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 +' Figure 3 (a-d): The Nal 0 1 and O2 region in the spectra of four stars in our sample: (a) HO telluric lines using the synthetic telluric ~ 74195, a member of the cluster Ie 2391 which is at a distance of 170 pc (b) HO 63922, a spectrum in the region of the 0 1 and O2 member of Vela 082, located at ~450 pc (c) HO 76534, a member of Vela R2, located at lines, constructed by Lundstrom et al. ~ 700 pc and (d) HO 75149, a member of Vela 081 (Humphreys, 1978) located at ~ 1500 pc. (1991). Note that the spectra have not been corrected for the influence of telluric lines. We intend to pursue this observation al programme and observe the Call ab sorption in the line of sight to our sample stars, to obtain Na IICa II ratios, when ated a high-resolution (R ~ 80,000) and the UV-coated Ford Aerospacel ever possible for each component. The study of the Na I 0 1 and O2 absorption Loral 2048x2048 CCO (#27) was used Na IICa II ratio traces changes in the cal lines in the line of sight to stars in the for all our observations. This CCO has a cium abundance in the absorbing gas. region of the IRAS Vela Shell. pixel size of 15 [Lm x 15 [Lm, a low dark Calcium is released in the gaseous Our sample consists of ~ 75 early current (3 e-/pixel/hour), a low readout phase when grain destruction occurs type stars (spectral types 09 to B7) noise (~6e- rms) and appears to have due to violent events such as, for exam whose locations with respect to the few defects. The net efficiency of this ple, supernovae. Therefore, in addition IRAS Vela Shell and the Gum Nebula are system is: 3.8 % at 5400 Aand 4.6 % at to determining the distribution and shown in Figure 2. The distances to 6450 A (Pasquini et aI., 1992). We have kinematics of the absorbing gas, we ex these stars were determined using pub used this instrument configuration both pect to learn more about the history of lished spectroscopiclphotometric data at La Silla and by means of remote the IRAS Vela Shell through these ob as well as proper motion data obtained control from Garching, with satisfactory servations. from the Hipparcos Input Catalogue. results. There is fairly good agreement between We have used MIOAS for our data the values of the distances obtained by reduction. After the standard bias and these two methods for the case of rela background subtractions and flat field References tively nearby stars. The distances for the ing procedures, we extracted orders by Chanot, A., Sivan, J.P.: 1983, Astron. Astro ~ stars in our sample range from 150 to using only the central 4 to 6 columns phys. 121, 19. 2000 pc and galactic altitudes (I z I) which had the highest SIN ratios, in the D'Odorico, S., Ghigo, M., Ponz, D.: 1987, range up to ~ 160 pc. stellar and thorium lamp frames. This ESO Scientific Report NO.6. was done to avoid the contribution due Eggen, O.J.: 1982, Astrophys. J. Suppl. Ser. to background scattered light. We opted 50, 199. Observations and Future to use a weighted average (weighted Feitzinger, J.V., StOwe, J.A.: 1984, Astron. Prospects according to their SIN ratio values) of Astrophys. Suppl. Ser. 58, 365. We have obtained fairly high SIN these central 4 to 6 columns rather than Fitzpatrick, E.L., Spitzer, L.: 1993, Princeton Observatory Preprint No. 541, (~60 to 500) Na 0 and O spectra for all a simple average since the SIN ratios for 1 2 Hauck, B., Mermilliod, M.: 1991, obtained the stars in our sample. The 1.4-m the case of the weighted average through the SIMBAD data retrieval facility. Coude Auxiliary Telescope (CAT) and method was typically higher than the Hawarden, 1.G. and Brand, P.W.J.L.: 1976, the Coude Echelle Spectrograph (CES) simple average method by a factor ~ 1.5 Monthly Notices Roy. Astron. Soc. 175, in combination with the Long Camera to 2. Thus, normalized two-dimensional 19P. 50 Humphreys, R.M.: 1978, Astrophys. J. Suppl. Ruprecht, J., Balazs, B., White, R.E.: 1981, University of Groningen. 