Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM 87801 ... We present observations of CO J \ 1»0 emission toward 3C 391, ...
THE ASTRONOMICAL JOURNAL, 115 : 247È251, 1998 January ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
CO OBSERVATIONS TOWARD THE SUPERNOVA REMNANT 3C 391 D. J. WILNER1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
S. P. REYNOLDS Department of Physics, North Carolina State University, Box 8202, Raleigh, NC 27695
AND D. A. MOFFETT Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM 87801 Received 1997 August 26 ; revised 1997 September 24
ABSTRACT We present observations of CO J \ 1È0 emission toward 3C 391, a supernova remnant whose radio continuum and X-ray emission suggest evolution near a strong density gradient in the surrounding medium. The CO maps reveal that the remnant is located at the edge of a molecular cloud, and the CO emission shows a striking correspondence with the ““ breakout ÏÏ morphology of the remnant traced by radio continuum emission. These data support the idea that the progenitor star exploded within a dense molecular cloud and that the supernova blast wave has now broken out through the cloud boundary. Bright radio emission coincident with strong CO emission suggests that the blast wave accelerates electrons to energies of at least 20 GeV, even as it moves into dense neutral material. Key words : ISM : individual (3C 391) È ISM : molecules È supernova remnants 1.
INTRODUCTION
brightened thermal X-ray emission (Rho & Petre 1996). The apparent discrepancy between the column density of H I derived from 21 cm observations (Caswell et al. 1971) and H atoms derived from X-ray observations (Wang & Seward 1984 ; Rho & Petre 1996) may be resolved if the shell part of the remnant lies behind a greater depth of molecular hydrogen than the ““ breakout ÏÏ region. Given the morphological and X-ray clues to the possible presence of molecular gas, RM93 examined the CO J \ 1È0 line survey of the Ðrst quadrant of the Galaxy made with the FCRAO 14 m telescope (Sanders et al. 1986). Although poorly sampled on a 3@ ] 3@ grid with a beam size of 45A, the survey data show extensive CO emission at velocities 90È110 km s~1 near the location of 3C 391, consistent with molecular gas at the D9 kpc distance of the remnant inferred from H I emission and absorption data (Caswell et al. 1971 ; Radhakrishnan et al. 1972). However, a deÐnitive physical association between the molecular gas and the remnant could not be made, on account of the widespread nature of the CO emission at low Galactic latitude near the tangent point. Motivated by the proposed association of 3C 391 with molecular material, we have performed new observations of the CO J \ 1È0 line toward the remnant using the NRAO 12 m telescope. The new data reveal a clear association between 3C 391 and the edge of a molecular cloud.
Although the progenitors of Type II supernovae are born in dense cores of giant molecular clouds, there are few documented examples of supernova remnants interacting with molecular material. This is presumably because ionization and stellar winds disrupt the natal molecular environment, and random motions result in migration over the short lifetimes of the stars. While many associations between supernova remnants and molecular clouds have been suggested on the basis of near positional coincidence (see, e.g., Huang & Thaddeus 1985, 1986), convincing cases have been made for only a handful of remnants that have been extensively mapped in molecular tracers (e.g., W44, Wootten 1977 ; W28, Wootten 1981 ; Puppis A, Dubner & Arnal 1988 ; G84.2[0.8, Feldt & Green 1993 ; CTB 109 [G109.1[1.0], Tatematsu et al. 1990a ; HB 21, Tatematsu et al. 1990b). The most spectacular example involves molecular clumps at the periphery of the nearby evolved remnant IC 443 (DeNoyer 1978), which has become a favorite target for studies of shock structure and chemistry (see, e.g., van Dishoeck, Jansen, & Phillips 1993). In most of these examples, the molecular material is found in concentrations that are considerably smaller than the supernova remnant. A recent VLA study of the radio-bright supernova remnant 3C 391 revealed a morphology consistent with expansion into a medium of strongly varying density, possibly the edge of a molecular cloud (Reynolds & Mo†ett 1993, hereafter RM93). The radio morphology of 3C 391 is that of a partial shell of radius 5@, with a faint extension out the open end of the shell suggestive of ““ breakout ÏÏ into a region of signiÐcantly lower density. The X-ray morphologyÈa maximum in the midst of the faint radio extension and a weaker maximum in the middle of the partial shellÈreÑects strongly varying X-ray absorption across the face of the remnant. 3C 391 is a member of the class of ““ thermal composite ÏÏ remnants, with center-
2.
