Revised classification of the SBS carbon star candidates including the discovery of a new emission line dwarf carbon star
aa r X i v : . [ a s t r o - ph . S R ] M a y Astronomy&Astrophysicsmanuscript no. Carbon˙astroph c (cid:13)
ESO 2018October 8, 2018
Revised classification of the SBS carbon star candidates includingthe discovery of a new emission line dwarf carbon star ⋆ (Research Note) C. Rossi , K. S. Gigoyan , M. G. Avtandilyan , and S. Sclavi. Department of Physics, University La Sapienza, Piazza A.Moro 00185, Roma, Italye-mail: [email protected] V. A. Ambartsumian Byurakan Astrophysical Observatory(BAO)and Isaac Newton Institute of Chile, Armenian Branch, Byurakan0213, Aragatzotn province, Armenia,e-mail: [email protected] Armenian State Pedagogical Uniiversity After Kh. Abovyan and Isaac Newton Institute of Chile, Armenian Branch, Armenia.e-mail: mar [email protected]
Received February 28, 2011 ; accepted
ABSTRACT
Context.
Faint high latitude carbon stars are rare objects commonly thought to be distant, luminous giants. For this reason they areoften used to probe the structure of the Galactic halo; however more accurate investigation of photometric and spectroscopic surveyshas revealed an increasing percentage of nearby objects with luminosities of main sequence stars.
Aims.
To clarify the nature of the ten carbon star candidates present in the General Catalog of the Second Byurakan Survey (SBS).
Methods.
We analyzed new optical spectra and photometry and used astronomical databases available on the web.
Results.
We verified that two stars are N − type giants already confirmed by other surveys. We found that four candidates are M typestars and confirmed the carbon nature of the remaining four stars; the characteristics of three of them are consistent with an early CHgiant type. The fourth candidate, SBS 1310 +
561 identified with a high proper motion star, is a rare type of dwarf carbon showingemission lines in its optical spectrum. We estimated absolute magnitudes and distances to the dwarf carbon and the three CH stars.
Conclusions.
Our limited sample confirmed the increasing evidence that spectroscopy or colour alone are not conclusive luminositydiscriminants for CH − type carbon stars . Key words. surveys – stars: carbon – stars: emission line – stars: individual SBS 1310 +
1. Introduction
The faint high latitude carbon stars (FHLCs , R > | b | > ◦ )are rare objects. The observed spatial distribution indicates a sur-face density less than 0.06 deg − (eg. Downes et al. 2004 andreferences therein); for this reason they are seldom the specifictarget of dedicated photometric or spectroscopic surveys. It is ofinterest that their colours are very similar to those of the muchmore numerous QSOs; in surveys aimed to search QSO, FHLCsare a small fraction of contaminants initially ”flagged” as highredshift QSO candidates and then identified as galactic objectsby their spectra showing the characteristic absorption features ofC2 Swan bands (eg. Totten et al. 1998, Christlieb et al. 2001,Downes et al. 2004). In this way distant giants in the halo areusually discovered, but a non negligible fraction has a significantproper motion indicative of a close object having the luminosityof a main sequence star.The discoveries of red faint stars from the prism surveysconducted at the Byurakan Astrophysical Observatory (BAO)are typical examples for this kind of follow up. The results ob-tained by the first spectroscopic survey conducted at BAO stim-ulated the need for a deeper survey aimed to reach fainter mag- ⋆ Based on observations made at the 1.52m telescope of the BolognaObservatory and 1.83m telescope of the Asiago Observatory. nitudes in searching for extragalactic objects with UV excessesand emission lines. The plates of the SBS were obtained at theBAO between 1974 and 1991 with the 1m Schmidt telescopeand three objective prisms in the range 7 h m < α < h m and + ◦ < δ < + ◦ (Markarian & Stepanian, 1983, Stepanian et al.1990); follow up spectroscopic programs with better resolutionstarted soon after and, as a by-product a number of stellar objectswere also found, mainly white dwarf (WD) and sdB subdwarfs.In some cases cool stars were also serendipitously discovered,the most important being SBS 1517 + +
561 which we discovered to be a dwarf carbon star(dC) showing emission lines in the spectrum, therefore being themost interesting object of this sample. \authorrunning and / or \titilerunning prior to \maketitle Table 1.