38,309. Catalogue of Star Clusters and AssocIa Srinivasan Sahu, M. and Blaauw, A.: 1994, Lundstrom, I., Ardeberg, A., Maurice, E., tions, Supplement I, Part B2, ed. B. Balazs Astron. Astrophys. Main Journal (subm.). Lindgren, H.: 1991, Astron. Astrophys. (Akedemai Kiado, Budapest), p. 471. Srinivasan Sahu, M. and Sahu, K.C.: 1993, Suppl. Ser. 91, 199. Srinivasan Sahu, M.: 1992, Ph. D. thesis, Astron. Astrophys. 280, 231. A Radial Velocity Search for Extra-Solar Planets Using an Iodine Gas Absorption Cell at the CAT + CES M. KURSTER.. 1,A.P. HATZES2, WO. COCHRAN,2 C.E. PULLIAM,2 K. OENNERL 1 and S. OOBEREINER 1 1Max-Planck-Institut fUr Extraterrestrische Physik, Garching, Germany 2McOonald Observatory, The University of Texas atAustin, Austin, U.S.A. Introduction Radial Velocity Technique The origin of the solar system is a minimized by superimposing the fundamental problem in astrophysics for Traditional radial velocity measure wavelength reference on top of the stel which many basic questions remain to ment techniques rarely exceed a preci lar observation. One means of accom be answered. Is planet formation a com sion of 200-500 m S-1. The reason for plishing this is to pass the starlight mon or rare phenomenon? Is it a natural this is that the wavelength reference is through an absorbing gas prior to its extension of the star formation process taken at a different time and often entrance into the spectrograph. The gas or is a different mechanism involved? traverses a different light path than that produces its own set of absorption lines Unlike most stars, the Sun is not found of the stellar spectrum. Use of radial against which velocity shifts of the stel in a binary system. Is its single status velocity standard stars circumvents the lar spectrum are measured. Since in related to the fact that it has a planetary problem of different light paths for the strumental shifts now affect both the system? Unfortunately, the answers to reference and stellar spectra, but the wavelength reference and stellar spec these and other important questions are standard observation is still made at a trum equally, a high degree of precision hampered by the fact that the only different time and there is always the is achieved. known example of a planetary system danger that the standard star is a low Griffin and Griffin (1973) first pro (around a non-degenerate star) is the amplitude variable. posed using telluric O2 lines at 6300 A one around the Sun. Clearly, before one The instrumental errors can be as a radial velocity reference. In this can develop general theories of planet formation, one must collect a large body of astronomical data that includes the frequency of planetary systems, the planetary mass function, and the corre .8 lation of such systems with mass, age, .6 stellar composition, etc. of the primary .4 star. .2 Although a number of direct and indi rect techniques have been proposed for extra-solar planet detections, radial ve X ::l locity measurements are proving to be a u:: .8 Q) cost effective means of using ground > .6 .~ based facilities to search for planets. Qi .4 0: What radial velocity precision is needed .2 to detect Jovian-sized planets? Natural ly, one uses the Jupiter-Sun system as a guide where the Sun orbits around the barycentre with an average velocity of .8 12 m S-1 and a period of 12 years. Thus, .6 an instrument capable of measuring rel .4 ative stellar radial velocities to a preci .2 sion better than 10 m S-1 and with a decade-long stability should be able to 5360 5365 5370 5375 5380 5385 5390 5395 5400 5405 detect the presence of a Jovian-massed Wavelength (A) planet orbiting 5 AU from a solar-type Figure 1: (Top) Absorption spectrum of the 12 cell obtained by taking a dome f1atfield through star. Lower mass objects can be de the cell. (Middle) Spectrum of a Cen B without the 12 cell from 5360 A to 5407 A. (Bottom) tected in orbits with smaller semi-major Spectrum of a Cen B taken with the 12 cell in front of the entrance slit of the ESO CES. All axes. spectra have been normalized to the continuum. 51