OBSERVATIONS
We observed the 12CO J \ 1È0 line toward 3C 391 with the NRAO 12 m telescope on 1994 June 16. Spectra were obtained with the facilityÏs dual polarization SIS receiver on a 12 ] 13 grid of points spaced 55A apart. This spacing corresponds to the beam size at the line frequency (115.271204 GHz) ; the linear resolution is about 2.4 pc. The grid center was located at a(1950) \ 18h46m48s, d(1950) \ [00¡59@00A. The spectrometers consisted of a pair of Ðlter banks with 128 channels of 500 kHz width, centered at LSR velocity 90 km s~1. These provided 1.3 km s~1 velocity resolution over the LSR velocity range 7È173
1 Hubble Fellow.
247
248
WILNER, REYNOLDS, & MOFFETT
km s~1. A position-switching mode was used, with reference position a(1950) \ 18h50m42s. 67, d(1950) \ [01¡20@52A. 6, chosen from the FCRAO survey to be free of signiÐcant CO emission at the velocities of interest. System temperatures were typically 500 K and integration times 1È2 minutes at each position, which resulted in an rms sensitivity of about 0.1 K (T *) per channel. Pointing was checked on Jupiter at R the start of the observations, and the grid center position was reobserved after the completion of each map row to check for pointing drifts. The CLASS package was used to subtract linear baselines from the raw spectra and to co-add individual spectra obtained at common positions. 3.
RESULTS
3.1. Molecular Cloud Morphology Figure 1 shows the CO J \ 1È0 spectrum at the central position of the grid, where the signal-to-noise ratio is highest. Several velocity components are visible in this direction, in particular, the prominent complex at 90È110 km s~1 argued by RM93 to lie at the distance of 3C 391. The small dip visible in the spectrum near 60 km s~1 results from weak emission at the reference position ; this weak feature is far removed (in velocity) from emission associated with the remnant. Figure 2 shows the grid of CO J \ 1È0 spectra over the face of the remnant, restricted to the velocity range 85È125 km s~1, superposed on VLA 1446 MHz continuum contours reproduced from RM93. Figure 3 shows the integrated CO emission in gray scale, including only the 90È110 km s~1 feature, centered on the remnant, together with the 1446 MHz contours from Figure 2. The improved spatial sampling and signal-to-noise ratio of the new CO map compared with the old FCRAO survey data reveal a striking
Vol. 115
morphological correspondence between the CO emission and the ““ breakout ÏÏ morphology of the radio remnant. Figures 2 and 3 show very close agreement between the steeply dropping CO intensity and the steeply rising radio continuum intensity across the inner edge of the radio shell. In addition, extensions of the bright limb of radio continuum align perfectly with the edges of strongest CO emission. This detailed agreement is unlikely to be coincidental : a density gradient in the surrounding medium naturally explains both the radio continuum and CO emission properties. To the northwest, the remnant expands into a moderately dense molecular cloud and appears in the radio as a bright hemispherical shell ; to the southeast, the remnant has broken through the molecular cloud boundary and emerges as lower surface brightness extensions in the radio. Does the molecular cloud observed in CO account for the gradient in hydrogen column density required to explain the X-ray spectral variations ? A crude estimate of the density gradient implied by the CO emission can be made by adopting the standard conversion from CO integrated intensity to molecular hydrogen column density, i.e., N(H )/W (CO) D 4 ] 1020 cm~2 (K km s~1)~1 (Scoville et 2 al. 1987). This conversion yields N D 0.8 ] 1022 cm~2 for H 10 K km s~1, the integrated intensity observed at the remnantÏs center. For a line-of-sight path about 10 pc at the edge, the average volume density is D300 cm~3, a value typical for giant molecular clouds, which gives some conÐdence that the standard conversion applies in this potentially unusual environment. The gradient across the face of the remnant, *W (CO) D 10 K km s~1, implies *N D 0.8 H the ] 1022 cm~2. This value compares favorably with *N D 0.9 ] 1022 cm~2 derived from modeling the H ROSAT Position Sensitive Proportional Counter data
FIG. 1.ÈCO J \ 1È0 spectrum toward the center of the 3C 391 supernova remnant. Several velocity features are present at this low Galactic latitude. The complex at 90È110 km s~1 is associated with the remnant ; the bar indicates the velocity range expanded in Fig. 2. The small dip near 60 km s~1 is due to weak emission at the reference position.
