Interstellar Ti II Absorption Toward the Magellanic Clouds

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Session 35 -- ISM: Abundances and Dust
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[35.02] Interstellar Ti II Absorption Toward the Magellanic Clouds

Katherine C. Roth, J. Chris Blades (STScI), and C. Elise Albert (U.S. Naval Academy)

We present high-resolution ($\Delta v = 5$ km s$^{-1}$) spectra of interstellar and extragalactic Ti II $\lambda 3384$ absorption toward several early-type stars in the Magellanic Clouds. The data were obtained with the CTIO 4-meter telescope and echelle spectrograph during three nights in November of 1994, taking advantage of the newly refigured secondary mirror, enhanced blue optics, and blue-sensitive TI 3 CCD detector. We observed seven stars in the LMC and four stars in the SMC, including Sk~$-$78 (HD 5980) which was undergoing an outburst during which it brightened by two magnitudes ($m_V \approx 9.5$). With a typical signal-to-noise ratio of $\approx$ 30, we are sensitive to absorption features as weak as 5 m\AA\ (2.5$\,\sigma$). Toward Sk $-$78 our measured signal-to-noise ratio is $\geq$ 50, more than four times higher than had been obtained previously toward this star and twice that of existing data toward any Magellanic Cloud member. This combination of high resolution and signal-to-noise ratio enables the detection of Ti II gas within the Milky Way halo and additionally probes the neutral ISM of the Magellanic Clouds themselves.

Ti II is a reliable tracer of H I gas because of the nearly identical ionization potentials of singly-ionized titanium and neutral hydrogen. Ti~II is also highly refractory and readily depletes onto dust grains. Lastly, Ti II absorption features are significantly weaker than the often saturated lines of Ca II and Na I, the other primary optical tracers of neutral material observable from the ground, which allows for the derivation of accurate column densities. For these reasons, Ti II has been used to measure the extent of neutral gas in the Milky Way halo and the depletion characteristics thereof. Our observations add to such existing scale height determinations, and additionally provide spatial information on the clumpiness of low velocity ($v \approx 0$ km s$^{-1}$) neutral halo material. We also measure gas distributions and dust depletions within the high velocity ($v \approx 100 - 350$ km s$^{-1}$) LMC and SMC absorbing material as well as lower velocity intergalactic gas originating within the Magellanic Stream.

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