The new HI supershell GS262-02+45 and the OB Association Bochum 7 ...... close relatives?

M. A. Corti¹ ² & E. M. Arnal¹ ³

1 - Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Argentina. 2 – Instituto de Astrofísica de La Plata (IALP), CONICET, Argentina.3 - Instituto Argentino de Radioastronomía (IAR), CONICET, Argentina.

BACKGROUND

Neutral hydrogen (HI) shells and supershells are usually identified, in a given velocity range, as a brightness temperature minimum in the HI distribution that is surrounded by walls of HI emission. Their physical dimensions are in the range from a few tens of parsecs to a few kiloparsecs (Rand & Stone 1996, AJ, 111, 190); (de Block & Walter 2000, ApJ, 537, L95). These structures may be formed by differents processes like stellar winds from massive stars and supernovae explosions (Spangler 2001, Space Sci. Rev.,99, 261), energetic event connected to a gamma-ray burst (Perna & Raymond 2000, ApJ, 539, 706) and the infall of a high velocity cloud into the HI disc of the Galaxy (Tenorio-Tagle 1981, A&A, 94, 338).During its evolution these HI shells and supershells may hit a nearby molecular clouds triggering further star formation. McClure-Griffiths et al. (2002, ApJ, 578, 176) reported the discovery of 19 new HI shells and supershells. Among them they mentioned GSH263+00+47, located at (l,b) = (263º.0, 0º.0). The OB association Bochum 7 (l,b) = (265º.0, -2º.0) is seen in projection onto the outer border of GSH263+00+47.In order to further explore the possibility that both objects were somehow related to each other, we study in more detail the large scale HI distribution in the area.

630pc

6 km s-¹

GS262-02+45:• HIMass = 1.1 ± 0.5 x 10exp(6) Msol.• TotalMass = 1.5 ± 0.7 x 10exp(6) Msol• no = 0.6 ± 0.3 cm-³• Age = (1 – 3) x 10exp(7) years• Ek = 0.5 x 10exp(51) Ergios

Bochum 7: OB association TE: O6.5V, O7.5V,O9V,O9.5V,

WN8, UCHIR.

Distance: 4.5 ± 0.6 kpc,

Age: 2-7 x 106 yr

VLSR = 44 ±10 km s-1,

μl = -8.8 ± 7 mas yr-1, μb = -2.0 ± 2 mas yr-1

RESULTS

Bearing in mind the baricentral radial velocity of GSH263+00+47, V = 47 km s-¹ and its velocity coverage, V = 26 km s-¹, special attention was paid to the HI distribution spanning the velocity range from about 30 to 60 km s-¹. All radial velocities are relative to the Local Standard of Rest (LSR). A series of four HI images covering the velocity range from 30 to 61 km s-¹ are shown in Figure 1. Each image is a mean of eight individual HI images.In the HI image at 41.5 km s-¹ a large shell-like feature is easily observable. At 49.5 km s-¹ the shell observed at lower velocity is still detectable and a new shell-like feature becomes the dominant structure. The later is the shell GSH263+00+47 discovered by McClure-Griffiths et al. (2002).

To disclose in more detail the behaviour of the HI distribution along the velocity range 38 to 58 km s-¹, a mosaic of eight HI images was constructed (see Figure 2). The large shell shown in Figure 1b) begins to be detectable at 40.5 km s-¹ and at velocities higher than 48.5 km s-¹ begins to lose its identity. At 46.5 km s-¹ the McClure-Griffiths et al. (2002) shell begins to be observable.

The mean HI brightness temperature distribution in the range 39 to 49 km s-¹ is shown in Figure 3. A least square fit of a circumference to the HI peaks defining the shape of the large HI shell is shown as a thin line. The centre of this circumference lies at (l,b) = (262º.6,-1º.8) and has an angular radius of 3º.6 ± 0º.4. This large shell is designated GS262-02+45. In Figure 3 the shell GSH263+00+47 also is shown. The supershell GS262-02+45 has a local minimum close to the location of Bochum 7. In Figure 4 a blowout of the region delimited in Figure 3 by a dash rectangle is shown. This figure was constructed using data from the HI Southern Galactic Plane Survey (SGPS) McClure-Griffiths et al. (2002). The mean radial velocity of this HI image is V = 45 km s-¹, and covers a velocity range of 10 km s-¹. Remarkably enough, Bochum 7 is located within a large local HI minimum that belongs to GS262-02+45. The HI minimum has an elliptical shape and major and minor axis of 3º and 0º.8, respectivaly. The radial velocity of Bochum 7 is coincident with the mean radial velocity of the HI image. The position angle (PA) (at a 2 level) of the mean proper motion of Bochum 7 spans the range -105º PA 140º. PA is measured from the north counterclockwise.

The expansion velocity is an important shell characteristic. One way of determining this parameteris from the use of position-position HI images (similar to those shown in Figure 2) of the feature under study. Using the maximum approaching (Vmax) and maximum receding (Vmin) radial velocity of those HI features associated with the expanding shell, under the assumption of a spherically symmetric expansion, Vexp = |0.5 (Vmax – Vmin)|. In this way a lower limit of Vexp ~ 6 km s-¹ is found. Another way of determining this parameter was described by McClure-Griffiths et al. (2002). Following their procedure a HI profile towards the center of GS263-02+45 was obtained (Figure 5). After identifying in this profile the front and rear walls of HI, an expansion velocity of 15 km s-¹ was obtained.

