Faint High Latitude Carbon Stars Research Articles

New carbon stars found in the Digitized First Byurakan Survey

Faint high-latitude carbon stars (FHLCs) are distant, luminous giants. For this reason, they are often used to study the dynamical and chemical properties of the Galactic Halo. However, a more accurate investigation of photometric and spectroscopic surveys has revealed an increasing number of such objects with luminosities of main sequence stars. We report the first results of a systematic search of FHLCs using the Digitized First Byurakan Survey (DFBS) low-resolution spectral database. The DFBS low-resolution spectral plates are analysed to find new faint early and late-type carbon star candidates. Twenty new faint high-latitude carbon star candidates have been detected in an area of ∼4030 sq. deg. Moderate-resolution slit spectroscopy was carried out for all candidates, confirming the carbon star nature for thirteen of them (nine are new discoveries), while the remaining objects are M-type stars. The carbon star sample consists of five N-type carbon stars, for which the estimated mass-loss rates agree well with the typical ones found for such carbon stars; five comparatively bright CH-type carbon stars; and three are probably dwarf carbon (dC) stars. The distance determination indicates that the dwarf carbon stars belong to the thin disk, while the other stars belong the Galactic halo. These results demonstrate that one can efficiently find new faint high-latitude carbon star in the Digitized First Byurakan Survey database.

Revised classification of the SBS carbon star candidates including the discovery of a new emission line dwarf carbon star

Faint high latitude carbon stars are rare objects commonly thought to be distant, luminous giants. For this reason they are often used to probe the structure of the Galactic halo; however more accurate investigation of photometric and spectroscopic surveys has revealed an increasing percentage of nearby objects with luminosities of main sequence stars. Aiming at clarifyin the nature of the ten carbon star candidates present in the General Catalog of the Second Byurakan Survey we analyzed new optical spectra and photometry and used astronomical databases available on the web. We verified that two stars are N-type giants already confirmed by other surveys. We found that four candidates are M type stars and confirmed the carbon nature of the remaining four stars; the characteristics of three of them are consistent with an early CH giant type. The fourth candidate, SBS1310+561 identified with a high proper motion star, is a rare type of dwarf carbon showing emission lines in its optical spectrum. We estimated absolute magnitudes and distances to the dwarf carbon and the three CH stars. Our limited sample confirmed the increasing evidence that spectroscopy or colour alone are not conclusive luminosity discriminants for CH$-$type carbon stars .

HE 1015–2050: DISCOVERY OF A HYDROGEN-DEFICIENT CARBON STAR AT HIGH GALACTIC LATITUDE

Medium resolution spectral analysis of candidate Faint High Latitude Carbon (FHLC) stars from Hamburg/ESO survey has given us the potential to discover objects of rare types. Two primary spectral characteristics of R Coroanae Borealis (RCB) stars are hydrogen deficiency and weaker CN bands relative to C_{2} bands. They are also characterized by their characteristic location in the J-H, H-K plane with respect to cool carbon stars. From a spectral analysis of a sample of 243 candidate FHLC stars, we have discovered a hydrogen-deficient carbon (HdC) star HE 1015-2050, at high Galactic latitude. A differential analysis of its spectrum with that of the spectrum of U Aquarii (U Aqr), a well-known cool HdC star of RCB type, provides sufficient evidence to put this object in a group same as that of U Aqr. Further, it is shown that HE 1015-2050 does not belong to any of the C-star groups CH, C-R, C-N or C-J. Cool RCB stars form a group of relatively rare astrophysical objects; approximately 51 are known in the Galaxy and some 18 in the Large Magellanic Clouds (LMC) and five in Small Magellanic Cloud (SMC). The present discovery adds a new member to this rare group. Although its spectral characteristics and its location in the J-H vs H-K plane places HE 1015-2050 in the same group to which U Aqr belongs, extended photometric observations would be useful to learn if there is any sudden decline in brightness, this being a characteristic property of HdC stars of RCB type.

