Target background stars in the field of IRC +10$^\circ$ 216

Deep images of IRC $+10\degr$ 216 in the B and V passbands, obtained by Mauron & Huggins (1999), have revealed its CSE as seen in dust-scattered, ambient galactic light. The envelope is structured in thin circular incomplete shells and extends out to $\sim$3, which corresponds roughly to the CO extension (Huggins 1995). Within 15$\arcsec$  of the centre, IRC $+10\degr$ 216 is opaque, but from roughly 15$\arcsec$ to 200$\arcsec$, it is possible to see through the CSE at optical wavelengths. Potential background targets are galactic field stars or very faint V=20-24 mag galaxies (see Fig. 1). Based on UBV photometry of the field obtained with the CCD camera of the 1.2 m telescope of Haute-Provence Observatory (to be reported in Mauron et al. 2002), a first selection of targets was performed. Provided it can be shown that such targets are located behind the CSE, obvious candidates are those stars being angularly the nearest to IRC $+10\degr$ 216 (to maximize circumstellar column density), the brightest in apparent magnitude (to permit high signal-to-noise ratio, high resolution spectroscopy) and the bluest in colour index (to minimize confusion by photospheric features in the spectrum).

Table 1 lists the targets studied in this paper, together with photometric data and our estimates of distances which are discussed below. The targets are identified in Fig. 1. The relevant circumstellar quantities are also listed in Table 1. The impact parameter r and tangential (line of sight) circumstellar column densities of hydrogen are derived using the following values for IRC $+10\degr$ 216: $v_{\rm outflow}=15$ km s^-1, d=130 pc and $\dot{M}_{\rm H}=2\times10^{-5}~M_{\odot}$ yr^-1 (Glassgold 1996; Groenewegen et al. 1998).

 

 

Table 1: Parameters for the observed stars and circumstellar quantities. The distances d are best estimates derived from our photometric and spectroscopic data (see Sect. 4); the angle $\Theta$ ($\arcsec$) is the angular offset from IRC +10$\degr$ 216; $N_{\rm H}^{\rm tang}$ is the expected line-of-sight circumstellar column density; $A_{\rm v}$ is the circumstellar extinction derived from $N_{\rm H}^{\rm tang}$when assuming a $N_{\rm H}$/$A_{\rm v}$ ratio typical of diffuse interstellar matter (see Sect. 2).

USNO 0975-0633-	name	V	B-V	Spec.	d/pc	$\Theta$/$\arcsec$	r/cm	$N_{\rm H}^{\rm tang}$/cm-2	$A_{\rm v}$/mag
6975	Star 6	16.00	0.78	G2:	$\sim$1400	37	$6.8\times 10^{16}$	$1.8\times 10^{21}$	0.9
6812	Star C	12.4	-	F8:	$\sim$500	153	$3.0\times 10^{17}$	$4.1\times 10^{20}$	0.2

Figure 1: Left panel: the $12.4 \times 12.4\arcmin$ field around IRC +10$\degr$ 216, imaged in the V-band at the 1.2 m Haute-Provence telescope. Stars 6 and C are labelled. Right panel: deep $2 \times 2\arcmin$ B-band CFHT image showing the IRC +10$\degr$ 216 envelope in dust scattered ambient galactic light. Star 6 is the bright star located 37$\arcsec$  below and slightly to the left of IRC +10$\degr$ 216.

The uncertainty in the $N_{\rm H}$ (exactly N(H+2H₂)) values demands examination. First, there is some uncertainty in $\dot{M}_{\rm H}$ and d; however, it can be remarked that $N_{\rm H}$ scales as $\dot{M}_{\rm H}$/d, and this ratio does not change by more than 30% between the case d=110 pc and d=190 pc (from Tables 2 and 6 of Groenewegen et al. 1998), because it is well constrained by observations. A more serious uncertainty is due to the fact that the CSE is inhomogeneous (clumpy). More precisely, the envelope density distribution observed from optical images is obviously not consistent with the smooth r^-2 law assumed for these $N_{\rm H}$ estimates. Hence the listed values of $N_{\rm H}$ are perhaps incorrect by as much as a factor of 2 in both directions, i.e., the true $N_{\rm H}$ may be higher or lower than these estimates. We note, however, that the mass loss rate measured at offsets $\Theta > 50$$\arcsec$ is found to be a factor of 5 larger than the one adopted above, as found from many observations probing, in general, layers nearer to the centre (Groenewegen et al. 1998). Consequently, our $N_{\rm H}$ values could in fact be underestimates of the true hydrogen column densities. We have also listed in Table 1, for illustration, the corresponding $A_{\rm v}$ values if one adopts the relation for the diffuse ISM, $N_{\rm H}$/ $A_{\rm v}=2.1\times10^{21}$ cm^-2 mag^-1 (Bohlin et al. 1978), but we note the actual circumstellar dust absorption could be lower (Omont 1991).

Concerning the distance of Star 6, examination of its B-V and U-B colour indices (0.78 and 0.31 respectively), faintness and high galactic latitude ( $b=+43\degr$) suggests that it may be a G-type (or reddened F) dwarf, or perhaps a metal-poor subdwarf. It is too blue to be a nearby M dwarf which would have $B-V\sim 1.5$. Estimating a distance relies on the adopted absolute magnitude and estimation of the interstellar and/or circumstellar absorption and reddening. Because of the high galactic latitude, the interstellar extinction is very low, and the maps of Burnstein & Heiles (1982) suggest $A_{\rm v}< 0.1$ mag. in a large zone around IRC +10$\degr$ 216. For Star 6 at least, the circumstellar column density should be larger than the interstellar one if our estimate of circumstellar column density above is correct. Making various assumptions concerning the amount of circumstellar dust extinction and reddening law (which is uncertain), and adopting subdwarf luminosities, e.g. $M_{\rm v} = +6.8$ (Allen 1973), we obtain for Star 6 a distance between 430 and 1100 pc, i.e. substantially greater than the distance to IRC +10$\degr$ 216. Larger distances would be inferred if one assumes a solar-like dwarf type, or a giant type.

No UBV data could be obtained for Star C because of saturation. The USNOC catalogue provides R = 13.0 and B-R = 0.2, which is relatively blue and suggests, with considerable uncertainty, an A3 spectral type. If this is the case, with $V \sim 13$, and $M_{\rm v}\sim +1.7$, one derives a distance of 2100 pc. If however, the USNOC colour is in error by 0.5 mag, which is not unlikely, then the star is nearer a F2-type with B-R = 0.7, then again with $V \sim 13$ and now $M_{\rm v}\sim +3.6$, its distance is about 800 pc. Only if this star were as late as a K7V, with B-R = 2.15 (which is unlikely) and $M_{\rm v}\sim +7.3$, would its distance be $\sim$140 pc.

In conclusion, the two targets seem to lie at favourable distances beyond IRC +10$\degr$ 216, i.e. beyond $\sim$130 pc. Unfortunately, neither Star C or 6 is present in the Hipparcos catalogue, preventing distance estimates using accurate parallaxes. However, the distances we estimate from photometry are very well confirmed by the spectral analysis described below. We note also that the expected circumstellar H column density for Star 6 would correspond to $A_{\rm v}=0.9$ mag in the ISM: this is comparable to that towards stars with intervening diffuse clouds in the spectra of which DIBs are readily detected, as evidenced by studies of DIBs in lines of sight which intersect only a single intervening diffuse cloud, with resultant low reddening (Cami et al. 1997; Weselak et al. 2000; Galazutdinov et al. 1998; Galazutdinov et al. 2000).