Dave Meyer Northwestern University Optical/UV Absorption Line Observations of Small-Scale Interstellar Structure

UV Absorption-Line Measurements of Interstellar Abundances GHRS: Cardelli & Meyer (1997)STIS: Cartledge, Meyer, & Lauroesch (2003) UV absorption-line studies of many nearby (d < 1 kpc) diffuse ISM sightlines show remarkable elemental abundance homogeneity, apart from dust depletion effects  Cartledge et al. find a [Kr/H] abundance spread of about 0.1 dex

Multi-Epoch H I 21 cm Absorption Observations of Pulsars Frail et al. (1994) find H I structure on scales of AU toward the 6 pulsars in their study and conclude that 10-15% of the cold H I gas could be in this dense form

High-Resolution Optical Spectroscopy of Interstellar Na I Welty et al. (1994) survey interstellar Na I absorption toward 38 stars at a spectral resolution of 0.5 km/sec Each star constitutes an absorption “beam” of about arc sec through the intervening diffuse ISM They identify 276 “clouds” of which many are cold enough to resolve the 1.05 km/sec Na I hyperfine splitting Could such cold Na I components be related to the Frail et al. small-scale H I variations?

Observations of Interstellar Na I Toward Resolvable Binary Stars Meyer & Blades (1996) – AAT UHRF 0.4 km/sec resolution spectroscopy of Na I absorption toward the binary  Cru (B2 V & B5 V stars with a projected separation of 6600 AU (38.8 arc sec at d = 170 pc)) The most striking Na I variations match the narrowest components (T < 200 K) that have large N(Na I)/N(Ca II) ratios Assuming simple cloud geometry and that N(Na I) traces N(H I), the variations imply small-scale molecular cloud densities Are small-scale Na I variations ubiquitous?

Watson & Meyer (1996) – 1.4 km/sec resolution Na I spectroscopy of 17 binaries spanning types O6 - A5, separations ,000 AU, & distances pc  all show interstellar Na I line strength and/or profile variations

Questions to Investigate: Does the Na I small-scale structure trace variations in interstellar H I?  Na I is not a dominant ion: N(Na I) = (  /  ) n(e) N(Na II)  few interstellar species are observable at optical wavelengths  the dominant ion resonance lines of many elements are in the UV Are there spatial patterns (shells, filaments, etc.) in the Na I structure?  probe larger scales thru multi-object spectroscopy of open star clusters  probe smaller scales thru integral field spectroscopy of globular clusters Down to what scale are there variations in the interstellar Na I absorption?  probe solar-system scales thru multi-epoch stellar spectroscopy

 Cru Lauroesch et al. (1998) HST GHRS data 6600 AU separation HD 32039/40  Lauroesch et al. (2000) HST STIS data 4800 AU separation Results: Na I variations seen in other neutral species but not dominant ions Lack of C I fine structure excitation indicates variable components are not high density

The Complex Interstellar Na I Absorption Toward h and  Per Points et al. (2004) – WIYN Hydra multi-object spectroscopy of interstellar Na I at 12 km/sec resolution toward 172 stars in the h and  Per double star cluster

The Na I profiles toward the 150 h &  Per stars showing high-velocity absorption from Perseus arm gas – no two profiles are identical. The 12 to 4950 arc sec separations correspond to 0.1 to 50 pc at the distance (about 2000 pc) of the Perseus arm gas.

Higher resolution (3 km/sec) spectra of 24 h &  Per stars show that the high-velocity Perseus arm gas breaks up into multiple components with dramatic variations in strength, velocity, & number from sightline to sightline with no apparent pattern

Mapping Interstellar Na I Absorption Toward the M15 Core Meyer & Lauroesch (1999) – WIYN Na I spectroscopy of central 27 ” x 43 ” of the globular cluster M15 at 4 ” spatial resolution (3 ” fiber diameter) & spectral resolution of 14 km/sec (5 ’ x 5 ’ image of M15 above courtesy of NOAO/KPNO)

The derived Na I column densities vary by a factor of 16 across the M15 IV cloud map  significant variations are seen down to the resolution limit of 6000 AU (assuming a distance of 1500 pc for the IV cloud)

Andrews et al. (2001) Comparison of Na I maps of two clouds toward M92 & M15 LISM & IVC maps M92 #1: 1600 AU scale M92 #2: 3200 AU scale M15 LISM: 2000 AU scale M15 IVC: 6000 AU scale All maps exhibit similar fiber-to-fiber N(Na I) variations – may reflect small-scale turbulence in Na ionization equilibrium Larger-scale, higher column variations in M15 maps suggest real H I structures

The Smallest-Scale Interstellar Na I Absorption Structure HD 28497: ~10 AU (Blades et al. 1997)  Ori: ~10 AU (Price et al. 2000)  Vel: ~15 AU (Crawford et al. 2000, 2003) HD : case of new Na I component strengthening on scale of ~20 AU/year (Welty & Fitzpatrick 2001)

KPNO Coude Feed observations of the Na I absorption toward 23 Ori (HD 35149) and its binary companion (9600 AU separation) at 1.4 km/s resolution  observations in 2004 reveal a component at +7 km/s toward HD that was not present in 1996 (Welty et al. 1996)  further observations of this component in 2005 reveal a doubling of its N(Na I)  the weaker +7 km/s Na I absorption toward HD constant in 2004/5 The projected transverse motion of HD over corresponds to  7 AU  the N(Na I) doubling during 2004/5 was on a star motion scale of  0.8 AU!  even allowing for a transverse gas flow of 40 km/s relative to the star, the doubling scale is still only  8 AU

Using the KPNO Coude Feed, we have been monitoring the Na I absorption toward 40 stars for the past decade  HD is the only star to exhibit temporal variations in N(Na I) on scales < 7 AU  15% of the 20 sightlines probing scales of 7 – 50 AU show variable N(Na I) Two of these latter cases are also in binaries:  HD  4800 AU binary separation  50 AU motion over 10 yrs  HD 36408A  2100 AU binary separation  25 AU motion over 10 yrs

Crawford (2003) Five of the ten cases found to date of temporal variations in N(Na I) lie in the direction of the Orion-Eridanus Shell  the Ori-Eri Shell is a large (r  150 pc), expanding bubble of gas seen in H  & 21 cm emission that has arisen from Orion OB1 SNe and stellar winds  two of the other cases are toward the Vela SN remnant (Cha & Sembach 2000) Thus, it appears that temporal Na I variations indicative of interstellar structure on scales less than  50 AU are rather rare & are preferentially associated with dynamic interstellar regions such as supernova remnants  Weisberg & Stanimirovic (2007) have recently reviewed recent multi-epoch studies of the H I 21 cm absorption toward high-velocity pulsars which find only a few H I variations on scales < 100 AU – they conclude that such structure is not ubiquitous

A Cold Nearby Cloud Inside the Local Bubble Meyer et al. (2006) – Na I absorption-line observations toward 33 stars in Leo place an upper limit of 45 pc on the distance of the 20 K cloud seen in H I 21 cm data

At 1.3 km/sec resolution, the Na I absorption lines corresponding to the cold Leo cloud are strong & narrow - easily detected toward even late-type stars!