Revised data for the ten C star candidates of the SBSGeneral Catalog.
SBS Coordinates (GSC2.3) sp. SIMBAD refnumber α δ class association0748 +
540 07 52 04.0 +
53 56 54 N CGCS 1889 [1]0759 +
533 08 03 12.4 +
53 11 34 CH0832 +
534 08 36 27.6 +
53 17 52 N CGCS 2244 [1]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 dC1444 +
503 14 46 36.4 +
50 11 28 M4V1537 +
571 15 38 39.3 +
57 01 33 M AY Dra [2]1543 +
555 15 45 22.4 +
55 21 33 CH1701 +
555 17 02 05.6 +
55 27 13 CH
Notes.
Description of the columns: 1) SBS number according toStepanian (2005); 2) coordinates in the Guide Star Catalogue (Laskeret al.2008) of the identified objects; 3) revised spectral classification;4) other association according to SIMBAD database; 5) References: [1]Alksnis et al. (2001); [2] Samus et al. (2010); [3] Maciejevski et al.(2004).
2. SIMBAD association
The coordinates of the ten carbon star candidates published inthe SBS General Catalogue refer to the equinox 1950, with un-certainties of the order of 1 − + + ff erent names and were already confirmed as N typegiants on the basis of spectroscopic or photometric follow up(Knapik et al. 1999). These two stars are already included in theGeneral Catalogue of Galactic Carbon Stars (GCGCS, Alksniset al. 2001). Four of the remaining eight stars are in fact M typestars with erroneous classification as C stars in the SBS GeneralCatalog. Two of them could be identified with known M typevariables, already included in the General Catalogue of VariableStars (X Uma is a Mira and AY Dra is a Semi Regular Variablestar, Samus et al. 2010). We classified the other two objects as Mstars with our recent observations described below. The positionof SBS 0854 +
530 corresponds to a variable star with no previ-ously published spectral classification (Maciejevski et al. 2004).The last M star, SBS 1444 +
503 is a faint object not associatedwith known sources.The remaining four carbon star candidates were not includedin GCGCS; although their HQS spectra showed strong absorp-tion bands, the resolution was not su ffi cient to derive moredetailed information. We better clarified their carbon natureand kinematical properties by the use of modern astronomicaldatabases and by our recent dedicated observations described inthe following sections.In Table 1 we give the revised data for all the ten SBS car-bon candidates, including the four M and the two known carbonstars. Table 2.
CCD optical photometry of our confirmed carbon stars.
