DIFFUSE ATOMIC AND MOLECULAR GAS NEAR IC 443

IC 443 seems to be an example of the interaction of a supernova remnant (SNR) with an interstellar (IS) cloud or complex of clouds. As such it provides the opportunity to observe the effects of the SN shock penetrating the high-density gas of the clouds. This involves heating the cloud to X-ray temperatures, followed by rapid cooling leading to a significant increase in the cloud's density. IC 443 has been a very popular target and comparison object primarily for X-ray observations and as an object with emission by excited molecules. According to Set of Identifications, Measurements, and Bibliography for Astronomical Data (SIMBAD) there have been 619 published studies or references to published studies between 1850 and 2007.

IC 443 has provided an opportunity to compare observations and theory of the hydrodynamics and radiative transfer associated with the penetration of clouds by an SN shock. Specific calculations were first made by McKee & Cowie (1975) and reviewed by Chevalier (1977). Early detailed spectroscopic observations of absorption from the SN shocked clouds associated with the Vela Remnant were described by Jenkins et al. (1976). Here we describe measurements on atomic and molecular absorption seen in the spectra of stars behind IC 443 and attempt to link them to the wealth of results already available.

Observations of SNRs may be made by a wide variety of instruments each of which defines the spectral resolution, spectral coverage, and spatial resolution. We present new data on IS absorption from atoms and molecules at visible wavelengths. Such observations provide the finest spatial resolution and high spectral resolution, but are limited by the availability of background stars. The stars must be of sufficient brightness and have a relatively simple absorption spectrum to permit a clear definition of the IS features. Members of the Gem OB1 association afford us the opportunity to probe significant portions of the sky in the vicinity of IC 443.

Early molecular observations with good spatial resolution of phenomena associated with IC 443 were obtained by Cornett et al. (1977) and Scoville et al. (1977) using the CO line at 2.6 mm. The former achieved a resolution of 24'' × 27'' while the latter had a half-power beamwidth of 2'. Both studies compared their contours with the red image of the Palomar Sky Survey where the emission is dominated by Hα. The strongest emission was observed to come from the region between the two most prominent Hα emitting clouds, hinting that the CO was associated with a molecular cloud rather than with the nearby H ii regions. In fact most papers since then have referred to IC 443 as the interaction of a SNR with a molecular cloud. As such it is probably the most readily observable example of an SNR associated with a molecular cloud because both the Vela Remnant and the Cygnus Loop seem to involve atomic gas with only a small amount of associated molecules.

Two papers within the past few years have attracted attention to IC 443 once again. Snell et al. (2005) used the Submillimeter Wave Astronomy Satellite to observe previously inaccessible lines of H₂O, O₂, C i, and ¹³CO along with ground-based observations of CO and HCO⁺. The authors invoke a combination of a fast J-type shock with velocity of about 100 km s⁻¹ and a slow shock (10–20 km s⁻¹) that could be either J-type of C-type. Using the Very Large Array and Arecibo telescopes Lee et al. (2008) observed 21 cm H i emission with a resolution of 40 arcseconds, comparable to that of the CO emission of Cornett et al. H i emission with a velocity range from −100 to +50 km s⁻¹ was found. The systemic velocity was −5 km s⁻¹, like that found for the ambient molecular cloud. The high spectral resolution allowed Lee et al. to resolve the shocked H i into filaments, and they proposed that the main shock propagated through a uniform medium before reaching the edge of the cloud and freely expanding toward the southeast. Much of the H i emission in the northeast portion was attributed to recombining gas behind the shock front, while the southern emission was coincident with the dissociative molecular shock.

Our main goal is to examine how results from atomic and molecular absorption fit into the above picture. We also build upon the earlier measurements on absorption from Na i D and Ca ii K by Welsh & Sallmen (2003), who studied four stars at relatively high spectral resolution. They detected absorption from numerous components over a velocity range from −100 to +50 km s⁻¹, much like that seen in H i emission. Our sample enlarges the number of sight lines probed and incorporates measurements on Ca i, K i, and the molecular species CH⁺, CH, C₂, and CN. These species help us make connections between the diffuse molecular gas and the ambient molecular cloud. The result is a more complete understanding of the interaction between a SN shock and a molecular cloud.

