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Interstellar Na I Absorption Towards Stars in the Region of the IRAS Vela Shell 1 3 M

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Interstellar Na I Absorption Towards Stars in the Region of the IRAS Vela Shell 1 3 M 4. Towards the Galactic Rotation Observatory and is now permanently in­ the rotation curve from 12 kpc to 15 kpc Curve Beyond 12 kpc with stalled at the 1.93-m telescope of the and answer the question: ELODIE Haute-Provence Observatory. This in­ "Does the dip of the rotation curve at strument possesses an automatic re­ 11 kpc exist and does the rotation curve A good knowledge of the outer rota­ duction programme called INTER­ determined from cepheids follow the tion curve is interesting since it reflects TACOS running on a SUN SPARC sta­ gas rotation curve?" the mass distribution of the Galaxy, and tion to achieve on-line data reductions The answer will give an important clue since it permits the kinematic distance and cross-correlations in order to get about the reality of a local non-axi­ determination of young disk objects. the radial velocity of the target stars symmetric motions and will permit to The rotation curve between 12 and minutes after the observation. The investigate a possible systematic error 16 kpc is not clearly defined by the ob­ cross-correlation algorithm used to find in the gas or cepheids distance scale servations as can be seen on Figure 3. the radial velocity of stars mimics the (due for instance to metal deficiency). Both the gas data and the cepheid data CORAVEl process, using a numerical clearly indicate a rotation velocity de­ mask instead of a physical one (for any References crease from RG) to R= 12 kpc, but then details, see Dubath et al. 1992). This Baranne A, Mayor M., Poncet J.L., 1979, technique allows us to extract easily the the gas data outline a flat or rising curve Vistas in Astronomy 23, 279. at Vrot = 230 km S-1 for R > 12 kpc. The radial velocity of cool stars (later than Blitz, L., Fich, M., Stark, AA, 1982, ApJS49, present cepheid sample suffers an FO) from spectra having a signal-to­ 183. effective cutoff in radial velocity meas­ noise ratio of about one and thus to get Clemens, D.P., 1985, ApJ 295, 422. urements around V= 12.5 mag, so that the velocity of Cepheids of 15th mag­ Dubath, P., Meylan, G., Mayor, M., 1992, ApJ the range in galactocentric distances nitude with an exposure time of about 400,510. that it covers is limited to the one indi­ one hour or so (see Fig. 4). Feast M.w. & Walker AR. 1987, ARM 25, cated in the figure. We initiated last winter a programme 345. Fux, R, 1994, in preparation. ELODIE is a high-resolution (45,000) of about 25 cepheids to cover the Mermilliod J.-C., Mayor M., Burki G., 1987, fibre-fed spectrograph with a fixed 11 -15 kpc range of galactocentric dis­ A&AS 70, 389. wavelength range from 3900 to 6800 A. tances. The approximate positions of Pont F., Mayor M., Burki G., 1994a, A&A in It was built by collaboration between the the target cepheids are indicated as press. Haute-Provence Observatory, the crossed hexagons on Figure 2. The Pont F., Mayor M., Burki G., 1994b, A&AS in Marseilles Observatory and the Geneva analysis of this sample should constrain press. Interstellar Na I Absorption Towards Stars in the Region of the IRAS Vela Shell 1 3 M. SRINIVASAN SAHU ,2andA. BLAAUW 1Nordic Optical Telescope Group, La Palma, Spain; 21nstituto de Astrof(sica de Canarias, Tenerife, Spain; 3Kapteyn Laboratorium, Groningen, The Netherlands Introduction positive galactic latitudes is not clearly ring-like structure is ~ 450 pc and its The Ha emission associated with the seen due to confusion with the estimated mass is -106 MG). Both from Gum Nebula is confined to a circular background galactic emission. The half the morphology on IRAS maps as well region which has a diameter of approxi­ circle of dark clouds at negative as from a study of the kinematics of the mately 36° (Chanot and Sivan, 1983). latitudes coincident with the IRAS Vela ionized gas in Puppis-Vela (Srinivasan In the southern brighter part of the Shell is apparent in the maps by Feit­ Sahu and Sahu, 1993) the IRAS Vela nebula, near ·l Velorum, the emission zinger and StOwe (1986), in their study Shell and the Gum Nebula appear to be measure is higher by a factor of ~3 as of the projected dark cloud distribution two separate structures which just compared to the fainter, outer regions. of the Milky Way. The IRAS maps clearly happen to overlap in projection. We examined the IRAS SuperSky Flux show that the cometary globules and We have analysed proper motions of maps in the entire Gum Nebula region dark