No. 1, 1998
SNR 3C 391
249
FIG. 2.ÈGrid of CO J \ 1È0 spectra from the NRAO 12 m telescope, restricted to the velocity range 85È125 km s~1, superposed on the VLA 1446 MHz image of the 3C 391 remnant from RM93. The intensity scale for the spectra ranges from [0.5 to 7 K. Note the remarkable edge of molecular emission aligned with the ““ breakout ÏÏ morphology of the radio shell. The stars mark the positions of the two 1720 MHz OH masers cataloged by Frail et al. (1996).
(Rho & Petre 1996). Thus the gradient implied by the CO emission appears compatible with that estimated from X-ray absorption. 3.2. Kinematic Evidence for Interaction Despite the clear morphological correspondence between the radio remnant and the molecular cloud, unambiguous kinematic evidence for interaction is difficult to discern in the CO J \ 1È0 data. The CO emission from 3C 391 roughly divides into two velocity components, a blueshifted one that shows a sharp edge from northeast to southwest across the remnant, and a redshifted one that shows a more di†use shape and appears to wrap around the bright parts of the radio shell. However, neither the spectra in Figure 2 nor the associated channel maps show any gross distortions that can be obviously linked to the action of the supernova remnant. This stands in stark contrast to the spectacular e†ects of blast-wave propagation on the molecular clumps in IC 443, where CO lines exhibit locally broad widths that
clearly mark extreme kinematic disturbances (see, e.g., White et al. 1987). Age estimates of the 3C 391 remnant range from D103 yr up to D104 yr, depending in detail on assumptions about the environment. The relative youth of 3C 391 and concomitant high shock velocities may play a role in suppressing kinematic signatures of interaction. For shock velocities in excess of 50È55 km s~1, molecules are expected to be rapidly dissociated, and they may not have had time to re-form in the postshock gas (McKee & Hollenbach 1980). Thus, if the shock velocities are as high as the 600 km s~1 inferred from X-ray observations (Rho & Petre 1996), we might not expect to Ðnd distortions in the bulk of the cloud traced by CO J \ 1È0 emission. Dissociation would also make difficult the recognition of high-velocity features in the CO spectra. The most compelling kinematic evidence for interaction comes from the two shock-excited 1720 MHz OH masers in the southeast part of the remnant discovered by Frail et al.
250
WILNER, REYNOLDS, & MOFFETT
Vol. 115
FIG. 3.ÈThe CO J \ 1È0 emission integrated from 90 to 110 km s~1, together with the contours of the VLA 1446 MHz image from Fig. 2. The gray scale ranges from 10 to 45 K km s~1. The 55A beam of the NRAO 12 m telescope is illustrated in the bottom right corner.