SBS GSC2.3
B V R E( B − V )number number mag mag mag mag0759 +
533 N8T4000463 14.15 13.23 12.50 0.0341310 +
561 N4BP000199 16.6 14.76 14.00 0.0151543 +
555 N4G7000252 16.42 14.83 14.20 0.0111701 +
555 N4ID000249 16.24 15.19 14.12 0.016
3. Recent observations
We observed the four C star candidates on 18 and 19January 2010 with the 1.52 m Cassini telescope of the BolognaAstronomical Observatory equipped with the Bologna FaintObject Spectrometer and Camera BFOSC, and a EEV P129915CCD detector in the spectroscopic and photometric ( B , V , R Johnson bands) modes. We obtained moderate resolution spec-tra for all the stars in the range 3700 − / pixel). We also obtained spectra in therange 4000 − / pixel) forSBS 0759 +
533 and SBS 1310 + +
530 with the same equip-ment with grism +
503 on 9 February 2011 with the 1.82 m telescope ofthe Asiago Astronomical Observatory equipped with the AsiagoFaint Object Spectrometer and Camera AFOSC and a TK1024CCD and grism / pixel in the range3900 −
4. Data analysis
The spectra of SBS0854 +
530 and SBS1444 +
503 confirm theirnature of M type stars. SBS 0854 +
530 presents features typicalfor very late giant, very similar to those of the M7.5 giant RXBoo; the most prominent absorptions belong to the TiO bandsat 4761, 4954, 5167, 5448, 5862, 6159, 6700, 7055, 7600Å andthe VO bands of the red system with several bandheads in therange 7334 − − +
503 can be classified as a M4 dwarf similar toGJ402 and LHS486; its most prominent features are: the TiO ab-sorption bands at 4954, 5167, 5448, 6700, 7100, 7600Å; the Mg b triplet 5167,5173,5184Å; very strong NaD doublet; the CaOHdi ff use bands centered at 5550 and 6230Å. the MgH bands at4780, 5211Å; the CaH at 6382, 6908, 6946Å. We will study inmore details these stars in a forthcoming paper dedicated to theother 29 late type star candidates of the General Catalog of theSBS.Concerning the four carbon candidates, the raw spectral clas-sification from HQS was confirmed by our new spectra which al-lowed the determination of the carbon type class, finding in onecase very interesting results. All the six low resolution spectrain the range 3850-7600 Å, are presented in Figure 1, includingthose of the two M stars. The spectra of the four carbon stars \authorrunning and / or \titilerunning prior to \maketitle in the wavelength region 3900 − +
555 and SBS 1701 +
555 for which we only havegrism +
561 and of SBS 0759 +
533 havebeen plotted in this figure. The better resolved spectra of thesetwo stars are shown in Figure 3. We classified our stars on thebasis of a number of spectroscopic and photometric character-istics. At a first look the spectra could indicate either C − R orCH type stars. Although our spectral resolution does not allowus to derive isotopic ratios, in any case it is su ffi cient to detectand measure the prominent spectral features defining the carbontype by comparing our targets with those of known standards.In Figure 2 important lines and molecular bands are indicatedwith vertical bars. Three stars, SBS 0759 + + +
555 are characterized by a number of absorptionfeatures which can be summarized as follows:Atomic lines : Ca ii K and H; Fe i ii +
555 where the CN bands are faint and thestrength of the isolated 4935Å line is similar in the three starswith an equivalent width ≃ i at 4226 Å is not detectable (marginally visiblein the grism + C C bands at 4737, 5165,5635, 6122, 6192 Å ; C C bands, if present are faint and notresolved. Anyhow C C band head at 4744 Å is marginallydetected in the grism + +
533 and SBS 1543 + + ff erences in the CN bands SBS 1701 + − H 3, C + +
555 are similar to that of the CH star V Ari, clas-sified as C − H 3.5, C − R star RV Sct (Goswami et al. (2010) andreferences therein). The close inspection of the spectral lines andthe supplementary diagnostics of the infrared colors (see below)have been fundamental in discarding this second hypothesis.In summary, the spectroscopic and photometric characteris-tics of SBS 0759 + +
555 and SBS 1701 +
555 ledus to classify these stars as belonging to the CH class, with thedi ff erences in the strength of CN bands most probably dependingon the chemical composition of the atmospheres of individualstars (see Wallerstein & Kanpp 1998, Lloyd Evans 2010).The most interesting object is SBS 1310 +
561 which showsa number of di ff erences with respect to the other stars: the Na Ddoublet is very strong (EW = ± i at λ λ ± ii doublet at λ = ± ± / N ratio at the very blue end of the spectrum and the spec-tral resolution we could not evaluate the contribution from theabsorption components. While not common, emissions are notunexpected in carbon giants (see for example Totten et al. 2000 and references therein), but in our case there are at least two in-dications for it being a main sequence object: one is the highproper motion µ α = −
118 mas / y and µ δ = +
28 mas / y (PPMXLcatalog, Roesner et al. 2010); the other three stars have negli-gible proper motions in both directions. Another promising lowluminosity good indicator for CH stars is the strength of the twoCaH bands at λ λ − α and H β we found -50 and -30 km / s with uncertainties of about20 km / s. The optical colours are typical for CH-type stars with the dwarfbeing the reddest. We can assume that these colours are veryclose to the intrinsic values because at the Galactic latitudes ofour stars the interstellar reddening is negligible (see Table 2).All the data from our new photometry agree quite well withthose of GSC2.3 within the errors, indicating that no majorchanges occurred between the di ff erent epochs of observations.For SBS 0759 + +
561 and SBS 1543 +
555 thereare also photometric data obtained during the Sloan Digital SkySurvey. The first star is saturated; for the other two stars the orig-inal photometry, converted to the Johnson bands with the trans-formation equations by West et al. (2005), yields the followingmagnitudes: for SBS 1310 + B = . , V = . , R = . + B = . , V = . , R = .
