We report the initial results of a study to probe the 

small-scale structure in diffuse interstellar clouds by observing optical absorption lines of CH+ at 4232 °A and Ca II K at 3933 °A towards both components in resolvable binary systems. The data analysis is still in progress. To date, 70% of the Ca II spectra have been analyzed for the 19 observed systems. For half of these, a difference in the line strength or in the number of calcium components is found. The sightlines have been selected so that the observing method provides a potential probe of the 

small-scale structure of the interstellar medium in the range 200-4000 AU

Attolini, M. R.

Crane, P.

Palazzi, E.

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO VOL. 20 C, N. 5 Settembre-Ottobre 1997Ca II and CH+ interstellar absorption observationsin the direction of resolvable binary stars()E. PALAZZI(1), M. R. ATTOLINI(1) and P. CRANE(2)(1) Istituto TESRE/CNR - Bologna, Italy(2) ESO - Garching, Germany(ricevuto il 5 Dicembre 1996; approvato il 18 Marzo 1997)Summary. — We report the initial results of a study to probe the small-scale structurein diffuse interstellar clouds by observing optical absorption lines of CH+ at 4232 A˚and Ca II K at 3933 A˚ towards both components in resolvable binary systems. Thedata analysis is still in progress. To date, 70% of the Ca II spectra have been analyzedfor the 19 observed systems. For half of these, a difference in the line strength or inthe number of calcium components is found. The sightlines have been selected so thatthe observing method provides a potential probe of the small-scale structure of theinterstellar medium in the range 200-4000 AU.PACS 98.38 j – Interstellar medium (ISM) and nebulae in Milky Way.PACS 98.70 – Other sources and radiations (including cosmic rays outside the SolarSystem).PACS 01.30.Cc – Conference proceedings.1. – IntroductionOne of the most powerful means to study the diffuse and cool components of the in-terstellar medium (ISM) is provided by the optical observations of atomic and molecularabsorption lines (Cowie and Songaila, 1986; van Dishoeck, 1990). In addition to the col-umn densities of the observed elements, it is possible to get information on the velocitystructure, density and inhomogeneity of the interstellar clouds where these species lie.However, the range of cloud sizes is still not well known and the extent to which the mat-ter is clumped within individual clouds is very uncertain. In the past only a limited numberof observations have been devoted to this subject.A search of small-scale structure in the ISM has been carried out (Crovisier et al.,1985; Kalberla et al., 1985; Diamond et al., 1989; Deshpande, 1992) by observing HI 21cm absorption line in the few targets that are available (extragalactic radio sources as() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.G Societa` Italiana di Fisica 707708 E. PALAZZI, M. R. ATTOLINI and P. CRANETABLE I. – Observed binary systems.HD Right ascension Declination mVSpectral No. of Ca IIK Wof Ca IIKnumber (1950.0) (1950.0) type components main component(mA˚)HD24071 03h 46m 44.7s  374602600 5.42 A1V 5 8:7  0:6HD24072 03h 46m 45.0s  374602000 4.86 B9V 7 7:0  0:8HD32039 04h 57m 54.7s +033203300 7.03 B9Vn ... ...HD32040 04h 57m 56.1s +033203600 6.66 B9Vn ... ...HD34798 05h 17m 05.8s  183401400 6.39 B5IV ... ...HD34797 05h 17m 07.0s  183303700 6.50 B8IV ... ...HD35149 05h 20m 12.2s +032905200 5.00 B1V ... ...HD35148 05h 20m 13.0s +033002000 7.18 B3V ... ...HD36486 05h 29m 27.0s  002000400 2.24 O9.5 12 13:5  0:4HD36485 05h 29m 27.0s  001901200 6.85 B2V 12 42:7  1:7HD37043 05h 32m 59.1s  055602800 2.77 O9III 7 23:9  0:7HD37043B 05h 32m 59.5s  055603500 7.00 B7IV 7 21:2  0:7HD37742 05h 38m 14.04s  0158002:9700 1.70 O9I 6 36:4  0:2HD37743 05h 38m 14.00s  0158003:000 4.21 B0III 6 22:3  0:7HD45725 06h 26m 23.4s  065905800 4.60 B3V 7 8:9  0:4HD45726 06h 26m 23.9s  070000000 5.40 B3 7 8:3  0:5HD59499 07h 26m 55.9s  314404000 6.39 B3V 4 56:2  1:2HD59500 07h 26m 56.3s  314403400 7.12 B4V 2 54:2  0:7HD61555 07h 36m 46.3s  264101300 4.50 B6V no Ca II ...HD61556 07h 36m 46.8s  264102000 4.62 B5IV no Ca II ...HD66005 07h 57m 47.6s  495002100 6.32 B2IV ... ...HD66006 07h 57m 48.7s  495001000 6.34 B2IV ... ...HD77002 08h 55m 45.0s  590200800 4.92 B2IV 2 11:9  0:1HD77002B 08h 55m 50.2s  590105800 6.87 B9.5V 