5 Element abundances in the high velocity component

Supernovae explosions can significantly alter the elemental abundances of the ambient interstellar gas through both the dissemination of the newly nucleosynthesized material and through the sputtering and evaporation of interstellar dust grains by the incident SNR shock front. WJ reported an anomalous N(Na I)/N(Ca II) ratio of $\sim$0.7 for the HV absorption component towards HD 47240 which is typical for high velocity gas in the general ISM (Siluk $\&$ Silk 1976). Since sodium depletion is thought to be approximately constant in the diffuse ISM (Jura 1976), then the low Na I/Ca II ratio can be attributed to the removal of adsorbed calcium atoms from dust grain surfaces by interstellar shock waves. However, our higher resolution visible observations reveal the HV component to be split into (at least) two components whose Na I/Ca II ratios are 0.29 (V = + 60 kms^-1) and 1.3 (V = + 70 kms^-1) respectively. This variation in Na I/Ca II column density ratio between the two cloud components can be easily seen in Figs. 4 and  5, and suggests that the gas in the inner (slower) regions of the SNR may well have been more affected by the SNR blast wave.

 

 

Table 4: Comparison of depletions for the line-of-sight to HD 47240.

Element	Mon Loop	Vela SNR	Cold ISM Gas	Warm ISM Gas
	HVC Depletion	HVC Depletion(1)	Depletion(2)	Depletion(2)
S	0	0	0	0
Mg	N/A	N/A	-1.24 to -1.56	-0.73 to -0.9
Si	-0.57 (0.17)	-0.5	-1.31	-0.35 to -0.51
Fe	-0.96 (0.21)	-0.55	-2.09 to -2.27	-1.19 to -1.24
N	-0.98 (0.21)	-1.72	-0.07	-0.2
O	-1.21 (0.16)	-1.14	-0.39	-0.4
P	-0.17 (0.24)	-0.58	-0.5	-0.23
Al	-1.05 (0.24)	-1.2	-2.4	-1.1
Ar	-0.77 (0.24)	N/A	-0.48	N/A
(1)- Jenkins et al. (1984), Jenkins et al. (1998).
(2)- Savage & Sembach (1996), Welty et al. (1999).

Assuming that solar abundances reflect the present element abundances in the interstellar medium, then the depletion factor ($\delta$) of a given element in the gas phase can be defined as $\delta _{\rm X}=\log \left( N({\rm X})/N({\rm H})\right)_{\rm cloud} -\log (N({\rm X})/N({\rm H}))_{\rm sun}$. Element abundances may be thus determined if the column density of hydrogen, N(H), is known for each cloud component along the line-of-sight to HD 47240. Unfortunately such values are, as yet, unavailable from Ly$\alpha$ or H I radio 21 cm studies. Thus, assuming that sulphur (S) is undepleted in the interstellar gas (as is normally found for lines-of-sight with relatively low reddening), and that it has a solar abundance relative to hydrogen, then the (equivalent) hydrogen column densities for each absorption component may be determined. This also assumes that the S II lines (around 1255 Å) are the dominant ionization state of interstellar sulphur in line-of-sight towards the Monoceros Loop.

The HV component at V = +65 kms^-1 was clearly detected in the SII (1253 Å) line of the $\it IUE$ spectrum of HD 47240. It was also seen in the SII 1259 Å line profile, but was contaminated by the nearby strong interstellar line of Si II (1260 Å). We failed to detect an HV component in the weaker SII line at 1251 Å line. Using the derived column density, log N(S) = 14.46 cm^-2, we calculate an equivalent value of log $N{\rm (H I)} =19.20$ cm^-2 for the HV component. Using this value of N(H I) we derive other element abundances under the assumption that the observed ionization stages for each element are the dominant ones in the ISM. The resultant element depletion patterns (relative to that of S) for the HV component are shown in Table 4. For comparison purposes we also show the patterns of element depletion for the (younger) Vela SNR (Jenkins et al. 1984, 1998) and the depletion patterns associated with both warm and cool interstellar disk gas as listed by Savage & Sembach (1996) and Welty et al. (1999).

In regions of the interstellar medium that have been highly disturbed one may expect that O, N and S will be relatively undepleted, and that Mg, Si and Fe will be depleted onto refractory grains that are gradually returned to the gas phase by sputtering and by grain-grain collisions. We note that for both the Monoceros Loop HV component and the Vela HV gas that the refractory elements of Fe, Si and Al are less depleted than that found for the cold ISM disk gas, suggesting that grain sputtering has infact returned these elements back to the ambient SNR interstellar gas. These levels of Fe, Si and Al depletion in both of the Vela and Monoceros Loop SNR HV gas clouds are more consistent with that found in the warm disk gas of the interstellar medium, which is generally characterized by gas with temperatures of $\sim$8000 K.

In contrast, we find that the elements of N, O and Ar are more depleted in both the Vela and Monoceros Loop HV gas than that generally found for both the cold and warm ISM. Jenkins et al. (1998) have argued that the strong deficiencies of both N and O in HV gas components can probably be attributed to ionization effects in which the observed lines of N I, Ar I and O I are not the dominant states for these elements in the HV gas. However, we note from Bohigas (1983) that a gradual dilution of nitrogen-rich SNR gas filaments is expected as a SNR ages and the supernova ejecta are enriched with swept-up elements from the surrounding ISM. Finally, we note the relatively small depletion of phosphorus in the Monoceros Loop HV gas which is similar to that found for the warm phase of the ISM disk gas. This pattern of near solar-system abundance for P (and Zn) has also been observed towards the disk stars $\mu$ Columbae and $\zeta$ Oph (Howk et al. 1999).