Neutral Hydrogen in the Direction of the Vela Supernova Remnant

THE ASTRONOMICAL JOURNAL, 116 : 813È822, 1998 August ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

NEUTRAL HYDROGEN IN THE DIRECTION OF THE VELA SUPERNOVA REMNANT G. M. DUBNER1 Instituto de Astronom• a y F• sica del Espacio, CONICET, Universidad de Buenos Aires, C.C. 67, 1428 Buenos Aires, Argentina ; gdubner=iafe.uba.ar

A. J. GREEN Department of Astrophysics, School of Physics, University of Sydney, NSW 2006, Australia ; agreen=physics.usyd.edu.au

W. M. GOSS National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801 ; mgoss=nrao.edu

D. C.-J. BOCK Department of Astrophysics, School of Physics, University of Sydney, NSW 2006, Australia ; and Radio Astronomy Laboratory, University of California, Berkeley, Berkeley, CA 94720 ; dbock=astro.berkeley.edu

AND E. GIACANI1 Instituto de Astronom• a y F• sica del Espacio, CONICET, Universidad de Buenos Aires, C.C. 67, 1428 Buenos Aires, Argentina ; egiacani=iafe.uba.ar Received 1998 March 17 ; revised 1998 April 22

ABSTRACT We have carried out a study of the distribution and kinematics of the neutral hydrogen in the direction of the Vela supernova remnant (SNR). A Ðeld of 6¡.8 ] 5¡.4 centered at l \ 264¡.1, b \ [1¡.6 was surveyed using the Parkes 64 m radio telescope (half-power beamwidth [email protected] at 21 cm). Nearly 2300 H I proÐles were obtained with a grid spacing of [email protected]. The presence of a thin, almost circular H I shell, centered at v \ 1.6 ^ 0.8 km s~1, is revealed. This shell delineates the outer border of the X-ray emission as shown in the ROSAT observations of Aschenbach, Egger, & Trumper and wraps around the receding part of the remnant. In addition, two higher velocity features possibly associated with Vela are observed at about [30 and 30 km s~1. These features are interpreted as gas accelerated by the expansion of the supernova shock. The low systemic velocity observed suggests a distance shorter than 500 pc for the Vela SNR. The H I shell is D7¡ in diameter and expands at v D 30 km s~1. By assuming a distance of 350 pc, we calculate for this shell a linear radius of 22 pc, a swept-up mass of D1200È2300 M , and an _ into the atomic preshock density of D1È2 cm~3. The kinetic energy transferred by the supernova shock interstellar medium is D(1È2) ] 1049 ergs, while the initial energy of the explosion is estimated to be D(1È2.5) ] 1051 ergs. We present the distribution of the column density of the neutral material absorbing the X-radiation, an essential parameter in the analysis of X-ray data. A comparison between the H I and Ha emission suggests that the H I shell contains embedded dust that might be responsible for increased optical absorption in this region. On the other hand, the brightest arc-shaped optical Ðlaments associated with the western side of Vela show good correspondence with the H I features. From a comparison between the H I and Molonglo Observatory Synthesis Telescope 843 MHz radio continuum emission, we Ðnd that the outermost arched radio Ðlaments correlate well with the main ridge of the H I shell. No strong inhomogeneities were found in the ambient H I medium in the direction of Vela X (the central nebula, powered by the pulsar PSR B0833[45). Key words : ISM : H I È ISM : individual (Vela supernova remnant) È ISM : structure È radio continuum È supernova remnants 1.

INTRODUCTION

SNRs have shown many of these reciprocal e†ects (e.g., Routledge et al. 1991 ; Pineault et al. 1993 ; Wallace, Landecker, & Taylor 1994 ; Reynoso et al. 1995 ; Green et al. 1997 ; Wilner, Reynolds, & Mo†ett 1998 ; Dubner et al. 1998). The aim of this paper is to study the neutral hydrogen (H I) in the direction of the Vela SNR and its correlation with di†erent spectral regimes in order to determine the e†ects of mutual interaction between the SNR and its environment. It is now widely accepted that the Vela SNR is associated with the pulsar PSR B0833[45 (Large, Vaughan, & Mills 1968). At radio frequencies, the remnant appears as a large, roughly circular shell of synchrotron emission Ðlaments, with an o†set central pulsar-powered nebula, Vela X. Recent observations at various radio frequencies and resolutions have been carried out by Milne (1995), Duncan et al. (1996), and Bock, Turtle, & Green (1998). There is now