2 8:9  0:3HD91356 10h 29m 48.2s  444804300 6.11 B4 5 13:3  0:3HD91355 10h 29m 49.0s  444803200 5.74 B9 7 18:4  0:4HD108248 12h 23m 48.1s  624901900 1.58 B0.5VI 2 9:7  0:1HD108249 12h 23m 48.6s  624902000 2.09 B1V 2 9:8  0:1HD144217 16h 02m 31.5s  194001200 2.62 B0.5V 3 31:1  0:2HD144218 16h 02m 31.9s  194000000 4.92 B2V 3 33:0  0:2HD145501 16h 09m 04.0s  191901700 6.30 B8V 2 38:9  0:3HD145502 16h 09m 05.0s  191905600 4.01 B2IV 2 55:2  0:3HD147933 16h 22m 34.9s  232000100 5.02 B2V ... ...HD147934 16h 22m 34.8s  231905800 5.92 B2V ... ...HD150135 16h 37m 33.8s  484000300 6.88 O6 ... ...HD150136 16h 37m 35.1s  484000100 5.66 O5 ... ...HD170740 18h 28m 39.2s  104905500 5.72 B2V 2 57:6  0:3HD170740B 18h 28m 38.3s  104905700 9.4 B9V 2 24:0  6:0continuum background are necessary or pulsar observations). Significant variations inprofiles on scales above 40 000 AU and below 25 AU were measured.More recently, evidence for structures on a scale of  0.1 pc has been obtained fromobservations of closely spaced stars in the LMC (Molaro et al., 1989) and two globularclusters (Langer et al., 1990).Ca II AND CH+ INTERSTELLAR ABSORPTION OBSERVATIONS ETC. 709Fig. 1. – Ca IIK interstellar absorption lines towards the two stars HD145501 and HD145501. Thedifference in the strength between the main component line in the two spectra is greater than 20%.One of the major problems in using distant backgrounds (extragalactic radio sources,globulars, LMC) is the large uncertainty in the distance of the intervening clouds andhence the impossibility of determining the real scale length of any apparent structure.A technique to avoid this problem is to perform high-resolution observations of inter-stellar absorption lines towards both components of binary systems and to look for linestrength differences in the spectra. Using this method, Meyer (1990) reports only onecase, out of the 6 studied, in which the differences in the strength of the measured linestowards the two stars are greater than 50% resulting in an ISM structure on a scale of2800 AU.Our project is directed towards enlarging the sample of the observed resolvable binarystars in order to determine if the existence of small-scale structure (in the range 200–4000AU (0.001–0.02 pc)) in the interstellar medium is more the exception than the rule or viceversa. In the following sections we give details on the observations and we present thevery preliminary results.2. – Observations and data analysisThe high-resolution spectra were obtained at the 1.4 m Coude` Auxiliary Telescope ofESO on La Silla during two observing runs: in 1993 June, performed in remote modefrom Garching and in 1993 December, in loco. The adopted configuration, CCD plus longblue camera, provided a resolution of about 115 000 at the studied wavelengths.The sight lines were selected on the basis of the following constraints:1) both components are O or B stars: hot stars are preferred for their high ionization level710 E. PALAZZI, M. R. ATTOLINI and P. CRANEFig. 2. – Ca II K interstellar absorption lines towards the two stars HD36485 and HD36486. Thislatter star migth show in its spectrum a stellar calcium feature, but the contamination by the stellarcomponent would not compensate the strong difference observed in the two spectra.so that the stellar continuum is almost flat;2) separation between the two components and distance of the stars providing a check onISM structure on scale less than 4000 AU;3) stars for which a detection of interstellar absorption features already existed (this wasgenerally true for only one of the two components).Data reduction was performed using the MIDAS system following the standard pro-cedure. For each image we subtracted the bias level, divided by the nightly flat field andremoved the cosmic rays. The spectra extraction was performed summing only on therows with higher S/N. Stellar spectra were wavelength-calibrated using comparison spec-tra of a thorium/argon calibration lamp obtained during each night of the observing runs.The next step in the data analysis was the determination of the column densities ofeach detected line. Spectral line profiles were modelled using the MIDAS context CLOUDwere column densities are computed by using Voigt profile model.For most of the stars the observed profile shows more than one component for bothCa II K and CH+. The