Observations were obtained with the echelle spectrograph of the 3.5 m telescope of the Apache Point Observatory on 2006 March 21 and 2006 December 28, 2007 January 5 and 28, and 2008 January 3. Our targets are listed in Table 1, along with pertinent stellar data. The coordinates and spectral type, as well as the V and B–V magnitudes, are from the SIMBAD database, operated by Centre de Donées Astronomiques de Strasbourg (CDS), Strasbourg, France. The absolute magnitude and reddening are derived from these data (see Böhm-Vitense 1981). Figure 1 displays the position of the lines of sight relative to CO contours of Huang et al. (1986). The extent of the radio continuum emission is indicated by the dashed line. Bias frames, flat fields, and Th–Ar comparison spectra were acquired each night. The bias subtracted, flat fielded, wavelength calibrated, one-dimensional spectra were extracted with Image Reduction and Analysis Facility (IRAF) routines. Doppler corrections were applied to bring the spectra into the frame of the local standard of rest. All spectra have a resolving power of about 35,000 and a signal-to-noise ratio from about 200 at 8000 Å declining slowly to between 50 and 100 at the CN lines near 3875 Å. Representative spectra appear in Figures 2–4.

Table 1. IC 443 Objects

Name	R.A. (2000)	Dec. (2000)	V	B–V	Spectral Type	E(B–V)	MV
ALS 8828	06h16m133	+22°45'48''	10.90	0.60	B2V	0.82	−2.50
HD 254346	06 16 57.3	+22 11 42	9.74	0.39	B2:III:	0.63	−3.60
HD 254477	06 17 26.4	+22 25 38	9.77	0.59	B8V	0.68	0.10
HD 254577	06 17 54.3	+22 24 32	9.19	0.64	B0.5II-III	0.92	−4.95
HD 43582	06 18 00.3	+22 39 29	8.79	0.32	B0IIIn	0.62	−5.00
HD 254700	06 18 25.3	+22 57 34	9.83	0.33	B5V	0.49	−1.00
HD 254755	06 18 31.7	+22 40 45	8.91	0.42	O9Vp	0.73	−4.80
HD 43703	06 18 39.4	+23 00 28	8.65	0.33	B1IV:p	0.59	−4.10
HD 43753	06 18 59.7	+23 00 04	7.89	0.25	B0.5III	0.53	−4.70
HD 43871	06 19 34.5	+22 49 47	8.38	0.05	A0V	0.05	1.00
HD 43907	06 19 45.2	+22 06 38	8.70	0.30	B1V:p	0.56	−3.60

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The moderate-resolution spectra offered a glimpse at the component structure for the directions probing the vicinity of IC 443. Using standard routines in the IRAF environment, we extracted equivalent widths (W_λ) in the following way. We started the process by obtaining the structure seen in the K i λ7699 spectra; the features are relatively strong and many of the components are present. We then found the amount of absorption from the species showing much weaker absorption (Ca i and the molecules CH, CH⁺, CN, and C₂), making certain of the reality of these components by requiring consistency in v_LSR among the species in light of the spectral resolution. The high-resolution study by Pan et al. (2004) helped us determine the expected correspondences. The last step involved the extraction of the Ca ii components. The availability of lines from both Ca ii H and K aided the process, especially when the number of components was much greater than seen in other species. The Ca ii absorption toward HD 43582 was particularly challenging; in this case we were guided by the results from Welsh & Sallmen (2003), but we note that our final component structure differed somewhat from theirs. Our survey does not incorporate results on the Na i D lines; the substantial optical depth at line center and our coarse spectral resolution did not allow meaningful interpretation. Tables 2 and 3 present the compilation of our set of W_λs.

Special care is required when deriving column densities from spectra that may contain unresolved component structure. We applied several methods to minimize the potential impact of unresolved structure. First, the "effective" b-value for the dominant, molecule-bearing components was determined by requiring the absorption from K i λ7699 and from the much weaker doublet at 4044 and 4047 Å, which was seen toward HD 254755, to yield the same column density. Because many of the lines are not optically thin, this was necessary to account for optical depth effects in curves of growth. A self-consistent column density indicated an "effective" b-value of 2.2 km s⁻¹. The detailed analysis of component structure in Ca ii, Ca i, K i, CH⁺, CH, and CN by Pan et al. (2005) revealed a typical b-value for K i lines of 1 km s⁻¹, suggesting that unresolved structure is indeed present in our spectra. It appears that there are two components separated by about 2 km s⁻¹ hidden within the features seen in our data. According to Pan et al. (2005), K i, Ca i, CH, and CN lines have comparable b-values, and we adopted a b-value of 2.2 km s⁻¹ for the latter three species in an analysis based on curves of growth. For CH⁺, whose typical b-value is somewhat larger (Pan et al. 2005), we chose b = 2.5 km s⁻¹. The suite of molecular oscillator strengths used here comes from (Federman et al. 1994) and the atomic f-values are from Morton (1991), for consistency with our earlier work. The differences with Morton (2003) are only at the level of a few percent.

Second, the complete spectra afforded by the echelle spectrograph allowed us to compare results from strong and weak lines from CH⁺, CH, C₂, and CN. In particular, there are (1) CH⁺λλ3957,4232, (2) the A−X and B−X transitions in CH, (3) P, Q, and R lines for C₂, and (4) the P and R lines of CN. Furthermore for CH, the lines at 3878 and 3890 Å arise from the same Λ-doubling component of the ground state, and earlier studies at visible wavelengths (Danks et al. 1984; Lien 1984; Jura & Meyer 1985) indicate that the populations for both Λ-doubling components are the same. Thus, the column density obtained from λ3886 should equal that from the two lines of the other Λ-doubling component. Within the mutual uncertainties of the features detected in our spectra, self-consistent column densities are obtained in all the cases. In the cases of C₂ and CN, where absorption from multiple rotational levels is seen, total column densities are also obtained for the chemical analyses described below. The summary of results for C₂ is given in Table 4. When only J = 4 shows detectable absorption, or when absorption is only seen for J = 0–4, the total C₂ column density is estimated from the distribution of levels seen toward HD 254577. In the former case, the result for J = 4 is multiplied by 3.3, while the sum for J = 0–4 is multiplied by 1.5 in the latter. When comparison is made with high-quality results based on the D−X (0,0) band at 2313 Å (Lambert et al. 1995; Sonnentrucker et al. 2007), the multiplicative factors yield total column densities that are 25%–40% smaller. However, most of the difference arises from levels with J > 10. When only the J = 0 line of CN is detected, the CN column density is inferred by multiplying the result for the ground state by 1.5, a result consistent with excitation from the 2.7 K Cosmic Background.

For Ca ii H and K, we adopted the Doublet Ratio Method (Strömgren 1948; Münch 1968). A listing of the set of column densities for each component seen in Ca ii absorption appears in Table 5; the velocities come from the results for Ca ii K. A comparison of the Ca ii column densities toward HD 254577, HD 43582, and HD 254755 found by Welsh & Sallmen (2003) generally shows reasonable agreement, with the most optically thick velocity components agreeing to better than 50%. With our limited spectral resolution and their limited signal noise, better correspondence cannot be expected.

3.1. General Results

3.2. Molecular Analysis

3.3. High-velocity Gas

Our measurements of atomic and molecular absorption toward stars lying behind IC 443 reveal connections to the gas studied via H i and CO emission. The molecular rich diffuse gas has a velocity that associates it with the ambient molecular cloud. Its density is considerably lower than that inferred for the molecular cloud, but this is not unexpected because absorption probes the diffuse envelope of the cloud. High-velocity gas is revealed by Ca ii absorption, which along the eastern edge of the nebula spans a range from −120 to +80 km s⁻¹. This range is similar to that seen in the shocked molecular gas and its associated 21 cm emission. One difference is that individual velocity components in the Ca ii spectra are discerned, suggesting that the expanding SNR created a 100 km s⁻¹ shock. The densities derived in the analysis of 21 cm emission, ∼10 cm⁻³, probably pertain to the high-velocity gas that we observe. The optical results presented here provide another facet of the interaction between a SNR remnant and a molecular cloud surrounding it and will aid future modeling efforts.

We thank Ron Snell for permission to use a digitized version of the figure in Huang et al. (1986). G.W. acknowledges an enlightening conversation with Bruce Draine regarding the origin of CH⁺. The research by Alec Hirschauer and Tim Means was supported by the Kenilworth Fund of the New York Community Trust. We made use of the SIMBAD database, operated at Centre de Donées Astronomiques de Strasbourg, Strasbourg, France.