# Presentation: 498630

##  Physical Conditions of the Nearby Gas

- Edward B. Jenkins
- Princeton University Observatory

## What Kind of Gaseous Material are we Considering?

- Warm clouds (*T*** ~** 7000 K) with *n***H** **~** 0.2 cm**-3** that is partially ionized, with a surprisingly high fractional ionization of He, indicating the presence of a radiation field that has energetic photons
- These clouds are similar to the material that is in the immediate vicinity of the Sun (just outside the heliosphere)

## What Kind of Gaseous Material are we Considering?

- The clouds are confined by the pressure of an external medium, presumably gas with *T* **~** 10**6** K

## Fundamental Issues

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

## Fundamental Issues

**Fundamental Issues**

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Thermal pressures**

**Total pressures**

## Fundamental Issues

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

## Most Clouds are Within ~ 15 pc

- Number of identifiable velocity components as a function of distance

- Redfield & Linsky, 2004, ApJ, 602, 776

- Number of sightlines used in each bin

## Most Clouds are Within ~ 15 pc

- This is supported by the observations of astrospheres around nearby stars, seen as Lα absorption at a displaced velocity.
- They are seen toward 59% of the stars in the sample that have d **<** 10 pc, but a much smaller fraction is seen for stars with d **>** 10 pc.
- This conclusion might be compromised by the fact that astrospheres are harder to detect as the foreground Lα from interstellar H starts to mask the astrospheric component.

- Wood et al. ApJS, 159, 118

## Layout Within the Local Bubble

- Lallement et al. (2003), A&A, 411, 447

## Local Clouds

- Redfield & Linsky, 2008, ApJ, 673, 283

**Measure absorption features toward nearby stars and interpret them as arising from contiguous clumps of gaseous matter, all of which have a kinematic behavior resembling a rigid body**

- The latest and most comprehensive survey, building on earlier work by various investigators, such as Crutcher, Lallement et al. & Frisch et al.

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 2.6 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 1.3 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 2.6 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 3.5 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 3.5 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 3.5 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 5.0 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 5.1 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 5.1 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 6.7 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 8.5 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 11.1 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 11.7 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 14.9 pc**

## Local Clouds

**Velocity comp. ident. with cloud and used to calculate *****v***

**Consistent velocity but ident. with another cloud**

**Entirely wrong velocity**

****

**Upwind direction**

**Downwind direction**

- Redfield & Linsky, 2008, ApJ, 673, 283

**Closest star: 15.5 pc**

## Fundamental Issues

**Locations**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

## Conclusions about the Local Clouds 

- Volume filling factor of the clouds: ~5.5—19%
- What’s between these clouds and how are they confined?
- Probably very hot gas (T~10**6**K)
- For the most part, the velocities seem to share a common direction
- Nevertheless, the relative motions are supersonic and the clouds could be colliding with each other, creating interaction zones that may have their own identity

## Conclusions about the Local Clouds

- Cloud interactions are difficult to identify specifically, but velocity differences in general terms can be calculated:

- Typical sound speed in clouds

- ρv**2**/k ≈ 5000 cm**-3**K

## Fundamental Issues

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

## Temperatures and Turbulent Velocity Dispersions

- Basic Formula – Doppler velocity dispersion adds in quadrature with turbulent velocities:

**This term varies with mass**

**This one doesn’t**

## Temperatures and Turbulent Velocity Dispersions

- Redfield & Linsky (2004), ApJ, 613, 1004

## Temperatures and Turbulent Velocity Dispersions

**Red portions indicate more precise measurements, i.e., those with errors less than the overall dispersion**

## Temperatures and Turbulent Velocity Dispersions

## However, there’s an interesting exception to this picture ....

## A Very Cold Cloud

- Observed in absorption at 21-cm & Na I
- Distance < 45 pc, hence within the LB
- Extraordinarily cold: *T* **=** 20 (+6, -8) K
- Very thin if in normal pressure equilibrium with LB surroundings

- Meyer et al. (2006), ApJ, 650, L67

## Thermal Equilibrium

- (Wolfire, Hollenbach, McKee, Tielens & Bakes 1995: ApJ, 443, 152.)

- Unstable Region

- Representative thermal pressure for gas in the LB

- Heating and cooling times

**2********10****6**** yr**

**2********10****4**** yr**

## Fundamental Issues

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

**Easiest quantity to measure: n****e**

## Fine-structure Level Populations

- Simple equilibrium equation for a 2-level ground state of atom or ion *X* that has an excited level *X**:
**(*****n******e******γ*****12*****e****** + n******H****** γ*****12*****H*****)*****n*****(*****X*****)***** =***** ****(*****A*****21***** + n******e******γ*****21*****e****** + ****** ******n******H******γ*****21*****H*****)*****n*****(*****X******)**
**For either case (collisions with electrons or H)                ****γ****12**** = (*****g*****1****/*****g*****2****)*****γ*****21****exp[ -(****Δ****E/kT)]**
**Usually, *****n******e******γ*****21*****e****** *****and***** n******H******γ*****21*****H******  << A*****21**
**Electrons: **
**γ****21e****= (slowly varying const vs. *****T*****) ******** T****-0.5**** **
**Hydrogen atoms:**
**γ****21H**** is more arbitrary**

**≈ ****1 for T > several ******** 10****3**** K**

## As a rule, the neγ12e  term is  important for  ions,  while the nH γ12H  term is important for neutrals
Available species with fine-structure splittings