minimum number of components necessary to reproduce the datahas been adopted in the analysis. Due to the spectral resolution we have in our spectra, itis not possible to resolve components separated by less than 1–2 km sec 1.3. – Results and discussionTable I lists the observed binary systems and, for spectra analysed to date, the numberof Ca II K components detected in each spectrum and the equivalent width measured forCa II AND CH+ INTERSTELLAR ABSORPTION OBSERVATIONS ETC. 711Fig. 3. – Ca IIK interstellar absorption lines towards the two stars HD91355 and HD91356. In thiscase in addition to the difference in strength of the observed features, the two spectra show also adifferent number of calcium components.the main component. The uncertainties associated to the equivalent width reflect both thestatistical and the continuum placement errors.The figures show some examples of the detected differences.Figure 1 shows the Ca II K interstellar absorption line towards both components of Sco system (HD145501, HD145502). The high-resolution observations indicate a multi-component profile with a line strength difference (> 20%) concetrated in the main compo-nent.Figure 2 shows the Ca II K absorption line towards the two stars HD36486 andHD36485. The latter is a B2V star so there might be a contribution to the line due tothe stellar calcium which show up in star of this spectral type, but even if this is the case itcould not account for the strong difference between the two profiles. Moreover, it seemsthat the main interstellar component do not coincide in velocity.Figure 3 shows a case in which the spectra of the two stars HD91355 and HD91356differ in the number of components. This means that the clouds dimensions would beeven smaller than the separation of the star.50% of the analysed systems (at the Ca II wavelength) show differences in the linestrength or in the number of components. Since data analysis is still in progress, we can-not establish if the presence of small-scale (< 4000 AU) structure in the diffuse interstellarmedium is the rule or the exception.Moreover, additional observations towards some of the stars in our sample are neces-sary to obtain a comparable signal-to-noise ratio in the two spectra. This will give us moreconfidence in the reliability of any detected difference in the interstellar features of the712 E. PALAZZI, M. R. ATTOLINI and P. CRANEobserved sightlines, even if the S/N does not seem to be a limit in the cases reported inthe figures.IRAS and CO maps, when available, in the observed direction of our sample will also beused in conjunction with the optical study. The complete set of observations will increasethe possibility to understand any detected difference in the measured column densities ornumber of components.It is clear that the sample of lines of sight has to be as large as possible (the numberof suitable binary systems that can give us information on the small scale is limited) inorder to establish a more complete range of sizes and to try to understand any possiblecorrelation between any of the species parameters, like linewidth or column density, andthe derived size of the clouds.REFERENCESCOWIE L. L. and SONGAILA A., Annu .Rev. Astron. Astrophys., 24 (1986) 499.CROVISIER J., DICKEY J. M. and KAZES I., Astron. Astrophys., 146 (1985) 223.DESHPANDE A. A., MCCULLOCH P. M., RADHARISHNAN V. and ANANTHARAMAIAH K. R.,Mon. Not. R. Astron. Soc., 258 (1992) 19P.DIAMOND P. J., GOSS W. M., ROMNEY J. D., BOOTH R. S., KALBERLA P. M. W. and MEBOLDU., Astrophys. J., 347 (1989) 302.KALBERLA P. M. W., SCHWARZ U. J. and GOSS W. M., Astron. Astrophys., 144 (1985) 27.LANGER G. E., PROSSER C. F. and SNEDEN C., Astron. J., 100 (1990) 216.MEYER D. M., Astrophys. J. Lett., 364 (1990) L5.MOLARO P., VLADILO G., AVILA G. and D’ODORICO S., Astrophys. J. Lett., 339 (1989) L63.VAN DISHOECK E.F., in Proceedings of the Conference The Evolution in Interstellar Medium,Berkeley, CA, June 21-23, 1989, edited by L. BLITZ (Astronomical Society of the Pacific, SanFrancisco) 1990, p. 207.

Ca II and CH+ interstellar absorption observations in the direction of resolvable binary stars

http://eprints.bice.rm.cnr.it/12302/1/ncc6540.pdf

Ca II and CH+ interstellar absorption observations in the direction of resolvable binary stars

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository