4 SNR G 315.4-2.30

We now briefly discuss a scenario for the origin of the SNR G 315.4-2.30 (a more detailed description will be presented elsewhere). We note that our scenario has much in common with the model proposed by Wang et al. (1993) to explain the origin of large-scale structures around the SN 1987A, and suggest that G 315.4-2.30 is an older version of the latter.

We believe that the SNR G 315.4-2.30 is the result of a cavity SN explosion of a moving massive star, which after the main-sequence (MS) phase (lasting $\sim$10⁷ yr) has evolved through the red supergiant (RSG) phase ($\sim$10⁶ yr), and then experienced a short ($\sim$10⁴ yr) "blue loop" (i.e. the zero-age MS mass of the star was $15{-}20~M_{\odot}$). During the MS phase the stellar wind (with the mechanical luminosity, L, of $\sim$ $10^{35} ~ {\rm erg} ~ {\rm s}^{-1}$) blows a large-scale bubble in the interstellar medium. The bubble eventually stalls at radius $R_{\rm st} \sim 12 (L_{35}/n)^{1/2} ~ {\rm pc}$ in time $t_{\rm st} \sim 10^6 (L_{35}/n)^{1/2} ~ {\rm yr}$, where $L_{35} = L/10^{35} ~ {\rm erg} ~ {\rm s}^{-1}$ and n is the number density of the interstellar gas. The current radius of SNR G 315.4-2.30 is $\simeq$16 pc. The motion of that star causes it to cross the stalled bubble and start to interact directly with the unperturbed interstellar gas. This happens at time $t_{\rm cr} \simeq R_{\rm st}/v_{\star} = 2.4\times 10^6 (L_{35}/n)^{1/2} v_{{\star},5} ^{-1} ~{\rm yr}$, where $v_{{\star},5}$ is the stellar velocity in units of $5 ~ {\rm km} ~ {\rm s}^{-1}$. The time is in agreement with the duration of the MS phase if $v_{\star} \simeq 1 ~ {\rm km} ~ {\rm s}^{-1}$. In this case the SN progenitor star enters in the RSG phase while it is near the edge of the MS bubble. During the relatively short RSG phase, the star loses most (two thirds) of its initial mass in the form of a dense, slow wind. The interaction of the RSG wind with the interstellar medium results in the origin of a bow shock-like structure with a characteristic radius, r, determined by the relationship: $\dot{M} _{\rm RSG} v_{\rm RSG} /4\pi r^2 \simeq n(2kT + m_{\rm H} v_{\star} ^2)$, where $\dot{M} _{\rm RSG}$ and $v_{\rm RSG}$ are, correspondingly, the mass-loss rate and wind velocity during the RSG phase, T is the temperature of the ambient interstellar medium, k is the Boltzmann constant, and $m_{\rm H}$ is the mass of a hydrogen atom. For $n\simeq 1~ {\rm cm}^{-3}$ (Smith 1997, Ghavamian et al. 2001), $\dot{M} _{\rm RSG} = 10^{-5} ~ M_{\odot} ~ {\rm yr}^{-1}$, $v_{\rm RSG} = 10 ~ {\rm km} ~ {\rm s}^{-1}$, T=8000 K, and $v_{\star} = 1 ~ {\rm km} ~ {\rm s}^{-1}$, one has $r\simeq 1.5$ pc. This value is in a comfortable agreement with the radius of the hemispherical optical nebula in the southwest of G 315.4-2.30 ($\simeq$1.6 pc).

About $\sim$10⁴ yr before the SN explosion the progenitor star becomes a blue supergiant whose fast wind sweeps the material of the RSG wind. We speculate that at the moment of SN explosion the blue supergiant wind was trapped in the southwest direction by the dense material of the bow shock-like structure, while in the opposite direction it was able to break out in the low-density MS bubble, so that the SN explodes inside a "hollow" hemispherical structure open towards the MS bubble.

The SN blast wave expands almost freely across the low-density MS bubble (for the explosion energy of 10⁵¹ erg and mass of the SN ejecta of $\simeq$ $3.5~M_{\odot}$, the expansion velocity of the blast wave is $v_{\rm bl} \simeq 5000 ~ {\rm km} ~ {\rm s}^{-1}$) and reaches the northeast edge of the bubble after $\sim$ $4.7\times 10^3$ yr. The density jump at the edge of the bubble results in the abrupt deceleration of the blast wave to a velocity of $\sim$ $(\beta n_{\rm b} /n)^{1/2} v_{\rm bl} \sim 600{-}800 ~ {\rm km} ~ {\rm s}^{-1}$ (the values derived from studies of Balmer-dominated filaments encircling the SNR G 315.4-2.30; Long & Blair 1990 and Smith 1997), where $\beta \simeq 5$ (e.g. Sgro 1975). This deceleration implies a density jump of a factor of $\simeq$200-300 or a number density of the MS bubble gas $n_{\rm b} \simeq 0.003{-}0.005 ~ {\rm cm}^{-3}$, i.e. a reasonable value given the mass lost during the MS phase is $\sim$ $0.5~ M_{\odot}$.

In the southwest direction, however, the expansion of the SN blast wave is hampered by the dense hemispherical circumstellar shell. The shocked remainder of this shell are now seen as the bright southwest protrusion (shown in Fig. 3). The existence of radiative filaments in this corner of the SNR implies that the blast wave slows down to the velocity of $\simeq$ $100 ~ {\rm km} ~ {\rm s}^{-1}$, that in its turn implies the density contrast of $\sim$15 000. For $n_{\rm b} =0.003 ~ {\rm cm}^{-3}$, one has the number density of the circumstellar material of $\sim$ $50 ~ {\rm cm}^{-3}$, i.e. in a good agreement with the density estimate derived by Leibowitz & Danziger (1983). On the other hand, the recent discovery of Balmer-dominated filaments protruding beyond the radiative arc (see Fig. 3 of Smith 1997 and Fig. 4 of Dickel et al. 2001) suggests that the SN blast wave has partially overrun the clumpy circumstellar shell (cf. Franco et al. 1991) and now propagates through the interstellar medium. We believe that this effect is responsible for the complicated appearance of the southwest corner of the SNR. For the radial extent of protrusions of $\simeq$2 pc, and assuming that their mean expansion velocity is $\sim$ $700 ~ {\rm km} ~ {\rm s}^{-1}$, one has that the blowouts occured $\sim$ $3\times 10^3$ yr ago.