(1996). The locations of these OH masers are marked by stars in Figure 2. As already reported by Frail et al. (1996), there is good agreement between the velocities of the OH masers and the CO velocities at the closest locations sampled in our data. Single Gaussian Ðts to the spectra yield CO center velocities 108.2 and 103.9 km s~1 (with widths of D3 km s~1) at the spots where the OH maser velocities are 110.2 and 104.9 km s~1, respectively. From the maser locations on bright radio continuum peaks, there can be no doubt that the remnant is interacting with the surrounding CO cloud. A close examination of the CO spectra in Figure 2 shows evidence for weak and broad emission, most prominent near the southernmost OH maser spot. This broad emission may result from nondissociative shocks or entrained cloud material, though the considerable confusion introduced by the surrounding cloud emission makes a deÐnitive attribution problematic. The recent detection of bright [O I] 63 km line emission associated with 3C 391 supports a shock interaction of the blast wave with dense molecular material (Reach & Rho 1996). This atomic line, a primary coolant for postshock gas, has also been observed at several positions in IC 443 (Burton et al. 1990). The shocked molecular gas, if present, may be highly excited and therefore weak in emission from the low-lying CO J \ 1È0 transition. The background cloud emission could easily mask such weak features. Additional confusion due to velocity crowding from gas at a
range of distances may also play a role in this direction close to the tangent point. Alternatively, the blast-wave interactions may be highly localized, as in IC 443, and therefore substantially beam-diluted at the modest angular resolution of the observations presented here. Mapping higher excitation transitions of CO would improve sensitivity to any warm, optically thin molecular component and at the same time discriminate against the extensive background cloud emission. A search for emission from highÈ dipole moment tracers known to highlight shocked zones, for example, lines of HCO` or SiO, might further suppress the background cloud emission and prove fruitful in pinpointing sites of blast-wave interaction. 3.3. Electron Acceleration It seems clear that the northwest rim of 3C 391Ïs radio shell represents the blast waveÏs impinging on dense molecular gas. The high average radio emissivity of this region of 3C 391 (D10~4 Jy arcsec~2 pc~1 ; RM93) is over 105 times the mean Galactic value in this direction. The latter estimate results from the 100 K brightness temperature observed at 408 MHz, an intensity of I \ 2kT /j2 D 1 l Kanback, B ] 10~17 ergs cm~2 s~1 Hz~1 (Beuermann, & Berkhuijsen 1985), corrected for the postshock compression of cosmic-ray electrons, whose individual energies increase as the compression ratio r D 4. Thus electrons radiating behind the shock at 1420 MHz are those that in front of
No. 1, 1998
SNR 3C 391
the shock had one-quarter the energy and therefore radiated at 1420/42 D 100 MHz. Using a spectral index of [0.5 for the Galactic background between 100 and 408 MHz (see Salter & Brown 1988 and references therein), we deduce a background intensity at 100 MHz of about 2 ] 10~17 ergs cm~2 s~1 Hz~1, or, for a path length D 20 kpc, S j T D l 10~9 Jy arcsec~2 pc~1. The huge increase, if obtained purely by compression of ambient cosmic-ray electrons and magnetic Ðeld, would require compression by about 2 orders of magnitude, since for remnants j P o17@6 (Reynolds 1988) for the observed spectral index of about [0.5 (Green 1991). This is not likely, as it would demand a highly radiative shock wave, inconsistent with the shock velocities inferred from the X-ray emission. Hence we draw the unremarkable, but well-founded, conclusion that 3C 391 is manufacturing new cosmic-ray electrons, new magnetic Ðeld, or both. The location of the maximum radio shell brightness at the very edge of the remnant strongly suggests that the emissivity increase occurs at the shock, rather than behind it in postshock turbulence, and supports shock acceleration as the origin of the relativistic electrons. The radio spectrum of 3C 391 maintains a power-law form to 30 GHz, the highest frequency observed, without evidence of a spectral break. Since an individual electron radiates at E D 15(l /B )1@2 GeV, the shock acceleration kG mechanism appearsGHz to operate to energies of tens of GeV without spectral steepening, for a mean magnetic Ðeld of order 10 kG. Such Ðeld strengths are expected just from compression of the ambient Ðeld, in the absence of strong internal turbulent ampliÐcation, and are also typical of equipartition values. This result is interesting, because the shock is entering molecular material, most likely preceded by a thin precursor ionized by postshock ultraviolet and soft X-ray emission, and the waves required to scatter particles ahead of the shock to produce Ðrst-order Fermi acceleration will be
251
strongly damped by ion-neutral friction outside the ionized precursor. Thus speculations have been made that shocks moving into dense neutral gas might be very poor particle accelerators (Draine & McKee 1993). However, the 3C 391 remnant seems to have no trouble producing a power-law distribution of electrons to energies of at least 20 GeV, with no sign of a roll-o†. Flux measurements at frequencies above 30 GHz would be useful for increasing, or bounding, the energy to which 3C 391Ïs blast wave is able to accelerate electrons. 4.