2. Takinginto account our measurement uncertainties and the fact that theSDSS conversions are not calibrated to carbon stars, the agree-ment is very good.From the infrared magnitudes published in the Two MicronAll Sky Survey (2MASS) catalogue (Skrustkie et al. 2006),we could check the position of our stars in the infraredcolour − colour diagrams. In Table 3 we present the 2MASS mag-nitudes and R − J colour. In the last two columns we reportthe colours J − H , H − K in the SAAO photometric systemobtained by transforming the 2MASS magnitudes according tothe formulae by Koen et al. (2007). In the infrared two − colourdiagram shown in Figure4 SBS 0759 + +
555 andSBS 1701 +
555 are placed inside the region occupied by thegreat majority of CH − type stars studied by Totten et al 2000,colors confirming the spectral classification. For these stars theneed for a spectroscopic classification originates from the factthat there is no obvious distinction in the colors of CH and earlyC − R stars. We will better discuss this argument in section 5.SBS 1310 +
561 lies on the ill defined boundary betweenCH and dwarf zone. Border line colours are not unique inthe small family of the emission line dC. To our knowledgethere are only three confirmed dC stars previously reportedshowing emission lines: CLS29, identified by Totten et al.(2000), which also lies on the boundary region between CHand dwarfs; PG0824 +
289 placed in the CH domain, has a com-posite spectrum (Heber et al. 1993); in this case most proba-bly the optically visible white dwarf companion a ff ects the in-frared emission of the system. The opposite case is the third star,SDSS J085853.3 + + +
555 and SBS 1701 +
555 by us-ing directly the 2MASS colours and the more general diagrams \authorrunning and / or \titilerunning prior to \maketitle Fig. 2.
Spectra of the four carbon stars in the wavelength region 3900 − Table 3.
Infrared photometry and colours of the four carbon stars. The last two columns are in the SAAO system.
SBS 2MASS
J H K s R − J J − H H − K number identification mag mag mag mag mag mag0759 +
533 J08031240 + +
561 J13124251 + +
555 J15452240 + +
555 J17020556 + of figures 2 ( R − J vs J − K s ) and 3 ( J − H vs H − K s ) ofLowrance et al. (2003). But these authors remark that the tracksfor giants and dwarfs overplotted in their figure 3 were obtainedby interpolating the data of figure 5 of Bessel & Brett (1988),based on colours of non carbon stars. Anyhow in these plotsSBS 1310 +
561 falls well inside the region defined by known dCstars.The photometric behavior of all the four stars has been mon-itored by the Northern Sky Variability Survey (NSVS, Wozniaket al. 2004) between March 1999 and May 2000. SBS 0759 + ±
5. Discussion
It is not necessary to repeat here the detailed discussion made byMargon et al. (2002) and Downes et al. (2004) on the reliabilityof the spectroscopic and photometric luminosity discriminantsfor CH-type stars. It can be su ffi cient to remember their finalcomment that at low dispersion ’spectra and colours of the dis-parate classes are frustratingly similar’. The same considerationis valid when the various types of carbon stars are considered. \authorrunning and / or \titilerunning prior to \maketitle Fig. 4.