clouds in the Puppis-Vela region, the early-type stars in the region and and discovered a ring-like structure catalogued from the ESO-SERC IllaJ there is strong evidence that the Vela (Fig. 1) coincident with the bright south­ plates (for example, Hawarden and OB2 stars are indeed part of a B-associ­ ern part, which we have termed the Brand, 1976), are part of this ring-like ation. The association nature is further IRAS Vela Shell (Srinivasan Sahu, 1992; structure. confirmed by the fact that both from Srinivasan Sahu and Blaauw, 1994). The members (known and probable) position in the galactic (I, b) diagrams This structure, centred around the Vela of Vela OB2 lie within the IRAS Vela and its distance, Vela OB2 appears to OB2 group of stars (the B-association of Shell which in fact just envelops the form an extension of the string of which y2Veiorum is a member, which we stars in this association. The Vela OB2 association subgroups known as the will refer to as Vela OB2, according to stars are therefore physically associated Sco-Cen association and thereby a part the nomenclature of Ruprecht et aI., with the IRAS Vela Shell. Based on dis­ of the Gould belt. Stromgren photome­ 1981), has an average radius of ~8°. tance estimates to y2 Velorum and Vela try by Eggen (1982) and an analysis by The section of the IRAS Vela Shell at OB2, the distance to the centre of this us using data from the homogeneous 48 _50 ---. rt.l Q) Q) H -100 OJ.) Q) ""0 .......... ,.c -150 2600 I (degrees) Figure 1: The IRAS Vela Shell at 60 ~tm, obtained from the IRAS SuperSky Flux maps. The positions of the Vela 082 stars are shown by "star" symbols, ~ Puppis by an open square and the Vela pulsar by a plus sign. ~ Puppis and the Vela pulsar are located near the edges of this shell, while the IRAS Vela Shell is clearly seen to envelop the Vela 082 stars. This suggests that the Vela 082 stars are physically associated with the shell and are the source of energy. The optically catalogued cometary globules and dark clouds in the Puppis-Vela region are part of the IRAS Vela Shell. The general location of the Vela Molecular Ridge (VMR) is also indicated in the plot. ubvy-H~ catalogue of Hauck and Mer­ milliod (1991), indicates that this associ­ ation is aged (~2 to 3 x 107 years old) and on the verge of disintegration. Gum Nebula Aim of the Study 10 The distribution and the kinematics of the Nal absorbing gas can help to understand this component of the inter­ stellar medium in the IRAS Vela Shell. The Goddard High-Resolution Spectro­ graph (GHRS) on the Hubble Space 0 " t " " d> Telescope with a resolving power of ~ ".. " ,," R ~ 80,000 and wavelength range from " " "" 1150 A to 3200 A has been used to " "" " " " study the properties of the highly " """ " " """ " ionized absorbing gas in each individual " 8"" o8lJ " .,." "" component for the case of l Velorum -10 " It " " (Fitzpatrick and Spitzer, 1993). How­ """ " ever, there are no good-quality optical " data with comparable resolution which " IRAS Vela Shell can be combined with the GHRS data to study the weakly ionized and neutral components in the IRAS Vela Shell. For -20 these reasons, in January 1993, we initi- Figure 2: Schematic figure showing the loca­ tions of the IRAS Vela Shell, the Gum Nebula 280 270 260 250 240 and the stars that we have observed. .e 49 1.1 "'-""--"'-'-1""" 'I""""1"" ,', spectra were obtained for stars and the thorium lamp. The stellar spectra were wavelength calibrated by using the thorium spectra taken either preceding (b) ~ 0,8 or following the stellar exposure. We HD 63922 identified typically 30 to 35 lines in the .~ 0.6 thorium spectra with the help of the +' <il Atlas of the Thorium-Argon Spectrum oJ IL: 0.4 (O'Odorico et aI., 1987) and fitted poly­ 0,8 HD 74195 nomials to obtain relationships between 0,2 the wavelength and pixel number. The polynomial fits had residuals with an 0, 7 '-'-"--,---'-'--"--,-L-~-'-L-LJ~'-'-~ OLL.L.L...L.1.-LJLLL-L..L..L~-L-LL.LL-J rms scatter of < 0.0045 A for all the 5888 5890 5892 5894 5896 5898 5888 5890 5892 5894 5896 5898 spectra. Figure 3 (a-d) shows the Nal Wavelength (1) Wavelength (A) 0 1 and O2 region in the spectra of four of the stars in our sample. 0.6 ,.,-,-"'I"""--'---1rl'I'I'1'1'1'1--'---1rl'" The Na I 0 1 and O2 lines fall in a spec­ tral region which has numerous telluric lines mostly due to atmospheric water vapour. The strengths of the telluric lines x ;§ (d) caused by water vapour can varf by a (c) factor of two or more within a matter of a ill ill HD 75149 ,2: HD 76534 .2: +-' +' few hours, even at a high altitude obser­ <il <il oJ oJ vatory like La Silla (A.
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