaction was sug- troscopic analysis of 17 previously classified R stars. The high gested as the cause of the luminosity fluctuations for CLS29 and + resolution spectra and the use of the most recent stellar mod- PG0824 289. els allowed these authors to find that 40% of their sample were wrongly classified; this result confirms previous doubts about the Ultraviolet spectra might confirm the presence of a white real relative number of R and CH types among the carbon stars. dwarf companion and hence the binarity of SBS1310+561. We The importance from the evolutionary point of view has been can compare the photometry only with the SLOAN data for deeply discussed by Zamora et al 2009; we refer to that paper similar stars. The average u − g computed from 65 stars clas- for further reading. sified ”D” in table 1 of Downes et al (2004) gives u − g=2.7 We want to remark here that there are also a few minor, but with σ=0.6. For this star u − g = 2.3. We finally verified very common causes of confusion in the color plots: one is the that the Galex archive (Martin et al. 2005) (farUV∼1400−1800, fact that often data from different photometric systems are put nearUV∼1800−2800Å) contains data of our targets, of sev- together without taking into account the necessary transforma- eral known dCs and of the FHLC present in the list of Totten tions, increasing in this way the spread of the results; another one el al. (2000). Excluding the dwarfs with composite spectra, is that often previous schemes are strictly adopted without con- most of the stars, including SBS1543+555, with similar spectral sidering that the conditions are never tight and/or those schemes type and similar B magnitudes or brighter than SBS1310+561 are based on possibly different photometric systems. In our case, are not detected. Very few, including SBS0759+533 and dealing also with an emission line star we paid particular atten- SBS 1701+555, have the difference NUV−B ∼ 6.5 mag. For tion to the photometric system used to place our stars in a two the two dCs G77−61 and CLS50 the difference is ∼ 5.3 mag. color diagram. For the limited number of known emission line SBS 1310+561 has the smallest difference NUV−B = 3.7 ± 0.3 dwarf carbon the photometric situation is further confused by the mag, possibly indicating the presence of a low luminosity but possible presence of a companion, as discussed below. hot source. Also photometric fluctuations cannot be considered conclu- sive luminosity indicators: CLS29 and PG0824+289 were also At present we do not have enough information for deeper monitored by the NSVS. Their light curves are presented with speculations: there are no other data published for any of these our targets in Figure 5 which shows how the three dC stars are stars in the near or in the far IR, where an emission excess could variable in a similar way and with similar amplitude as the two witness the presence of a residual debris disk from the mass CH variable giants. transfer event (see Fazio & Lowrance 2008).

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Table 4. Absolute K magnitudes and distances to the three CH of dwarf carbon showing Balmer and Ca ii lines in emission. giants. Dealing with a possible close binary system close time spaced observations should be performed to shed more light on the ob-

SBS MK d ∆ d ject which shows photometric fluctuations, but the sparse data number mag kpc kpc do not allow to speculate too much about a possible light curve 0759+533 -4.9 10 1. and/or periodical spectral variations. 1543+555 -5.8 20 2. 1701+555 -4.3 20 2. Acknowledgements. K.S.G. and C.R. would like to thank the Bologna and Asiago Observatories staff for the logistic support and the technical assistance during the observations. The University La Sapienza supported the project with funds from the MIUR. This research has made use of SIMBAD database oper- 5.2. Luminosities and distances ated at CDS, Strasbourg, France, and also of the Two Micron All−Sky Survey database, which is a joint project of the University of Massachusetts and the From the data at our disposal we could estimate approximateval- Infrared Processing and Analysis Center/California Institute of Technology. ues of the distance to our targets. For the dC star SBS1310+561 GALEX is operated for NASA by the California Institute of Technology un- we started assuming for the in the K band a der NASA contract NAS5-98034. IRAF is distributed by the NOAO which is value +6.1 < MKs < +6.6 which is the range for dC stars with operated by AURA under contract with NFS. known parallaxes (Lowrance et al. 2003 and references therein). With the observed Ks=+10.80 mag the distance modulus is then 4.2 < DM < 4.7 and the distance 70

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Fig. 1. CCD spectra of the recently classified SBS stars in the range 4100−7500Å. Bottom panels: the two confirmed M stars; bars in the lowest panel indicate TiO bands common to both stars and VO bands of the late type; lines prominent in SBS1444+503 are only reported in the corresponding panel. Upper panel: the dwarf carbon and the three CH stars; vertical bars indicate prominent features listed in the text. Bars corresponding to lines of particular interest for SBS 1310+561 are vertically shifted of 0.15 units above the others. Please give a shorter version with: \authorrunning and/or \titilerunning prior to \maketitle, Online Material p 2

1.5

1

0.5

SBS1310+561 0

1

0.8

0.6

0.4

0.2 SBS0759+533

0 4000 4500 5000 5500 6000 6500

Fig. 3. Medium resolution spectra of the CH giant SBS0759+533 and the dC SBS1310+561. For this star an enlargment of the region 3900 - 4200Å from the lower resolution spectrum is plotted to emphasize Hδ and the CaII emissions, these last being not covered by the grism #7 spectral range . Ordinates are the same as in Figure 1. Please give a shorter version with: \authorrunning and/or \titilerunning prior to \maketitle, Online Material p 3

14.6 15 14.4 14.2 14.5 14 14 13.8 13.6 1300 1400 1500 1600 1300 1400 1500 1600 SBS1701+555 PG824+289

15.5 15.5 15 15 14.5 14.5 14

1300 1400 1500 1600 1300 1400 1500 1600 SBS1543+555 CLS29

14.8 12.85 14.6 12.8 14.4 12.75 14.2

12.7 14 1280 1300 1320 1340 1300 1400 1500 1600 SBS0759+533 SBS1310+561

Fig. 5. Light curves of our CH stars plus CLS29 and PG084+289. The dates are MJD-50000 days; the data are from the NSVS archive. The typical uncertainties are 0.03 mag for SBS 0759+533; 0.08 mag for SBS1310+561 and PG824+289; 0.11 mag for the other stars.