Using the linear fit and the power law fit of the galactic rotation model of Fich, Blitz & Stark (1989, ApJ 342, 272) and a baricentral velocity 45 km s-¹, a kinematic distance to GS262-02+45 of 5.3 ± 1.0 kpc is derived. Adopting this distance, the supershell radius turns out to be ~ 320 pc and the shell thickness is ~ 100 pc. The total mass of the HI supershell is 1.5 x 10exp(6) M¤. Under the assumption that this mass is uniformely distributed within the supershell, a mean volume density of n ~ 0.5 cm -³ is obtained. Bearing in mind the total mass and the expansion velocities of GS262-02+45, the kinetic energy of this supershell is (0.5 – 3) x 10exp(51) ergs. If GS262-02+45 was created by the stellar winds of a stellar aggregate located in its centroid, its dynamic age would be in the range (1.3 – 3.0) x 10exp(7) yr. Figure 6 shows some of the HI supershell GS262-02+45 caracteristics.

Bronfman et al. (1996, A&ASS 115, 81) found CS(2-1) emission, peaking at V = 43.8 km s-¹, associated with the IRAS08426-4601 point source. This source is located very close, angularly speaking, to Bochum 7. Based in the similitude of the radial velocities, it is very likely that IRAS08426-4601 is located in the neighbourhood of Bochum 7. The IRAS colours of this source indicate that it may be an ultracompact HII region (see Wood & Churchwell, 1989a ApJS 69, 831).

CONCLUSIONS

Summing up, it has been shown that:1) The kinematic distance of GS262-02+45 (5.3 ± 1.0 kpc) is compatible with the spectrophotometric distance of Bochum 7 (4.5 ± 0.6 kpc).2) The baricentral radial velocity of Bochum 7 (44 ± 10 km s-¹ ) is similar (within the errors) to the baricentral velocity of GS262-02+45 (44 ± 10 km s -¹ ).3) There seems to be evidence of the interaction between the massive stars of Bochum 7 and GS262-02+45.4) GS262-02+45 is much older (age (1.3 – 3.0) x 10exp(7)yr) than Bochum 7 (age (2.0– 7.0) x 10exp(6)yr).5) The proper motion of the stars of Bochum 7 do not rule out an outward motion (of the OB association) at a position angle of ~ 90º with respect to the centre of GS262-02+45.

Based on the points enumerated above, it appear as likely that the formation of Bochum 7 may have been triggered by GS262-02+45.

If IRAS08426-4601 were an ultracompact HII region created with Bochum 7, this region may be a new case for a sequential star formation scenario.

Within this framework, GS262-02+45 would represent the oldest structure (whose genesis is unknown for the time being) and the IRAS source would be the youngest object.The shell GSH263+00+47 (McClure-Griffiths et al. 2002) has an age similar to Bochum 7. Furthermore, the direction of the proper motion of Bochum 7 (even with their large errors) do not favour a physical association between Bochum 7 and GSH263+00+47.

FIGURE 1: Mosaic of HI mean brightness temperature in selected velocity ranges within30 to 61 km s-¹. The central LSR velocity of eachmap is indicated at the inner top right corner. The angular resolution of the HI survey is givenby the filled circle drawn in the upper left cornerof the HI image. The lower contour is 15K andthe higher contour is 65K.

FIGURE 2: As Figure 1 but for a narrower velocity range.

FIGURE 3: Grey – scale representation of the mean brightness temperature in the velocity range 39 to 49 km s-¹. The thin circunference represents the least square fit to the HI peaks defining GS262-02+45. The location GSH263+00+47 is also indicated. The dash – rectangle delimits the blowout shown in Figure 4.

FIGURE 4: The location of Bochum 7 is markedby a big filled black dot. This HI image was constructed using the SGPS data. Clearly enough,Bochum 7 is projected onto a large ellipsoidal HI minimum.

FIGURE 5: The spectrum of HI obtained towards the centre of GS262-02+45. The approaching (front wall) and receding (rear wall) parts of the HI shell are identified. The baricentral velocity is indicated by Vo. The expantion velocity is derived as half the velocity separation between the peak velocity of the front and rear walls.

FIGURE 6: Same as Figure 3. The physical dimentions of GS262-02+45 and the HI minimum associated with Bochum 7 are shown. A distance of 5.3 kpc was adopted for GS262-02+45 .

DATABASES

The large region delimited 255º l 270º and -8º b 5º was reanalyzed using the new high sensitivity HI survey of the southern sky. Arnal et al. (2000, A&AS 142, 35) describe the motivation and observing strategy of the IAR HI survey Bajaja et al. (2005, A&A 440, 767) describe the stray-radiation correction applied to thedata, and Kalberla et al. (2005, A&A 440, 775) present the data, as combined to the northern material of theLeiden/Dwingeloo Survey (LDS), via a link to the CDS.A smaller region, 263º l 268º and -5º b 1º, was analyzed making use of the SGPS (McClure-Griffiths et al. 2001, ApJ 551, 394). These data were retrived from de ATCA site.

Acknowledgment: I would like to thank the authorities of the IAR for the use of its facilities.