Small-Scale Structure in the Diffuse Interstellar Medium

Dave MeyerNorthwestern University

Fig. 15-28, p.308

Galactic H I 21 cm emission

Leiden/Dwingeloo survey data from Hartmann and Burton (1997) as published in Pasachoff and Filippenko (2004)

High Velocity Cloud WW187

ATCA observations at resolutions of 38 x 60 arc sec, 2 x 2 arc min, 4 x 4 arc min, 8 x 8 arc min

(Wakker et al. 2002)

For cloud distance of 50-100 kpc, smallest, unresolved H I cores correspond to sizes of 5-15 pc

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) sightlines in the diffuse ISM have shown remarkable elemental abundance homogeneity, apart from dust depletion effects in the case of Kr, 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 5-100 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 D absorption toward 38 individual stars at a spectral resolution of 0.5 km/sec

Each star constitutes an absorption-line “beam” of about 0.0001 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 IToward Resolvable Binary Stars

Meyer & Blades (1996) – 0.4 km/sec resolution AAT UHRF spectroscopy of Na I absorption toward the binary Cru (consisting of a B2 V and B5 V star with a projected separation of 6600 AU (38.8 arc sec at d = 170 pc))

The most striking Na I variations correspond to the narrowest components (T < 200 K) which also have large N(Na I)/N(Ca II) ratios

Assuming a simple cloud geometry and that N(Na I) traces N(H I), such variations imply small-scale molecular cloud densities

Are small-scale Na I variations ubiquitous?

Watson & Meyer (1996) – 1.4 km/sec resolution KPNO Na I spectroscopy of 17 binaries spanning O6 - A5 in type, 500 - 30,000 AU in separation, & 100 - 1200 pc in distance 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 in H I clouds: N(Na I) = (/) n(e) N(Na II) few interstellar species are observable via optical absorption-line spectroscopy the dominant ion resonance lines of many elements are detectable 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

What about the small-scale structure of interstellar Na I in other galaxies? probe nearby galaxy cores thru integral field absorption-line spectroscopy

The Diffuse Molecular Sightline Toward HD 206267

Pan et al. (2001)

Optical spectra of the multiple star system HD 206267 [E(B-V)=0.6] show variations in the interstellar Na I, Ca I, K I, CN, CH, CH+ (and some DIB) absorption on scales of 10,000 to 20,000 AU

Lauroesch & Meyer (1999)

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 and Per stars exhibiting absorption from the high-velocity Perseus arm gas – no two of these profiles are identical. The 12 to 4950 arc sec separation of these stars corresponds to 0.1 to 50 pc at the distance (about 2000 pc) of the Perseus arm gas.

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

Mapping Interstellar Na I Absorption Toward the M15 Core

Meyer & Lauroesch (1999) – WIYN DensePak Na I spectroscopy of central 27” x 43” of the globular cluster M15 at 4” spatial resolution (3” fiber diameter) and 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 scaleM92 #2: 3200 AU scaleM15 LISM: 2000 AU scaleM15 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 219188: case of new Na I component strengthening on scale of ~20 AU/year (Welty & Fitzpatrick 2001)

Lauroesch et al. (2005) – 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 35149 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 35148 has been constant in 2004/5

The projected transverse motion of HD 35149 over 1996-2004 corresponds to 7 AU the N(Na I) doubling during the past year occurred 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 30 stars for the past decade HD 35149 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 32040 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 Stanimirovic et al. (2003, 2004) have recently completed a new multi-epoch study of the H I 21 cm absorption toward several high-velocity pulsars and find only a few H I variations on scales < 100 AU – they conclude that such structure is not ubiquitous

Na I Clouds Toward M81

Maund et al. (2004)

Bowen et al. (1994) – find Na I absorption from 3 velocity subcomplexes toward SN1993J in M81 the v +120 km/s gas cannot be ascribed to the M81 disk or to the Milky Way

Mapping the Na I Absorption Toward the Core of M81

Meyer et al. (2005) – WIYN DensePak Na I spectra of central 27 x 43 (450 x 700 pc) of M81 at 4 spatial and 14 km/s spectral resolution detect high-velocity clouds at +156, +178, and +217 km/s with no corresponding H I emission

The strong +217 km/s Na I feature is “beam diluted” in strength as one moves from the central C fiber to spectral sums involving the surrounding fiber rings thus, it is not surprising that H I 21 cm surveys could miss such small-angular-scale features

The 7 arcsec2 beam of the central fiber projects to an area of 2000 pc2 at the distance of M81 in order for the +217 km/s Na I absorption to be so strong and saturated in this fiber, it must either cover this large area with a significant N(Na I) or represent an even denser, smaller Na I cloud

Possible Explanations:

1) An M81 Compact High-Velocity Cloud (CHVC) similar in size to Galactic H I CHVCs

2) Tidally-stripped gas in the M81 Galaxy Group perhaps linked to +120 km/s Na I absorption seen toward SN1993J (2.6 from M81 core)