The interstellar medium (ISM) is inhomogeneous on a wide range of scales, from fractions of a parsec to kiloparsecs. When supernova remnants (SNRs) interact with the ISM, they can be excellent probes of its physical conditions and morphology. This interaction has profound e†ects on the evolution of both the SNRs and the surrounding ISM. On the one hand, the structure of the ISM strongly inÑuences the morphology and dynamics of the SNRs. However, the supernova shocks may compress and accelerate the clouds they encounter, changing the physical and chemical characteristics of the ISM. Previous atomic and molecular line studies of the ISM near several Galactic ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Member, Carrera del Investigador Cient• Ðco, CONICET, Argentina.

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DUBNER ET AL.

evidence that Vela is a member of the composite class of SNRs (Dwarakanath 1991 ; Bock et al. 1998). Based on the work of Milne (1968a), the distance to this SNR has generally been assumed to be 500 pc. However, there have been several recent studies of Vela (Ogelman, Koch-Miramond, & Auriere 1989 ; Oberlack et al. 1994 ; Aschenbach, Egger, & Trumper 1995 ; Cha, Sembach, & Danks 1997 ; Jenkins et al. 1998) that provide strong evidence for a smaller distance. From an evaluation of these diverse results, we adopt in this paper a distance of about 350 pc. The ROSAT image of the Vela SNR in soft X-rays (Aschenbach et al. 1995) has the morphology of a circular disk. The geometric center of this D4¡ radius disk is close to the projected origin of the pulsar as given by Bailes et al. (1989). An earlier spectral analysis of the X-ray emission by Kahn et al. (1985) concluded that the radiation is mostly thermal, originating from a hot gas of nonuniform density and temperature, approximately in pressure equilibrium. Large-scale images in the ultraviolet from Miller (1973) and in Ha (Milne 1968b) show the Vela SNR to be a circular nebula with both di†use emission and Ðlamentary structure. There appears to be an area of greater absorption on the eastern boundary of the remnant. Miller (1973) ascribes the morphology of the wispy [O III] Ðlaments in this region to a collision with dense interstellar material. UV absorption and emission spectra have been obtained for several stars in the direction of Vela and show evidence of high-velocity gas (e.g., Danks & Sembach 1995). Observations of the B5 II/III star HD 72089 (Jenkins & Wallerstein 1995 ; Jenkins et al. 1998) reveal interstellar absorption lines at velocities ranging from v \ [50 to LSR 110 km s~1 for transitions produced in postshock recombined and cooled neutral gas. Raymond et al. (1997) have used resonance-line scattering to estimate a shock velocity of 165 km s~1, in agreement with previous results from Blair, Vancura, & Long (1995). In previous work on the neutral gas in the direction of the Gum Nebula by Dubner et al. (1992, hereafter Paper I), low-resolution (30@) H I images revealed the existence of a large shell (the ““ thick H I shell ÏÏ), with a diameter of D14¡, expanding at D8 km s~1, maintained by stellar winds from the hot central stars f Puppis and c2 Velorum. In Paper I, evidence is presented suggesting that the Vela SNR is located in a cavity with its northwest quadrant interacting with the large H I shell. The current project was undertaken to investigate the H I gas associated with the Vela SNR using higher angular and spectral resolution. To achieve this goal we used the Parkes telescope, with an angular resolution of [email protected] and a velocity resolution of 0.4 km s~1. Our aims include a reÐned estimate of the systemic velocity, total mass, and kinetic energy of the associated gas. We searched for high-velocity features at the forbidden negative values in order to determine the expansion velocity of the gas associated with the SNR. We have also calculated the column density of the absorbing material, which will constrain the X-ray spectrum. An additional aim is to correlate the H I distribution with existing radio (Bock et al. 1998), X-ray (Aschenbach et al. 1995), and Ha (Bock et al. 1998 ; Buxton, Bessell, & Watson 1998) data, including an evaluation of the Vela X region to investigate possible large-scale inhomogeneities, as proposed by Dwarakanath (1991) to explain the unexpected o†set between the pulsar and the center of the nebula Vela X.

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The observations and data reduction for the project are described in ° 2, and the results are given in ° 3. Conclusions are presented in ° 4. 2.