- Available species with fine-structure splittings

- Fine-structure Level Populations

**Atoms                                   Ions**

**C I (very weak, but detectable under favorable circumstances)**
**O I (even weaker)**

## C I Fine-structure Excitation

- Jenkins (2002) ApJ, 580, 938

## As a rule, the neγ12e  term is  important for  ions,  while the nH γ12H  term is important for neutrals
Available species with fine-structure splittings

**As a rule, the *****n******e******γ*****12*****e******  *****term***** *****is  important for  ions,  while the***** n******H****** γ*****12*****H  *****term is important for neutrals**
- Available species with fine-structure splittings

- Fine-structure Level Populations

**Atoms                                   Ions**

**C I (very weak, but detectable under favorable circumstances)**
**O I (even weaker)**

**Si II, Fe II (excited levels too weak to see in LISM)**
**C II is good, although absorptions out of the unexcited level are always saturated**

## First Measurement of C II* and C II

**Line of sight to Capella (****α**** ****Aur) over a distance of 13 pc**

- Wood & Linsky (1997), ApJ, 474 L39

## A Way to Overcome the C II Saturation Problem

- Use S II as a surrogate for C II – S II lines are not badly saturated and there are 3 lines of different strength available.
_*Assume*_ that we know the true ratio of S to C in the gas, but be open to the possibility that this is not true.

## Measurements of ne

- Redfield & Falcon (2008), arXiv 0804.1802

*T*

- n**H** **+** n**He**

- Assume the ionization rate to be the same everywhere and calibrate it by solving for the known values of n**e** and n**H** near the Sun.

- Correct?

**LIC**

## Most suitable element to study is Mg
Relevant equation:

- Most suitable element to study is Mg
- Relevant equation:

- Ionization Equilibrium

- (Γ **+** *n****e****C****e*** **+** *n***H+**C**X,H+** **+** *n***He+**C**X,He+**)*n*(Mg I) **=** [(*α****rr**** ****+**** **α****di****)n****e*** **+** α***g****n***H**]*n*(Mg II)

**Photoionization**

- Ionization

- Recombination

**Collisional ionization**

**Charge exchange**

- Dominant process

- Negligible

- Small effect

**Radiative recomb.**

**Dielectronic recomb.**

**Recomb.  dust grains**

- Simple power law with *T*

- Strong onset for *T > *5000 K

- Very minor effect for LISM

- sum

## Use Fine-structure Excitation and Ionization Equilibrium Together

- Three velocity components toward ε CMa

- Gry & Jenkins (2001), A&A,367,617

## Fundamental Issues

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

## After a Revision of the Mg Dielectronic Recombination Rate

- Slavin & Frisch (2006), ApJ, 651, L37

- New results from Altun, et al. (2006) A&A, 447, 1165

## After a Revision of the Mg Dielectronic Recombination Rate

- Slavin & Frisch (2006), ApJ, 651, L37

- New results from Altun, et al. (2006) A&A, 447, 1165

- Assume that C/S is larger than normal

## Correction to Old Ionizing Flux Determination

**Results from the ultraviolet sky-survey telescope (S2/68) in the TD-1 satellite reported by Gondhalekar, Phillips & Wilson (1980):**

**?**

## Diffuse Galactic Light for 1360 < λ < 1730 Å Measured by the SPEAR Mission

- Edelstein et al. (2006), ApJ, 644, L153

## Correction to Old Ionizing Flux Determination

**Results from the ultraviolet sky-survey telescope (S2/68) in the TD-1 satellite reported by Gondhalekar, Phillips & Wilson (1980):**

**6.2**

**18.3**

**39% increase**

**How much here as well?**

## After a Revision of the Mg Dielectronic Recombination Rate

- Slavin & Frisch (2006), ApJ, 651, L37

- New results from Altun, et al. (2006) A&A, 447, 1165

## After a Revision of the Mg Dielectronic Recombination Rate

- Slavin & Frisch (2006), ApJ, 651, L37

- New results from Altun, et al. (2006) A&A, 447, 1165

## A Comparison Between C II Measured Directly and C II Inferred from S II

- Redfield & Falcon (2008), arXiv 0804.1802

- Maybe Slavin & Frisch are right?

- C II

- C II*

## More Results

## Fundamental Issues

**Fundamental Issues**

**Locations**

**Bulk velocities**

**Non-thermal velocity dispersions**

**Temperatures**

**Densities**

**Compositions**

**Ionization fractions**

**Geometrical arrangement in the sky, distances ****→**** distributions in 3D space**

**Collisions between clouds, leading to interaction zones and ****ρ****v****2**** ****dynamical pressures?**

**Turbulence, MHD processes**

**Thermal equilibrium (& its time scale)**

**Apportionment of elements in gas phase vs. dust**

**Density of ionizing radiation, history of the gas (ionization/recombination time scale)**

- Next talk by Slavin will cover this topic

## Ionization Equilibrium or Lack Thereof

- Recombination time scale:

- Myr

- For a representative value of *n****e*** = 0.13 cm**-3** found by Redfield & Falcon (2008)

## Argument Against the Proposal that the Local Material is Out of Equilibrium and Still Recombining

- Argon is a useful substance for testing this:
- Should not deplete onto dust grains
- The recombination rate for ionized Ar is about the same as that of H
- The photoionization cross section of Ar is substantially higher than that of H (by about a factor of 10)
- If Ar/H is at about the cosmic abundance ratio, then a time-dependent recombination picture is appropriate.

## Argument Against the Proposal that the Local Material is Out of Equilibrium and Still Recombining

- Results from a survey using FUSE to measure Ar and O (O is a good surrogate for H):

- Lehner et al. (2003), ApJ, 595, 858