H i and optical spectroscopy towards the M15 intermediate-velocity cloud |
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| Authors: | J V Smoker N Lehner F P Keenan E J Totten E Murphy K R Sembach R D Davies B Bates |
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| Institution: | Astrophysics and Planetary Science Division, Department of Pure and Applied Physics, The Queen's University of Belfast, University Road, Belfast BT7 1NN;Department of Physics, University of Keele, Keele, Staffs ST5 5BG;John Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218-2695, USA;NRAL, Jodrell Bank, nr Macclesfield, Cheshire SK11 9DL |
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| Abstract: | We present single-dish Arecibo 21-cm H i observations, covering a 0 675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V LSR=70 km s−1 are investigated; it is found to be clumpy and has a peak surface density N H i ∼8×1019 cm−2. Additionally, we have performed a long H i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H i with a velocity exceeding 60 km s−1 was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H i pointings and low-resolution spectra in the Ca ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations. |
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| Keywords: | ISM: clouds ISM: general ISM: structure globular clusters: individual: M15 radio lines: ISM |
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