OBSERVATIONS AND DATA REDUCTION

An area of 34 deg2, centered on l \ 264¡.1, b \ [1¡.6, was observed using the Parkes 64 m telescope in three separate sessions : 1995 June 23È24, 1995 September 11È15, and 1996 May 21È23. The receiver used is wideband (1.2È1.8 GHz), with orthogonal linearly polarized feeds. The half-power beamwidth (HPBW) of the telescope at the frequency of the H I line is [email protected], and the pointing accuracy is ¹20A. The system noise temperature is 28 K, measured against cold sky. Each polarization was recorded with an instantaneous bandwidth of 4 MHz over 2048 channels, giving a spectral resolution of 2 kHz (0.4 km s~1) per channel. The total velocity coverage is ^400 km s~1 centered at 0 km s~1 with respect to the local standard of rest. After Hanning smoothing, the Ðnal velocity resolution per channel is 0.8 km s~1. The integration time for individual spectra was 50 s, and a reference spectrum for bandpass calibration was taken using frequency switching at [400 km s~1 at intervals of 25 minutes. Spectra were measured at positions on a grid of [email protected], the Nyquist sampling interval. The area 260¡.75 ¹ l ¹ 267¡.5, [4¡.25 ¹ b ¹ ]1¡.125 was surveyed using constant Galactic latitude scans, and 2259 spectra were obtained. A small section in the southeast quadrant of the region (264¡.75 \ l \ 267¡.5, [4¡.25 \ b \ [3¡.5) could not be observed in the time scheduled. Flux density calibration was made using scans across Hydra A, for which the Ñux density was assumed to be 43.5 Jy at 1.4 GHz. The brightness temperature scale was calibrated against the IAU standard position S8, assumed to have an integrated value of 897 ^ 66 K km s~1 (Williams 1973). For the three observing sessions, we found a ^2% variation in the measured values for S8. The conversion between Ñux density and brightness temperature is calculated to be 0.78 K Jy~1. The rms uncertainty in the observations is 150 mJy beam~1, equivalent to 117 mK beam~1. Initial processing was carried out using the SLAP and SDCUBE software (L. Staveley-Smith 1996, private communication). Final data analysis was performed using the AIPS and MIRIAD software packages. 3.

RESULTS

3.1. Neutral Hydrogen in the Direction of the V ela SNR 3.1.1. Distribution of the H I Emission

Figure 1 illustrates the general H I emission in the direction of the Vela SNR. The top panel shows an average H I proÐle of the observed region from the Parkes data, while the bottom panel displays the circular rotation model (Fich, Blitz, & Stark 1989) toward l \ 264¡, b \ [1¡.5 (assuming R \ 8.5 kpc and # \ 220 km s~1). The neutral hydrogen 0 0 emission is present between [10 and 130 km s~1. The peak around 60 km s~1 corresponds to H I associated with the Perseus arm at a distance of D7 kpc. Figure 2 shows the neutral hydrogen distribution in the surveyed region, covering the entire velocity range where signiÐcant H I emission is found. Each image is the average of eight consecutive spectral channels (spanning 6.6 km s~1), with the exception of the three lower images in Figure 2d, which have Ðner velocity resolution, as explained below. The two contour lines in black show the outer border of the

No. 2, 1998

H I IN THE DIRECTION OF VELA SNR

FIG. 1.ÈT op, typical H I proÐle of the observed region from Parkes ; bottom, Galactic rotation curve for l \ 264¡, b \ [1¡.5, according to the circular rotation model of Fich, Blitz, & Stark (1989).

Vela SNR X-ray emission, taken from the ROSAT image of Aschenbach et al. (1995), and are superposed to facilitate comparison. For the discussion below, the boundary of the X-ray emission is taken as the extent of the SNR, because the source appears to be complete in this spectral range. The plus sign shows the location of the Vela pulsar, PSR B0833[45. The asterisk indicates the location of the star HD 72089, where very high velocity interstellar absorption lines have been observed (Jenkins & Wallerstein 1995 ; Jenkins et al. 1998). In what follows, we analyze the H I distribution for the entire velocity range, looking for traces of an interaction between Vela and the surrounding H I gas. In the image obtained at v \ [0.4 km s~1 (Fig. 2a), the H I distribution shows a striking correspondence with the northern border of the Vela SNR. The neutral gas emission closely follows the upper half of the X-ray remnant. At v \ 6.2 km s~1, this feature becomes prominent toward lower Galactic longitudes, where it covers a large part of the remnant. This broad feature is still present in the images at 12.8 and 19.4 km s~1, and it is part of the thick H I shell reported in Paper I, centered at l \ 266¡, b \ [2¡.5. It was suggested in Paper I that the SNR was interacting with the thick H I shell. The more detailed study carried out in the present survey reinforces this hypothesis. In the images at v \ 26 and 32.6 km s~1, an extended cloud is seen projected onto the central part of the SNR, roughly centered near l \ 265¡.0, b \ [1¡.7. This feature is discussed below. At v \ 39.2 and 45.8 km s~1, the H I emission appears more widely distributed, with a few brighter clouds spread