The CH fraction of carbon stars at high Galactic latitudes

CH stars form a distinct class of objects with characteristic properties like iron deficiency, enrichment of carbon and overabundance in heavy elements. These properties can provide strong observational constraints for theoretical computation of nucleosynthesis at low-metallicity. An important question is the relative surface density of CH stars which can provide valuable inputs to our understanding on the role of low to intermediate-mass stars in the early Galactic chemical evolution. Spectroscopic characterization provides an effective way of identifying CH stars. The present analysis is aimed at a quantitative assessment of the fraction of CH stars in a sample of stars using a set of spectral classification criteria. The sample consists of 92 objects selected from a collection of candidate Faint High Latitude Carbon stars from the Hamburg/ESO survey. Medium resolution (R ~ 1300) spectra for these objects were obtained using OMR at VBO, Kavalur and HFOSC at HCT, IAO, Hanle, during 2007 - 2009 spanning a wavelength range 3800 - 6800 A. Spectral analysis shows 36 of the 92 objects to be potential CH stars; combined with our earlier studies (Goswami 2005, Goswami et al. 2007) this implies ~ 37% (of 243) objects as the CH fraction. We present spectral descriptions of the newly identified CH star candidates. Estimated effective temperatures, 12C/13C isotopic ratios and their locations on the two colour J-H vs H-K plot are used to support their identification.

CH stars at high Galactic latitudes

In the present work we report on several CH stars identified in a sample of Faint High Latitude Carbon stars from Hamburg survey and discuss their medium resolution spectra covering a wavelength range 4000 - 6800 \AA . Estimation of the depths of bands (1,0) $^{12}$C$^{12}$C ${\lambda}$4737 and (1,0) $^{12}$C$^{13}$C ${\lambda}$4744 in these stars indicate isotopic ratio $^{12}$C/$^{13}$C ${\sim}$ 3, except for a few exceptions; these ratios are consistent with existing theories of CH stars evolution. The stars of Hamburg survey, a total of 403 objects were reported to be carbon star candidates with strong C$_{2}$ and CN molecular bands. In the first phase of observation, we have acquired spectra of ninety one objects. Inspection of the objects spectra show fifty one objects with C$_{2}$ molecular bands in their spectra of which thirteen stars have low flux below about 4300 \AA . Twenty five objects show weak or moderate CH and CN bands, twelve objects show weak but detectable CH bands in their spectra and there are three objects which do not show any molecular bands due to C$_{2}$, CN or CH in their spectra. Objects with C$_{2}$ molecular bands and with good signals bluewards of 4300 \AA which show prominent CH bands in their spectra are potential candidate CH stars. Thirty five such candidates are found in the present sample of ninty one objects observed so far. The set of CH stars identified could be the targets of subsequent observation at high resolution for a detail and comprehensive analysis for understanding their role in early Galactic chemical evolution.

Faint High-Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: An Initial Catalog

A search of more than 3000 deg2 of high-latitude sky by the Sloan Digital Sky Survey has yielded 251 faint high-latitude carbon stars (FHLCs), the large majority previously uncataloged. We present homogeneous spectroscopy, photometry, and astrometry for the sample. The objects lie in the 15.6 < r < 20.8 range and exhibit a wide variety of apparent photospheric temperatures, ranging from spectral types near M to as early as F. Proper-motion measurements for 222 of the objects show that at least 50%, and quite probably more than 60%, of these objects are actually low-luminosity dwarf carbon (dC) stars, in agreement with a variety of recent, more limited investigations that show that such objects are the numerically dominant type of star with C2 in the spectrum. This SDSS homogeneous sample of ~110 dC stars now constitutes 90% of all known carbon dwarfs and will grow by another factor of 2–3 by the completion of the survey. As the spectra of the dC and the faint halo giant C stars are very similar (at least at spectral resolution of 103), despite a difference of 10 mag in luminosity, it is imperative that simple luminosity discriminants other than proper motion be developed. We use our enlarged sample of FHLCs to examine a variety of possible luminosity criteria, including many previously suggested, and find that, with certain important caveats, JHK photometry may segregate dwarfs and giants.

