"A bright star [in the image at left] is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star."
"U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds."
"Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the centre of the image. Its brightness, however, is enough to overwhelm the capability of Hubble’s Advanced Camera for Surveys making the star look much bigger than it really is."
"The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable (see for example Hubble’s images of Eta Carinae, potw1208a), the shell of gas expelled from U Cam is almost perfectly spherical."
"The image was produced with the High Resolution Channel of the Advanced Camera for Surveys [using the 606 nm and 814 nm filters]."
Abundance "estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in [the] carbon-rich, very metal-poor [star], [...] LP 706-7 [are reported]. [...] A Pb abundance is also derived for LP 706-7 by a re-analysis of a previously observed spectrum."
"LP 706-7 [was] observed with the University College London coudé échelle spectrograph (UCLES) and Tektronix 1024×1024 CCD at the Anglo-Australian Telescope. [...] the numbers of photons obtained around the Pb I λ4057 are [...] 3000 per 0.04Å pixel (S/N∼80) for [...] LP 706-7".
"The surface gravity of LP 706-7 (Norris et al. 1997a) was based on the requirement that Fe I and Fe II lines give identical abundances. More recently, a trigonometric parallax for this star has been published from the Hipparcos mission (ESA 1997), π = 15.15 ± 3.24 mas. Somewhat surprisingly, this surface gravity indicates an absolute magnitude MV = 8.0 ± 0.4, which is subluminous compared to both main sequence and subgiant Population II stars with Teff = 6000 K. A subgiant of MV = 3.0 or 4.0 would have a parallax of only 1.5 or 2.4 mas. Either the Hipparcos measurement of this star is significantly in error, or the star is far more bizarre than its CH-star status suggests. If the temperature estimate (based on photometric colors) and the Hipparcos parallax were both correct, we should be forced to infer a radius ten times smaller than for a subgiant and four times smaller than for a main-sequence star, but the surface gravity appears inconsistent with such a compact object (since g ∝ M/R2). It seems most likely that the Hipparcos parallax is simply incorrect, although an examination of the records (D. W. Evans, priv. comm.) revealed no concerns."
For "LP 706-7, because radial-velocity variations that might be expected for a star with a white-dwarf companion have not yet been detected (Norris et al. 1997a)."
We "found strong excesses of neutron-capture elements in the two metal-deficient satrs LP 625-44 and LP 706-7 with [Fe/H]= −2.7 and −2.74, respectively, which are interpreted as the result of s-process nucleosynthesis from a single site. Namely, the abundant material polluted by s-process nucleosynthesis dominates over the original surface abundances of neutron-capture elements. For instance, the Ba abundance in these two stars is a factor of several hundred times higher than the general trend of model predictions at [Fe/H]= −2.7. Even the abundance of Eu, which is usually interpreted as a signature of the r process, but should also be produced by the s-process as well, is enhanced by more than a factor of 10 in these two stars. Therefore, the neutron-capture elements in these two stars should present almost pure products of s-process nucleosynthesis at low metallicity. The exceptions to this are the abundances of Sr and Y in LP 706-7, which show no distinct excess. Therefore, the contribution of the s-process to these two elements may not be significant for this star."
There "is no evidence of binarity for [...] LP 706-7 (Norris et al. 1997a)."
The "precise mechanism for chemical mixing of protons from the hydrogen-rich envelope into the 12C-rich layer is still unknown, even for stars with solar metallicity, despite several theoretical efforts (Herwig et al. 1997; Langer et al. 1999). This makes it even harder to understand the peculiar abundance pattern of the s-process elements found in carbon-rich, metal deficient stars such as LP 625-44 and LP 706-7."
What "physical conditions are necessary to reproduce the observed s-process abundance profile of LP 625-44 and LP 706-7 without adopting any specific stellar model."
As "long as the same neutron exposure is adopted, the abundance patterns of LP 625-44 and LP 706-7 are reproduced with equivalent reduced χ2 values, even in extreme conditions of very high neutron density, Nn ≳ 1011cm−3. These parameter values simulate, more or less, the s-process conditions expected during the thermal pulse phase (Iben 1977)."
"Almost all elements, except for Pb, were found to be made in the first neutron exposure. Even the lead abundance converges after about three recurrent neutron exposures. This is consistent with the small overlap factor, r ≈ 0.1, deduced in our best-fit model. [...] fixed neutron exposure τ = 0.71 for LP 625-44. The observed Pb/Ba ratio is reproduced in the few-pulse model only for a small overlap factor, r ≲ 0.2, while the Ba/Sr ratio is rather insensitive to r and allows for a wider range, r ≲ 0.65. The Pb abundance is so sensitive to r that large r-values (0.2 ≲ r) are almost entirely excluded [...] This is a characteristic feature of the s-process pattern observed in LP 625-44 and LP 706-7."