815

over the Ðeld. For velocities in the range from v \ 52.4 to D100 km s~1, the distribution of the neutral gas is roughly parallel to the Galactic plane, corresponding to H I associated with an extension of the Perseus arm. At higher positive velocities, the emission is very weak and smoothly distributed. As mentioned above, in Figure 2d we display the H I distribution between about [2 and 7 km s~1 with Ðner velocity resolution to show the morphological agreement between the H I gas and the X-ray remnant. An excellent correspondence can be observed in the north at v \ 0 and 2.5 km s~1, where the H I emission outlines the border of the X-ray remnant. At higher velocities (v ^ 5 km s~1), the H I layer moves toward lower Galactic longitudes and broadens. This distribution of the H I emission suggests that the atomic gas encloses the northern part of the remnant and wraps around toward the far side of the expanding SNR. From the images at v \ 0 and 2.5 km s~1, we estimate the thickness of this H I shell to be 1¡.0 ^ 0¡.3. In addition, there is an excellent correspondence between the X-ray and H I emission near l ^ 261¡.5, b ^ [3¡.5 for the three lowvelocity images in Figure 2d. The weaker X-ray emission observed by Aschenbach et al. (1995) at position angle D140¡ is shown in our images as irregular black contours in the lower left quadrant. This region of faint X-ray emission roughly correlates with a minimum in the H I distribution present at v \ 0 and 2.5 km s~1. Based on the velocities at which the best X-ray/H I emission correspondence is observed, we propose that the systemic velocity for the associated H I gas is D1.6 ^ 0.8 km s~1, a value slightly lower than the 4 km s~1 proposed in Paper I. Swept-up H I gas is clearly detected from [3.3 to 6.6 km s~1. At higher positive velocities, H I emission that might be associated with the Vela SNR merges into the extended thick H I shell (Paper I). Between D7 and D15 km s~1, both the kinematic and morphological signatures of interaction become ambiguous. The low systemic velocity (D1.6 km s~1) precludes estimation of a reliable distance using a circular Galactic rotation model. However, it is consistent with our adopted value of D350 pc. 3.1.2. High-V elocity H I Gas

In addition to the velocity range described above, negative-velocity images were inspected for features showing a positional coincidence with the Vela SNR. At the Galactic longitude of Vela, negative velocities are forbidden. Hence, any negative-velocity feature is a good tracer of accelerated clouds, avoiding confusion with the H I Galactic background. At a velocity of approximately [29 km s~1, a feature with a brightness temperature of 0.76 K was detected. Figure 3 shows the H I proÐle in the direction l \ 264¡.5, b \ [0¡.5 and a contour image of the feature, which extends between [32 and [24 km s~1. We propose that this cloud represents gas accelerated toward the observer by the Vela shock. A possible positive-velocity counterpart for this structure was described in the previous section as an extended feature present in the images at v \ 26 and 32.6 km s~1, projected against the center of the remnant (Figs. 2a and 2b). If this cloud is associated with the Vela SNR, this would represent gas receding with an expansion velocity of D30 km s~1, in agreement with the negative-velocity cloud. However, this feature cannot be kinematically separated from gas located

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FIG. 2.ÈImages of the 21 cm H I emission between [7 and ]125 km s~1, each integrated over eight consecutive channels (approximately 6.6 km s~1) as obtained with Parkes radio telescope (HPBW [email protected]). The central LSR velocities are indicated in each image. The gray scale for these images varies between 0 and 20 Jy beam~1 with the exception of (a) at v \ 6.2 km s~1, where the range is from 7 to 20 Jy beam~1. The plotted H I contours (white lines) are 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, and 21 Jy beam~1 in all cases. The black contours show the lowest levels (in arbitrary units) of X-ray emission, from Aschenbach, Egger, & Trumper (1995), to depict the complete SNR. The three lower images displayed in (d) show the H I integrated over three consecutive channels (approximately 2.5 km s~1) in the velocity interval [0.8 to ]5.8 km s~1. For these three images, the gray scale varies between 8 and 20 Jy beam~1, and the plotted H I contours are 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 Jy beam~1. The plus sign shows the position of the pulsar PSR B0833[45 (l \ 263¡.55, b \ [2¡.79). The asterisk identiÐes the star HD 72089 (l \ 263¡.2, b \ [3¡.9).