Near infrared two − colour diagram in the SAAO pho-tometric system; the zones limiting the loci of the di ff erentcarbon classes are those defined in the Figure 3 by Totten etal. (2000). Open triangles: the three SBS − CH giants; filledtriangle: SBS1310 + + + − R giants are well separatedfrom CH and early − R stars , there is no clear distinction be-tween CH and early − R, as we noted in section 4.2, nor betweenN and late − R types. This problem has been discussed in detail byZamora et al 2009 who made an accurate photometric and spec-troscopic analysis of 17 previously classified R stars. The highresolution spectra and the use of the most recent stellar mod-els allowed these authors to find that 40% of their sample werewrongly classified; this result confirms previous doubts about thereal relative number of R and CH types among the carbon stars.The importance from the evolutionary point of view has beendeeply discussed by Zamora et al 2009; we refer to that paperfor further reading.We want to remark here that there are also a few minor, butvery common causes of confusion in the color plots: one is thefact that often data from di ff erent photometric systems are puttogether without taking into account the necessary transforma-tions, increasing in this way the spread of the results; another oneis that often previous schemes are strictly adopted without con-sidering that the conditions are never tight and / or those schemesare based on possibly di ff erent photometric systems. In our case,dealing also with an emission line star we paid particular atten-tion to the photometric system used to place our stars in a twocolor diagram. For the limited number of known emission linedwarf carbon the photometric situation is further confused by thepossible presence of a companion, as discussed below.Also photometric fluctuations cannot be considered conclu-sive luminosity indicators: CLS29 and PG0824 +
289 were alsomonitored by the NSVS. Their light curves are presented withour targets in Figure 5 which shows how the three dC stars arevariable in a similar way and with similar amplitude as the twoCH variable giants.
It is important to remember that, since according to the evolu-tionary models no carbon is produced by main sequence hydro-gen burning stars, the most reasonable, widely accepted explana-tion is that dCs belong to evolved binary systems; the presentlyobserved carbon stars are thought to be formed through masstransfer of carbon enriched material while the initial primarycomponent was ascending the asymptotic giant branch as a Cgiant (Green 2000 and references therein). deKool and Green(1995) constructed several models of binary system evolutionleading to dC star formation; their predictions have been con-firmed by radial velocity and chemical abundances studies ofseveral targets (Lucatello et al. 2005, Behara et al. 2010). Thecharacteristics of the individual spectra depend on the distancebetween the components of the binary system and the nature ofthe collapsed companion:the prototype G77 −
61 is a single line spectroscopic binary notshowing emission lines; the companion, invisible in the opti-cal tange is a WD with Te ff < + α inemission ( figure 5 of Totten and Irwin 1998) For PG 0824 + +
561 does not show a composite spectrum. Mostprobably the visible star has an optically invisible companion,possibly a close WD. This can be suggested by the emission ofthe Ca ii H and K lines which are normally indicators of strongchromospheric activity. For a single dwarf star this should notbe the case, but in a binary system a white dwarf companion inclose orbit can spin up the M dwarf via tidal locking and thustrigger chromospheric activity on the cool star. In this respectwe remember that mass transfer or a similar interaction was sug-gested as the cause of the luminosity fluctuations for CLS29 andPG0824 + + u − g computed from 65 stars clas-sified ”D” in table 1 of Downes et al (2004) gives u − g = σ = u − g = ∼ − ∼ − + B magnitudes or brighter than SBS 1310 + +
533 andSBS 1701 + ff erence NUV − B ∼ . −
61 and CLS50 the di ff erence is ∼ . +
561 has the smallest di ff erence NUV − B = . ± . \authorrunning and / or \titilerunning prior to \maketitle Table 4.
Absolute K magnitudes and distances to the three CHgiants. SBS M K d ∆ dnumber mag kpc kpc0759 +
533 -4.9 10 1.1543 +
555 -5.8 20 2.1701 +
555 -4.3 20 2.