SUMMARY
New maps of the CO J \ 1È0 line around the 3C 391 supernova remnant provide strong evidence that the progenitor star exploded near the edge of a molecular cloud. The gradient observed in CO emission accounts for both the ““ breakout ÏÏ morphology of the radio continuum emission and the gradient in hydrogen column density required to explain the X-ray spectral variations across the face of the remnant. The acceleration of electrons to 20 GeV to produce synchrotron emission is not hampered where the blast wave encounters dense molecular gas. Probably, 3C 391 is the youngest of the few documented remnant-cloud interactions, and the only one involving a cloud much larger than the remnant. The favorable geometry makes this source well suited for addressing issues of interaction between supernova remnants and molecular gas. We thank the NRAO sta† for enabling remote observations from the 12 m telescope, and Phil Jewell for invaluable assistance. Support for this work was provided by NASA through Hubble Fellowship grant HF-01086.01-96A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555.
REFERENCES Beuermann, K., Kanback, G., & Berkhuijsen, E. M. 1985, A&A, 153, 17 Reynolds, S. P., & Mo†ett, D. A. 1993, AJ, 105, 2226 (RM93) Burton, M. G., Hollenbach, D. J., Hass, M. R., & Erickson, E. F. 1990, ApJ, Rho, J. H., & Petre, R. 1996, ApJ, 467, 698 355, 197 Salter, C. J., & Brown, R. L. 1988, in Galactic and Extragalactic Radio Caswell, J. L., Dulk, G. A., Goss, W. M., Radhakrishnan, V., & Green, A. J. Astronomy, ed. G. L. Verschuur & K. I. Kellermann (2d ed. ; Berlin : 1971, A&A, 12, 271 Springer), 1 DeNoyer, L. K. 1978, MNRAS, 183, 187 Sanders, D. B., Clemens, D. P., Scoville, N. Z., & Solomon, P. M. 1986, Draine, B. T., & McKee, C. F. 1993, ARA&A, 31, 373 ApJS, 60, 1 Dubner, G. M., & Arnal, E. M. 1988, A&AS, 75, 363 Scoville, N. Z., Yun, M. S., Clemens, D. P., Sanders, D. B., & Waller, W. H. Feldt, C., & Green, D. A. 1993, A&A, 274, 421 1987, ApJS, 63, 821 Frail, D. A., Goss, W. M., Reynoso, E. M., Giacani, E. B., Green, A. J., & Tatematsu, K., Fukui, Y., Iwata, T., Seward, F. D., & Nakano, M. 1990a, Otrupcek, R. 1996, AJ, 111, 1651 ApJ, 351, 157 Green, D. A. 1991, PASP, 103, 209 Tatematsu, K., Fukui, Y., Landecker, T. L., & Roger, R. S. 1990b, A&A, Huang, Y.-L., & Thaddeus, P. 1985, ApJ, 295, L13 237, 189 ÈÈÈ. 1986, ApJ, 309, 804 van Dishoeck, E. F., Jansen, D. J., & Phillips, T. G. 1993, A&A, 279, 541 McKee, C. F., & Hollenbach, D. J. 1980, ARA&A, 18, 219 Wang, Z. R., & Seward, F. D. 1984, ApJ, 279, 705 Radhakrishnan, V., Goss, W. M., Murray, J. D., & Brooks, J. W. 1972, White, G. J., Rainey, R., Hayashi, S. S., & Kaifu, N. 1987, A&A, 173, 337 ApJS, 24, 49 Wootten, A. 1977, ApJ, 216, 440 Reach, W. T., & Rho, J. 1996, A&A, 315, L277 ÈÈÈ. 1981, ApJ, 245, 105 Reynolds, S. P. 1988, in Galactic and Extragalactic Radio Astronomy, ed. G. L. Verschuur & K. I. Kellermann (2d ed. ; Berlin : Springer), 439