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FIG. 2.ÈContinued

beyond 3 kpc, according to the Galactic rotation model from Figure 1. In what follows, we assume an expansion velocity of approximately 30 km s~1 for the swept-up H I shell. After inspecting the neutral hydrogen emission in the direction of the star HD 72089 for the whole velocity range,

no corresponding features were detected near the velocities at which the interstellar absorption lines were observed. 3.1.3. T he Absorbing H I Material

In order to accurately model the X-ray spectrum, knowledge of the distribution of the column density of the H I

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FIG. 3.ÈL eft : H I spectrum taken at l \ 264¡.5, b \ [0¡.5, showing the high negative velocity feature at [29 km s~1. The arrow points to this feature. Right : Contour diagram of the high-velocity feature integrated between [32 and [26 km s~1. Plotted contours are 0.16, 0.20, 0.40, 0.60, and 0.80 Jy beam~1.

absorbing material is required. In Figure 4 (top), we show the neutral hydrogen column density distribution, N , inteH grated between [3.3 and 2.5 km s~1. This is the velocity range assumed for all the H I gas between the observer and the systemic velocity of the remnant. The bottom panel of Figure 4 displays the X-ray remnant taken from Aschenbach et al. (1995), with the H I column density overlapped (using the same contours as in the top image). There is an indication that brighter X-ray regions coincide with lower H I column density, with the exception of the band of weak X-ray emission to the southeast (P.A. D 140¡), which roughly correlates with an extended hole in the H I gas, as mentioned in ° 3.1.1. This H I hole was probably created prior to the supernova explosion (Paper I), and the lower density of the interstellar gas in this direction may explain the weaker thermal X-radiation. Aschenbach et al. (1995) have shown the presence of six extended X-ray features outside the main boundary of the source, which they propose are fragments from the explosion, now moving ahead of the blast wave. The most prominent of these ““ bullets,ÏÏ near l \ 266¡.7, b \ 0¡.5, appears to surround a peak in the N distribution. This particular H an alternative origin for this correspondence suggests that feature might be a dense H I cloud that was engulfed by the expanding shock front and is presently being evaporated. Spectra taken near the eastern limb of Vela by W. P. Blair (1998, private communication) indicate that this bullet is probably a protrusion of recently shocked gas, in agreement with the H I data. 3.1.4. Physical Parameters of the Associated H I Gas

The observational evidence suggests that the expanding shock associated with the Vela SNR has swept up the interstellar gas, forming an H I shell that overlaps most of the periphery of the X-ray source. Table 1 summarizes the observed and derived physical parameters of the Vela H I shell. The center and radius correspond to the best Ðt to the observed portion of the H I shell. To estimate the amount of swept-up H I mass, we have integrated N between [3.3 and 6.6 km s~1, i.e., over the H in which the morphology strongly suggests velocity range an association. The total swept-up mass is about 1200 M . _ In this estimate, the major sources of uncertainty are the choice of the background level and the integration bound-

aries. The high-velocity features detected at about ^30 km s~1 contribute less than 5% of the total mass. However, as discussed above (° 3.1.1), there is neutral gas in the velocity range of 7.4 to 15.7 km s~1, for which the association with the SNR shell could not be clearly determined. If we include this higher velocity gas in our calculations, then the total amount of swept-up mass is about 2300 M . Accordingly, in Table 1 we quote lower and upper limits_for the parameters based on the best estimate for the minimum and maximum velocity ranges, respectively. The volume density was estimated by assuming that all the swept-up H I mass was uniformly distributed into a spherical volume D22 pc in radius. The kinetic energy transferred to the ISM was calculated from the expansion velocity derived from these observations. For the initial energy of the supernova explosion, we have applied ChevalierÏs (1974) formula, E \ 5.3 ] 1043n1.12v1.4R3.12 ergs , 0 0 sh where the ambient density, n , and the radius, R, are taken 0 the shock velocity, v , we from the present H I data. For sh have adopted 165 km s~1 from the Raymond et al. (1997) calculations, based on UV emission lines observed from a face-on shock, since this value is a direct measure of the shock associated with the SNR. This value is much higher than the expansion velocity of 30 km s~1 observed for the swept-up H I gas. If the observed H I shell is the recombined TABLE 1 PHYSICAL PARAMETERS OF THE ASSOCIATED H I SHELL Physical Parameter

