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Small-Scale Structure in the Diffuse Interstellar Medium. Dave Meyer Northwestern University. Galactic H I 21 cm emission. Leiden/Dwingeloo survey data from Hartmann and Burton (1997) as published in Pasachoff and Filippenko (2004). Fig. 15-28, p.308.

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Small-Scale Structure in the Diffuse

Interstellar Medium

Dave Meyer

Northwestern University

Galactic H I 21 cm emission

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

Fig. 15-28, p.308

High Velocity Cloud WW187

(Wakker et al. 2002)

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

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 I

Toward Resolvable Binary Stars

Meyer & Blades (1996) – 0.4 km/sec resolution

AAT UHRF spectroscopy of Na I absorption

toward the binary m 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: spectroscopy of 17 binaries

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) = (a/G) 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 spectroscopy of 17 binaries

Lauroesch & Meyer (1999)

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

Pan et al. (2001)

<- spectroscopy of 17 binariesm Cru

Lauroesch et al. (1998)


6600 AU separation

HD 32039/40 ->

Lauroesch et al. (2000)


4800 AU separation


 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 spectroscopy of 17 binariesc 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 c Per double star cluster

The Na I profiles toward the 150 h and spectroscopy of 17 binariesc 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 c 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 and

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 and

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) and

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 and

  • HD 28497: ~10 AU (Blades et al. 1997)

  • Ori: ~10 AU (Price et al. 2000)

    k 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 and

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, and

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

Five of the ten cases found to and

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)

Crawford (2003)

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 and



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 and

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” and

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)