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S-type AGB stars

	S stars are a group of red giants with enhanced s-process elements (strong ZrO, LaO lines, see Vanture & Wallerstain, 2002ApJ...564..395V). If the enhanced s-process elements come from the red giants themselves, they are intrinsic S stars and are in a transitional phase in the AGB evolutionary sequence from M to S to C stars. If, however, the enhanced s-process elements come from its companion in a binary system, they are extrinsic S stars. A important characteristics of intrinsic S star is the detectability of Tc lines (because of the short lifetime of this unstable element), while extrinsic S stars do not have (see Jorissen & Mayor, 1988A&A...198..187J, 1992A&A...260..115J).
	The General Catalogue of Galactic S Stars 2nd ED. (GCGSS2): present all 1347 S stars known at that time. (Stephenson, 1984PW&SO...3....1S)
	Other ways to descriminate intrinsic and extrinsic S stars: (1) periodic radial velocity variation that indicates a binary system (see Jorissen & Mayor, 1988A&A...198..187J, 1992A&A...260..115J); (2) UV emission from hot WD companion in a binary (see Johnson et al., 1990fmpn.coll..332J, 1993ApJ...402..667J); (3) He I 1083nm line that only appear in stars earlier than K type stars and thus should not appear in intrinsic S stars (see Brown et al., 1990AJ.....99.1930B).

Infrared Observations

They identifed USNO, MSX, 2MASS and IRAS counterparts of 528 S stars (among 1347 S stars in GCGSS2). They used the combination of two color-color diagrams [H-12]-[K-12] and [K-12]-[J-25] to statistically identify 256 candidate intrinsic and 147 candidate extrinsic S stars, thus increased the total number of intrinsic and extrinsic S star candidates to 287 and 176, respectively. (from Yang et al., 2006AJ....132.1468Y)

They analyzed ISO SWS01 spectra of 17 S stars (20 stars in total, but with the other 3 with bad ISO data quality). 15 of these stars are intrinsic S stars while the rest 2 are extrinsic S stars. They divided the intrinsic S stars into 3 groups (I, II, III) with increasingly redder colors (i.e., increasing mass loss rates). S stars in each group show similar SED shape. The group I stars share similar SED with the 2 extrinsic S stars. They identified molecular features as follows:     2.36-2.65um, H2O+CO, absorption,     2.66-3.38um, H2O+CO2, absorption,     2.90-3.30um, HCN, absorption,     3.86-4.35um, CS, absorption,     4.6um, H2O+CO+CO2, absorption,     6.38-9.70um, H2O, absorption,      8.4-15.6um, dust, emission. The dust feature show variable peak positions from 10.5um to longer than 11 um. (from Yang et al., 2007A&A...463..663Y) (figs: ISO spectra of the 17 S stars, grouped into I, II and III, according to increasing mass loss rate.)

Molecular Observations

	They surveyed 27 nearby (d<1kpc) S stars for their CO,1-0, 2-1, HCN, 1-0, and SiO, 2-1 lines. (from Bieging & Latter, 1994ApJ...422..765B)
	They detected HCN (0,1^1c,0) J=3-2 and (0,0,0) J=8-7 lines from the S star Chi Cygni. The high excitation temperature of these lines indicate that the emission should come from the inner CSE of the star (within 20 R*). Therefore, it support the idea that HCN is made by non-equilibrium chemistry in the shock regions close to the stellar surface. (from Duari & Hatchell, 2000A&A...358L..25D)