Value

Center (l, b) (deg) . . . . . . . . . . . . . . . . . . . . . . Adopted distance (pc) . . . . . . . . . . . . . . . . . Angular radius (deg) . . . . . . . . . . . . . . . . . . Linear radius (pc) . . . . . . . . . . . . . . . . . . . . . Systemic velocity (km s~1) . . . . . . . . . . . Velocity range (km s~1) : Minimuma . . . . . . . . . . . . . . . . . . . . . . . . . . . Maximuma . . . . . . . . . . . . . . . . . . . . . . . . . . Expansion velocity (km s~1) . . . . . . . . . Swept-up mass (M ) . . . . . . . . . . . . . . . . . . _ Ambient atomic density (cm~3) . . . . . . Kinetic energy (ergs) . . . . . . . . . . . . . . . . . . Initial energy (ergs) . . . . . . . . . . . . . . . . . . . .

263.8, [2.8 350 3.6 ^ 0.5 22 ^ 3 1.6 ^ 0.8

a See text for discussion.

[3.3 to 6.6 [3.3 to 15.7 30 ^ 3 1200È2300 1È2 (1È2) ] 1049 (1È2.5) ] 1051

FIG. 4.ÈT op : H I column density integrated between [3.3 and 2.5 km s~1, displayed in gray scale and contours. The gray scale varies between 1.5 ] 1020 and 15 ] 1020 cm~2. Bottom : Gray-scale ROSAT X-ray image (Aschenbach et al. 1995), in arbitrary units, overlaid with the same N contours as above. H

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postshock shell that is expected to form in the Ðnal stage of cooling behind the shock front, then the velocities measured from both the H I and UV lines should be the same. It has been observed that velocities derived from H I data are systematically lower than the equivalent optical, X-ray, or UV line velocities (Giovanelli & Haynes 1979 and references therein ; Pineault, Landecker, & Routledge 1987 ; Dubner & Arnal 1988 ; Routledge et al. 1991). Projection e†ects or local inhomogeneities in the ISM are usually invoked to explain the apparent inconsistencies. In the case of the Vela SNR, a contrast between the cloud and intercloud densities on the order of 30 is required in order to account for the velocity di†erences. Such a contrast is not observed, even taking into account the possibility that the signal may su†er from beam dilution and/or can spread over several channels because of anisotropy in the velocity Ðeld. A revision of the mechanisms at play in the SNR-ISM interaction is required. 3.2. Radio Continuum (843 MHz)ÈH I Comparison Figure 5 shows a comparison between our H I distribution and a radio continuum image at 843 MHz (Bock et al. 1998) observed with the Molonglo Observatory Synthesis Telescope (MOST ; HPBW 43A ] 64A). This telescope is a synthesis interferometer that is sensitive to emission on angular scales of ¹30@. A discussion of the artifacts present in the MOST image is given by Bock et al. (1998). A good correspondence is observed between the outermost arched radio Ðlaments and the main ridge of the H I

Vol. 116

shell. On the other hand, there are no strong inhomogeneities apparent in the H I features toward the direction of the Vela X Nebula, centered near l \ 264¡, b \ [3¡. There is an unexplained o†set between the birth position of the Vela pulsar (Bailes et al. 1989) and the radio peak of Vela X, the Ñat-spectrum radio nebula powered by this pulsar. Dwarakanath (1991) suggested that inhomogeneities in the ISM might inÑuence the morphology of this nebula, but at the resolution and sensitivity of our data, we do not Ðnd evidence to support this model. 3.3. HaÈH I Comparison Figure 6 shows an Ha image of the Vela SNR region with contours of the H I emission at v \ 2.5 km s~1. The Ha image is from the Mount Stromlo and Siding Spring Observatories Wide Field CCD Ha Imaging Survey (Buxton et al. 1998). Technical details speciÐc to the production of this image are explained in Bock et al. (1998). The bright emission regions observed at the top of the image are, from right to left, the H II regions RCW 32, RCW 33, and RCW 35 (Rogers, Campbell, & Whiteoak 1960), respectively. The di†use optical emission has a circular morphology in the northern part, which is outlined by the H I shell. There is a suggestion of stronger absorption of optical emission matching the location of the H I shell, possibly indicating the presence of dust embedded in the neutral gas layer. In the Ha image, features at di†erent distances are seen superposed. The H I cube was searched for counterparts to the Ha Ðlaments. A good correlation can be observed

FIG. 5.ÈIn gray scale, the MOST image of the radio continuum emission at 843 MHz (HPBW 43A ] 64A) from Bock, Turtle, & Green (1998). The gray scale is saturated outside the intensity range of [10 to ]10 mJy beam~1. Contours of H I emission at v D 2.5 km s~1 are plotted for 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 Jy beam~1.