From the data at our disposal we could estimate approximate val-ues of the distance to our targets. For the dC star SBS 1310 + + . < MK s < + . K s =+ . < DM < . < d <
87 pc.To compute the absolute visual magnitude we must take intoaccount the variability of the star. We adopted V = . V = .
76 and V = .
44 from GSC2.3and the two limits for the distance modulus obtained from the Kmagnitude which is less sensitive to variability. These data yieldthe absolute visual magnitude M V = + . ± .
2, in good agree-ment with the mean value of the dC stars with measured paral-laxes. Finally we can give a crude estimate of the space velocitycomponents and try to put some constraints on the Galactic pop-ulation membership of SBS 1310 + =
80 pc, the parallax is π = = (-6 ±
5, -10 ±
8, -30 ± / s, consistent with disk membership (in Galactocentric coor-dinates : -6, + / s). The values are corrected for thesolar motion (10, 15, 7)km / s with respect to the LSR; U is pos-itive in the direction of the Galactic center. The formulae usedare those described by Johnson & Soderblom (1987).To compute the absolute magnitudes M K and the distancesto the three CH stars we used the empirical fitting formula: Log ( M K + . = . − . J − K ) (1)obtained by Totten et al. (2000) from a selected sample of gi-ants (standard deviation = K -band absolute mag-nitudes in the SAAO system and the derived heliocentric dis-tances in are given in Table 4.Assuming the standard deviation of fitting formula as themain cause of the uncertainty on M K and therefore on the dis-tance modulus, the uncertainties on the distances are about 10%.In the 2MASS system the absolute magnitudes would di ff er byabout 0.04 mag.
6. Summary and Conclusions
We presented moderate resolution CCD spectra and new photo-metric data for four SBS candidate C stars. Spectra and coloursare consistent with early CH type classification. The most im-portant result is the discovery that SBS 1310 +
561 belongs to thesmall group of dwarf carbon stars. In spite of the limited sam-ple we could verify the increasing evidence that spectroscopy orcolours alone are inconclusive as luminosity discriminants forCH − type stars. In fact, only the high proper motion, and possi-bly the strength of the red CaH bands lead to the conclusion thatSBS 1310 +
561 is a main sequence star, an extremely rare case of dwarf carbon showing Balmer and Ca ii lines in emission.Dealing with a possible close binary system close time spacedobservations should be performed to shed more light on the ob-ject which shows photometric fluctuations, but the sparse datado not allow to speculate too much about a possible light curveand / or periodical spectral variations. Acknowledgements.
K.S.G. and C.R. would like to thank the Bologna andAsiago Observatories sta ff for the logistic support and the technical assistanceduring the observations. The University La Sapienza supported the project withfunds from the MIUR. This research has made use of SIMBAD database oper-ated at CDS, Strasbourg, France, and also of the Two Micron All − Sky Surveydatabase, which is a joint project of the University of Massachusetts and theInfrared Processing and Analysis Center / California Institute of Technology.GALEX is operated for NASA by the California Institute of Technology un-der NASA contract NAS5-98034. IRAF is distributed by the NOAO which isoperated by AURA under contract with NFS.
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Fig. 1.
CCD spectra of the recently classified SBS stars in the range 4100 − +
503 areonly reported in the corresponding panel. Upper panel: the dwarf carbon and the three CH stars; vertical bars indicate prominentfeatures listed in the text. Bars corresponding to lines of particular interest for SBS 1310 +
561 are vertically shifted of 0.15 unitsabove the others. lease give a shorter version with: \authorrunning and / or \titilerunning prior to \maketitle , Online Material p 2
Fig. 3.
Medium resolution spectra of the CH giant SBS 0759 +
533 and the dC SBS 1310 + δ and the CaII emissions, these last being notcovered by the grism lease give a shorter version with: \authorrunning and / or \titilerunning prior to \maketitle , Online Material p 3
Fig. 5.
Light curves of our CH stars plus CLS29 and PG084 + + +
561 and PG824 ++