Faint High-Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results

We report the discovery of 39 faint high-latitude carbon stars (FHLCs) from Sloan Digital Sky Survey (SDSS) commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0 and show a diversity of temperatures as judged by both colors and NaD line strengths. Although a handful of these stars were previously known, these objects are, in general, too faint and too warm to be effectively identified in other modern surveys such as the Two Micron All Sky Survey, nor are their red/near-IR colors particularly distinctive. The implied surface density of FHLCs in this magnitude range is uncertain at this preliminary stage of the survey because of completeness corrections but is clearly greater than 0.05 deg-2. At the completion of the Sloan survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper-motion measures for each object, indicating that the sample is a mixture of extremely distant (greater than 100 kpc) halo giant stars, useful for constraining halo dynamics, and members of the recently recognized exotic class of very nearby dwarf carbon (dC) stars. The broadband colors of the two populations are indistinguishable. Motions, and thus dC classification, are inferred for 40%–50% of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. The observed kinematics suggest that the dwarfs occupy distinct halo and disk populations. The coolest FHLCs with detectable proper motions in our sample also display multiple CaH bands in their spectra. It may be that CaH is another long-sought, low-resolution, spectroscopic luminosity discriminant between dC's and distant faint giants, at least for the cooler stars.

A search for faint galactic carbon stars from the First Byurakan Spectral Sky Survey. I.

We present and analyse in this paper the first results of a systematic search for Galactic carbon (C) stars using the First Byurakan Spectral Sky Survey (FBS). The present surveyed area extends over ~6140 deg2 on the sky corresponding to ~ of the complete FBS Survey. Accurate positions, spectral classifications, approximate B and R magnitudes, and finding charts are provided for 35 objects. Identification and spectral subclasses (R or N) of those stars, belonging to the magnitude range 10.5 ≤ R ≤ 16.4, were determined by scrutinizing the Byurakan Schmidt telescope objective-prism plates. Subsequent slit-spectroscopic observations were obtained by us for 21 stars, confirming the carbon nature of them all. From this result, it is assumed that the remaining C star candidates are most likely carbon-rich stars as well. 18 of the listed objects at a Galactic latitude (°) are faint enough () to be considered as Faint High Latitude Carbon (FHLC) stars according to the definition of Totten & Irwin ([CITE]). JHK magnitudes for 23 objects of our list are provided by the 2MASS database. Approximate distances are estimated from R and/or JHK data. Most of the stars are located between 7 and 25 kpc from the Sun while their distances z to the Galactic plane range from about 2 to 25 kpc. The most distant object of our sample is found at 45 kpc and kpc. It is interesting to note that the rate of newly discovered FBS FHLC stars -one object per 200 deg2 -is comparable to that achieved by other previous surveys. Consequently it is reasonable to expect that the full exploitation of the FBS plates might result in the identification of about 45 new FHLC stars.

The APM survey for cool carbon stars in the Galactic halo -- I

A by-product of the APM high-redshift quasar survey was the discovery of several distant (20–100 kpc) N-type carbon stars at high galactic latitude. Following on from this, we have started a systematic all-sky survey at galactic latitudes ⊢b⊢>30° to find further examples of these rare objects, and we report here on the results from the first season of follow-up spectroscopy. Faint, high-latitude carbon (FHLC) giants make excellent probes of the kinematic structure of the outer Galactic halo. Therefore, in addition to detailed spectrophotometry covering a wide wavelength range, we have obtained high-resolution (∼1 A) spectra centred on the CN bands at ∼8000 A, and have derived accurate (≲10 km s−1) radial velocities for the known FHLC stars. From the initial phase of our survey covering ≈6500 deg2, we find a surface density of faint N-type carbon stars in the halo of ≈1 per 200 deg2, roughly a factor of 4 less than the surface density of CH-type carbon stars in the halo. Intermediate-age, N-type carbon stars seem unlikely to have formed in the halo in isolation from other star-forming regions, and one possibility that we are investigating is that they either arise from the disruption of tidally captured dwarf satellite galaxies or are a manifestation of the long-sought optical component of the Magellanic Stream.