No. 2, 1998

821

b

=

-2

FIG. 6.ÈLow-resolution (25A) gray-scale image of the Ha emission in arbitrary units, from Bock et al. (1998). Continuum emission has not been subtracted. Contours of H I emission at v D 2.5 km s~1 are plotted for 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 Jy beam~1. The bright extended emission regions seen at the top of the image are, from right to left, the H II regions RCW 32, 33, and 35, respectively.

-42 30

DECLINATION (J2000)

b

=

-4

-43 00

30

-44 00

l=

26

1

30

between the H I at v \ 2.5 km s~1 and the brightest arched Ha Ðlaments. Namely, in the bottom right sector of Figure 6, near l \ 262¡, b \ [3¡, the two bright, nearly circular arcs appear to be outlined by the outer boundary of the H I shell. However, the correlation does not extend to the other bright Ðlaments. Superposed on the southernmost of these arcs is another Ðlament with opposite curvature, concave to the outer edge of the remnant (near l \ 262¡.5, b \ [3¡.5). As shown in Figure 7, this Ðlament has a morphology that correlates well with the geometry of a small H I cloud (centered at about R.A. \ 08h26m, decl. \ [44¡45@, J2000.0) at v ^ 32 km s~1. It is worth noting that this small cloud occurs at the same velocity range as the feature discussed in ° 3.1.1 that is likely to be associated with the Vela SNR. 4.

-45 00

30

-46 00 08 36

34

32

30 28 26 24 RIGHT ASCENSION (J2000)

l = 22 26 3

20

FIG. 7.ÈOverlay of Ha emission of the southwest part of Vela with H I contours corresponding to emission integrated between 29.7 and 35.3 km s~1, at levels of 5, 6, 7, 8, and 9 Jy beam~1. A few Galactic coordinate lines are included for reference.

CONCLUSIONS

We have discovered the existence of a relatively thin H I shell coinciding with the border of the Vela SNR, as traced by the thermal X-ray emission. The shell is about 22 pc in radius and is centered at (l \ 263¡.8, b \ [2¡.8, v \ ]1.6 km s~1). The low systemic velocity derived from the present observations suggests a distance of less than 500 pc. The H I shell expands at about 30 km s~1 into a medium with n D 0 1È2 cm~3. During its lifetime, it has injected (1È2) ] 1049 ergs of kinetic energy into the ISM. The initial energy of the explosion is estimated to be (1È2.5) ] 1051 ergs. The typical

822

DUBNER ET AL.

physical parameters obtained and the evidence that at different wavelengths the SNR has a well-deÐned spherical shape suggest that the surrounding medium has not strongly a†ected either the morphology or the dynamics of the remnant. This conclusion is compatible with previous results (Paper I) showing that the Vela SNR is expanding in the interior of a partly swept-up bubble, probably maintained by stellar winds from the massive stars c2 Vel and f Pup. In X-rays, the remnant has a Ðlled-in appearance (Aschenbach et al. 1995), which suggests heating by the reverse shock of some remaining gas clumps still surviving after the passage of the main shock. The mechanism of cloudlet evaporation (White & Long 1991) might be responsible for this X-ray emission. A comparison of the H I distribution with the Ha emission suggests that the swept-up H I shell contains dust responsible for obscuration at optical wavelengths. The spectacular arched optical Ðlaments observed in Vela toward the west correlate with enhancements in the H I emission. The H I distribution was searched for counterparts to the interstellar absorption lines observed in the UV range toward the B star HD 72089 ; these lines are good indicators of the shock velocity. At the present angular resolution and sensitivity, no H I features were discovered at the same location with matching velocities. From a comparison between the H I and the synchrotron emission at 843 MHz, a good correlation with the outer-