"The ratio is slightly higher in LP 706-7, [Pb/Ba] = +0.27 ± 0.24. This may indicate that a range of 13C amounts is indeed required in the most metal-poor AGB stars, as well as for the moderately metal-poor ones."
"Hipparcos trigonometric parallaxes and photometric data [are available] for about 40 bright carbon stars [...] Individual absolute visual and bolometric magnitudes, normal color indices [blue (B), violet (V)] (B − V)0, absorption values and distance moduli were determined. By comparison with stellar evolutionary tracks for initial mass 1 ≤ M/M⊙ ≤ 4 it is found that the majority of CH- and R-stars are on the giant and subgiant branches, but N-stars occupy a region −4 < MV < −1 and 1.6 < (B − V)0 < 3.6 and correspond to an advanced stage of thermally pulsing asymptotic branch giants."
"Using Hipparcos parallaxes and proper motions, three multiple stars with a carbon star component are examined. Hipparcos data confirms a physical link between W CMa and HD 54306 (B2V), both probable members of the association CMa OB1. Some stars are located below the subgiant branch for the mass 1M⊙ and a number of the N stars are below the theoretical limit for carbon stars on the AGB."
"The most straightforward method, i.e. through trigonometric parallaxes, has hitherto been of little value, owing to the considerable distances even to the nearest carbon stars, and the imperfectness of previous measuring methods [...] The situation has radically changed after the mission by the astrometric satellite Hipparcos. The mean error of about 1 mas – a characteristic value for parallaxes measured by Hipparcos – provides us with reliable distance estimates inside, say, the 0.5 kpc region around the Sun including some 100 carbon stars."
"Hipparcos also supplied us with precise photometric data, giving the mean brightness estimate from ~ 100 observations of each star, a circumstance, which because of the variability of carbon stars, is of special value. Here, a problem specific to carbon stars – stars with a peculiar spectral energy distribution –, to accurately correct ground-based photometry for atmospheric extinction, is irrelevant for Hipparcos data."
"Hipparcos results clearly confirm that the great range of observed scatter in the color index B−V is intrinsic and not caused by different amounts of interstellar reddening [...] A considerable stretch in the horizontal direction is a result of enhanced sensitivity of the color index (B−V)0 to small temperature changes in a cool extended atmospheres; also various degree of violet depression play a definite role."
"The spectra of six carbon stars increase in brightness shortward of 3900 Å, indicating that the violet opacity in these stars is dominated by C3, not SiC."
"Carbon stars are evolved stars, similar to the Sun, which are nearing the ends of their lives, in the so-called asymptotic giant branch phase. Many form dusty, carbon-rich envelopes, due to mass loss, which makes the very red in color, especially in the near-infrared. IRAS 06088+1909 is a very dusty carbon star toward the Galactic anticenter (Jura & Kleinmann 1990, ApJ 364, 663). 2MASS is particularly sensitive to carbon stars. Liebert et al. (2000, PASP, 112, 1315) report on several very cool carbon stars in or beyond the Galactic halo, some of which are heavily dust enshrouded. They conclude that 2MASS can be used to define a useful sample of carbon stars at high Galactic latitude as tracers of the halo out to distances comparable to the Magellanic Clouds. In the 2MASS image, the fainter reddish "stars" immediately east of due north and west of due south of both IRAS 06088+1909 and the bluer bright star to its northeast are known 2MASS "filter glint" artifacts; known diffraction spike and persistence artifacts are also seen associated with these two bright stars."
"ESO PR Photo 34g/04 [second down on the right] shows the stellar cluster NGC 2093, a comparatively rich aggregate of young stars, a few tens of millions of years old. The hot temperature of the most massive of such young stars is responsible for its predominantly blue colour. The sky field measures 5.6 x 5.1 arcmin. North is up and East is left. ESO PR Photo 34h/04 shows the stellar cluster NGC 2108, a rich "mid-aged" cluster, about 600 million years old. A careful comparison with its neighbour NGC 2093 (PR Photo 34g/04) shows that the brightest stars of NGC 2108 are fainter and whiter than the brightest members of NGC 2093; this indicates that NGC 2108 is older. Note also the distinctly red star to the East (left) of the center of this cluster. This is a member of a stellar class referred to as "Carbon stars", cool giant stars that are characterized by the presence of carbon molecules (C2) in their atmospheres and having extremely red colours. This sky field measures 2.8 x 2.6 arcsec; the orientation is the same."