Two mass-losing carbon stars in the Galactic halo

Observations of two mass-losing carbon stars in the Galactic halo, lRAS 08546 + 1732 and 12560 + 1656, are presented. These objects were discovered serendipitously, and stand out from the usual carbon stars at high galactic latitudes in that they have optical and lRAS colours consistent with current mass-loss. New optical spectra, CO observations, and modelling of the spectral energy distribution and of the CO lines are presented. Luminosities are derived using a period-luminosity relation. From the dust modelling lRAS 08546 + 1732 is found to be at 20 kpc from the Sun (11.3 kpc from the Galactic plane) and has a mass-loss rate of 3.3 x 10-6 Mo yc1• From the CO non-detection we deduce that it is probably oxygen-deficient, corroborating earlier work. lRAS 12560 + 1656 is found to be at 8.0 kpc from the Sun (7.8 kpc from the Galactic plane) and has a mass-loss rate of 1.3 x 10-6 Mo yr-1. The detection of the 12CO J = 2-1 transition in the spectrum of lRAS 12560 + 1656 after an integration time of 10 h makes it probably the longest 12CO integration on a stellar object. The detection itself makes the star one of the most distant stellar objects detected in the CO line. The outflow velocity of 3.2 km S-1 is very low, and the stellar velocity is + 88 km S-1 with respect to the LSR. Modelling of the CO line implies an oxygen abundance of 0.7 dex below solar. We examine existing data on the 'faint high-latitude carbon stars' and identify two additional distant, mass-losing, N-type AGB stars. The nature of halo carbon stars is discussed, and suggestions on how to find more mass-losing halo AGB stars are . presented.

A CCD survey for faint high-latitude carbon stars

We describe a wide-area CCD survey to search for faint high-latitude carbon (FHLC) stars. Carbon giants provide excellent probes of the structure and kinematics of the outer Galactic halo. We use two-color photometric selection with large-format CCDs to cover 52 sq deg of sky to a depth of about V = 18. Of 94 faint C star candidates from our own CCD survey, one highly ranked V = 17 candidate was found to have a strong carbon and CN bands. We estimate that, to a depth of V = 18, the surface density of FHLC stars is 0.02 deg(exp -2). An updated FHLC sample is used to constrain halo kinematic and structural parameters. Although larger samples are needed, the effective radius of FHLC giants, assuming a de Vancouleurs law distribution, is larger than that for Galactic globular clusters.

FAINT HIGH-LATITUDE CARBON STARS

ABSTRACT Since carbon giants are ideal for study of the structure and kinematics of the outer galactic halo, we have undertaken a wide-area survey to search for faint high-latitude carbon (FHLC) stars. We use two-color photometric selection with large format CCDs to cover 52 deg2 of sky to a depth of about V=18. Below this limit, we find good (< 20%) agreement between our object counts as a function of magnitude and the galactic models of Bahcall and Coneira (1984) at a variety of latitudes and longitudes. Our spectroscopic followup began with low-resolution spectra of 19 unconfirmed C-star candidates from the Case objective-prism photographic survey of Sanduleak and Pesch (1988). Four of these we find to be M stars. The 15 C stars we classify on the two-parameter Keenan-Morgan (1941) system as warm (color class < 4), with moderate to weak carbon band strengths (C class < 3). Of 94 faint C-star candidates from our own CCD survey, one highly ranked V=17 candidate was found to have strong carbon and CN bands. We estimate that to a depth of V=18, the surface density of FHLC stars is 0.0190.044-0.016 deg-2. We identify two FHLC stars with previously cataloged high-proper-motion objects. These objects are thus inferred to be dwarf carbon (dC) stars, supplementing the one previously known case, G 77-61. Not all dC stars will have detectable proper motions, so other luminosity/distance indicators are needed: we find that C dwarfs all have similar JHK colors, and possibly an unusually strong lambda-6191 bandhead of carbon. By comparing positions of the HST Guide Star Catalog and the original Palomar Observatory Sky Survey, we detect proper motions in two additional FHLC stars. Our proper-motion survey, spanning a 30-yr base line, thus identifies four new dC's, and provides proper-motion upper limits for another 44 FHLC stars. Kinematic simulations suggest that virtually all Population II dCs will have detectable proper motions in a survey as sensitive as our own, and that to a limit of V 18, at least 10% of FHLC stars are dwarfs. The local space density of dC stars may thus substantially exceed that of luminous C stars, so that the automatic assumption of giant luminosities for C stars is no longer valid.

Three newly recognized dwarf carbon stars

The identification of two faint high-latitude carbon stars with previously cataloged high proper motion objects is confirmed, and the discovery of proper motion in a third reported. The R = 15.5 star CLS 96 corresponds to LP 328-57, and C * 22 (Bothun et al., 1991, Astron. J., 101) is LP 765-18 = LHS 1075. These three objects are thus inferred to be dwarf carbon stars, supplementing the one previously known case. All of these objects have JHK colors similar to the prototype dward C star, G77-61, suggesting IR colors as a useful luminosity indicator. LP 328-57 and G77-61 also share certain spectral peculiarities