most arched Ðlaments of Vela are observed. However, there are no high-density contrasts or inhomogeneities that might a†ect the development of the Ñat-spectrum nebula Vela X that can explain the eccentric position of the pulsar inside this nebula. Finally, we have produced from this H I survey a map of integrated column density necessary to model the X-ray spectrum. We would like to acknowledge the unstinting help and support given by the late Bobbie Vaile during the observations in 1995. We would like to thank the sta† at Parkes for their tireless assistance, particularly E. Troup and H. Fagg. We are also grateful to L. Staveley-Smith of the Australia Telescope National Facility for his assistance with the data reduction and to B. Aschenbach for providing the ROSAT X-ray image in FITS format. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility operated by CSIRO. The Molonglo Observatory Synthesis Telescope is operated by the University of Sydney with support from the Science Foundation within the School of Physics and the Australian Research Council. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. This project was supported by a grant from the Antorchas Foundation and through CONICET grant PMT-PICT0107, both from Argentina.

REFERENCES Aschenbach, B., Egger, R., & Trumper, J. 1995, Nature, 373, 587 Large, M. I., Vaughan, A. E., & Mills, B. Y. 1968, Nature, 220, 340 Bailes, M., Manchester, R. N., Kesteven, M. J., Norris, R. P., & Reynolds, Miller, E. W. 1973, PASP, 85, 764 J. E. 1989, ApJ, 343, L53 Milne, D. K. 1968a, Australian J. Phys., 21, 201 Blair, W. P., Vancura, O., & Long, K. S. 1995, AJ, 110, 312 ÈÈÈ. 1968b, Australian J. Phys., 21, 501 Bock, D. C.-J., Turtle, A. J., & Green, A. J. 1998, AJ, submitted ÈÈÈ. 1995, MNRAS, 277, 1435 Buxton, M., Bessell, M., & Watson, R. 1998, Publ. Astron. Soc. Australia, Oberlack, U., Diehl, R., Montmerle, T., Prantzos, N., & von Ballmoos, P. 15, 24 1994, ApJS, 92, 433 Cha, A. N., Sembach, K. R., & Danks, A. C. 1997, BAAS, 191, No. 51.05 Ogelman, H., Koch-Miramond, L., & Auriere, M. 1989, ApJ, 342, L83 Chevalier, R. A. 1974, ApJ, 188, 501 Pineault, S., Landecker, T. L., Madore, B., & Gaumont-Guay, S. 1993, AJ, Danks, A. C., & Sembach, K. R. 1995, AJ, 109, 2627 105, 1060 Dubner, G., & Arnal, M. 1988, A&AS, 75, 363 Pineault, S., Landecker, T. L., & Routledge, D. 1987, ApJ, 315, 580 Dubner, G., Giacani, E., Cappa de Nicolau, C., & Reynoso, E. 1992, Raymond, J. C., Blair, W. P., Long, K. S., Vancura, O., Edgar, R. J., Morse, A&AS, 96, 505 (Paper I) J., Hartigan, P., & Sanders, W. T. 1997, ApJ, 482, 881 Dubner, G. M., Holdaway, M., Goss, W. M., & Mirabel, I. F. 1998, AJ, in Reynoso, E. M., Dubner, G. M., Goss, W. M., & Arnal, E. M. 1995, AJ, press 110, 318 Duncan, A. R., Stewart, R. T., Haynes, R. F., & Jones, K. L. 1996, MNRAS, Rogers, A. W., Campbell, C. T., & Whiteoak, J. B. 1960, MNRAS, 121, 103 280, 252 Routledge, D., Dewdney, P. E., Landecker, T. L., & Vaneldik, J. F. 1991, Dwarakanath, K. S. 1991, J. Astrophys. Astron., 12, 199 A&A, 247, 529 Fich, M., Blitz, L., & Stark, A. A. 1989, ApJ, 342, 272 Wallace, B. J., Landecker, T. L., & Taylor, A. R. 1994, A&A, 286, 565 Giovanelli, R., & Haynes, M. P. 1979, ApJ, 230, 404 White, R. L., & Long, K. S. 1991, ApJ, 373, 543 Green, A. J., Frail, D. A., Goss, W. M., & Otrupcek, R. 1997, AJ, 114, 2058 Williams, D. R. 1973, A&AS, 8, 505 Jenkins, E. B., et al. 1998, ApJ, 492, L147 Wilner, D. J., Reynolds, S. P., & Mo†ett, D. A. 1998, AJ, 115, 247 Jenkins, E. B., & Wallerstein, G. 1995, ApJ, 440, 227 Kahn, S. M., Gorenstein, P., Harnden, F. R., Jr., & Seward, F. D. 1985, ApJ, 299, 821