"NGC 2108 is a rich "mid-aged" cluster, about 600 million years old. A careful comparison with its neighbour NGC 2093 shows that the brightest stars of NGC 2108 are fainter and whiter than the brightest members of NGC 2093; this indicates that NGC 2108 is older. Note also the distinctly red star to the East (left) of the center of this cluster. This is a member of a stellar class referred to as "Carbon stars", cool giant stars that are characterized by the presence of carbon molecules (C2) in their atmospheres and having extremely red colours. This sky field measures 2.8 x 2.6 arcsec. North is up and East is left."
"There are different types of carbon-enhanced stars: (i) stars showing carbon enhancement with s-process element enhancement; (ii) carbon enhancement with r-process element enhancement; and (iii) carbon enhancement with normal n-capture element abundances. There is yet another type of very metal-poor stars with strong s-process enhancement that are only slightly carbon-enhanced ([C/Fe] = +0.2; Hill et al. 2002)."
"Christlieb et al. (2001) reported a sample of 403 faint high-latitude carbon (FHLC) stars identified by means of line indices – i.e. ratios of the mean photographic densities in the carbon molecular absorption features and the continuum bandpasses – which were the basis for the Hamburg catalogue of high Galactic latitude carbon stars. The identification was primarily based on the presence of strong C2 and CN molecular bands shortward of 5200 Å; it did not consider CH bands. At high Galactic latitudes, although the surface density of FHLC stars is low, different kinds of carbon stars are known to populate the region (Green et al. 1994). One kind is normal asymptotic giant branch (AGB) stars, carbon-enriched by dredge-up during the post-main-sequence phase, which are found among the N-type carbon stars. Another kind is FHLC stars showing significant proper motions and having the luminosities of a main-sequence dwarf, called dwarf carbon stars (dCs). A third kind of FHLC stars is the so-called CH giant stars, similar to the metal-poor carbon stars found in globular clusters and some dwarf spheroidal (dSph) galaxies (Harding 1962). Among these, at high Galactic lat- itudes, warm carbon stars (possibly some C-R stars) are also likely to be present. The sample of stars offered by Christlieb et al. (2001), being high-latitude objects, with smaller initial mass and possibly lower metallicity, is likely to contain a mixture of these objects."
"Among the carbon stars, the C-N stars have lower temperatures and stronger molecular bands than those of C-R stars. C-N stars exhibit very strong depression of light in the violet part of the spectrum. They are used as tracers of an intermediate-age population in extragalactic objects. The C-R stars as well as CH stars have warmer temperatures and blue/violet light is accessible to observation and atmospheric analysis. C-N stars are easily detected in infrared surveys from their characteristic infrared colours. The majority of C-N stars show ratios of 12C/13C more than 30, ranging to nearly 100, while in C-R stars this ratio ranges from 4 to 9. The strength/weakness of the CH band in C-rich stars provides a measure of the degree of hydrogen deficiency in carbon stars."
"The s-process element abundances are nearly solar in C-R stars (Dominy 1984), whereas most of the carbon and carbon-related stars show significantly enhanced abundances of the s-process elements relative to iron (Lambert et al. 1986; Green & Margon 1994)."
“A red giant is a luminous giant star The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are the so-called red giant branch stars (RGB stars) ... Another case of red giants are the asymptotic giant branch stars (AGB) ... To the AGB stars belong the carbon stars of type C-N and late C-R. ... The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii.”
"The lithium content of red-giant stars is highly variable (Wallerstein and Conti 1969). The largest amounts of lithium are found in three carbon stars, WZ Cas, WX Cyg, and T Ara, being of the order of 10-2 of calcium. ... Boesgaard (1970) has found a similar high lithium abundance in the S star T Sgr. This is a higher ratio of lithium to calcium than is found in T Tauri stars or in meteorites."
Lithium has an infrared line at 812.6 nm.
Carbon has infrared emission lines: "C III at 0.971 µm and C IV at 2.075 µm".
The orange band from molecular CaCl is "observed in the spectra of many carbon stars."
"[T]he concentration of CaCl is strongly temperature and pressure dependent, but almost independent of the C/O ratio at a fixed pressure."
"The probable absence of CaCl bands in spectra of carbon stars with C/O ≫ 1 can be explained by CN opacity effects near 6000 Å, ... whereas the absence of CaCl bands in spectra of the coolest M and S stars can probably be attributed largely to molecular band masking."
"Temperatures [are] estimated from intensity ratios of atomic lines (used mainly for early C stars), color in the orange region of the spectrum, strength of the Na D-lines, and C2 band intensity gradients."
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- orange sphere of the sun
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