The Local Interstellar Medium
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[edit] The Local Interstellar Medium
THE IONIZATION OF THE ‘LISM’ AS REVEALED BY FUSE OBSERVATIONS OF N,O, AND Ar TOWARDS WHITE DWARF STARS .
The Astrophysical Journal, 538:L81–L85, 2000 July 20
E. B. Jenkins, W. R. Oegerle, C. Gry, J. Vallerga, K. R. Sembach, R. L. Shelton, R. K. C. Roth, A. K. Dupree, and J. Edelstein
[edit] Important Points:
1.INTERSTELLAR MEDIUM
2.INTERSTELLAR CLOUD
3.INTERSTELLAR DUST
4.IONIZATION OF ISM
5.UV ABSORPTION SPECTRA
[edit] The Interstellar Medium(ISM)
Matter that exists between the stars within a galaxy. Consists of extremely dilute mixture of ions, rays and magnetic field. Contains 99% gas and 1% dust by mass. 15% of the visible matter in our Galaxy is composed of interstellar gas and dust. Density - few thousand to a few hundred particles per cubic centimeter.
Besides abundant hydrogen and helium, 114 interstellar gaseous molecules are currently identified in the interstellar medium
[edit] INTERSTELLAR DUST
The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice .Each dust flake is roughly the size of the wavelength of blue light or smaller. Image:Dust.jpg
[edit] Interstellar Cloud
Atoms, molecules, and cosmic dust particl es clumped together more densely to form Interstellar cloud.They are divided into 2 categories. One is Molecular vcloud and the other H I region. Molecular clouds contain high fraction of molecules� And they are of 100 different types, Where as, H I region is composed of hydrogen atoms as detected by 21cm emission line.They can be cold or hot clouds.
[edit] MOTIVATION
To have a clear understanding of the physical processes that are responsible for high fractional ionization of He in the LISM.
To Understand the origin of ionization of the LISM.
[edit] FINDINGS
1) Ar I / H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. 2) The result favors the interpretation that the ionization of the local medium is maintained by a strong EUV flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition (flash from near by supernova or its shock waves).
[edit] RESULTS BASED ON
Far Ultraviolet Spectroscopic Explorer spectra of the white dwarf stars (FUSE; Moos et al. 2000; Sahnow et al. 2000) a)) G191-B2B b)) GD 394 c)) WD 22112495 d)) WD 23312475 at a resolution of 20–25 km s-1.All of the stars are at distances that range between approximately 50 and 80 pc.
[edit] SUPPORTING DATA:
A )-- (Dupuis et al. 1995)
< n (He I) / n (H I) > = 0.07 ; with the assumption ,He/H = 0.10 in all forms. => He is usually slightly more ionized than H.
B)-- (Wolff, Koester, & Lallement 1999)
Helium shows less variability in how strongly it is ionized from one region to the next.
C)-- (Vallerga 1998)
The EUV radiation from white dwarfs and other stars can explain the observed fractional ionization of H but these sources do not produce enough photons with energies above 24.6 eV to explain the ionization of He.
[edit] PROPOSALS TO RESOLVE
A) (Breitschwerdt & Schmutzler 1994). Ionization of He is maintained by recombination radiation from highly ionized but cooled gases that might surround us.
B) (Reynolds 1986; Frisch & Slavin 1996) LISM is not a steady state condition; it is returning from a much more highly ionized state produced by an energetic event in the recent pastt (t ≤106 yr).(eg the flash from nearby supernova or its shock wave.)
C) (Slavin 1989; Slavin & Frisch 1998)
The ionization of He is maintained in a steady state by the diffuse EUV radiation arising from conductive interfaces between the cloud edges and the surrounding hot medium at
T ~ 106K.
d) (Sofia & Jenkins (1998) The abundances of N I, O I, and Ar I ; that are generally very lightly depleted in the ISM (if at all)—can help to unravel the mystery of the He ionization.
The photoionization of H and He in the (LIC) is thought to be strongly dominated by : 1) Radiation from £ CMa (Vallerga & Welsh 1995) 2) Nearby white dwarf stars (Dupuis et al.1995) 3) Possibly line emission from hot (T ~106 K) gas that surrounds the LIC (Cheng & Bruhweiler 1990) 4) Radiation from the conductive interface at the boundary between the LIC and this hot gas (Slavin 1989).
[edit] Why Ar ?
1)Noble gases can adhere to grains only through the very weak Vander waals force (Watson 1976). 2)Neutral argon has two strong resonance lines 1048.220 A0 and 1066.66 A0 in the FUV range. 3)The interstellar Ar features are easily recognizable in most spectra, their strength turns out to be a liability as the lines become strongly saturated when dense clouds are being investigated. 4)Neutral Ar has an ionization potential greater than that of H, but its photoionization cross section is extraordinarily high. 5)Ar is a good tracer element to indicate how strongly the apparent abundances of other elements could be influenced by ionization, and this information is crucial for research on the chemical evolution of the gases in different contexts.
[edit] OBSERVATIONS
Star l(deg ) b( deg ) log N(H)of LIC λ coverage(Å)
G191-B2B 156.0 +7.1 18.25 987-1082 GD 394 91.4 1.1 17.68 987-1082 WD 22112495 345.8 -52.6 16.73 905-1082 WD 23312475 334.9 -64.8 17.12 905-1082
N(H) is from the LIC, surrounding our solar system, (Redfield & Linsky (2000))
Far-ultraviolet spectra of WD 22112495 recorded by FUSE.
Species G191-B2B GD 394 WD 22112-495 WD 23312-475
H I 18.36 18.65 18.76 18.93 N I 13.90(-0.26) 13.85 ± 0.15(-0.6) 14.02 ± 0.15(-0.54) 14.61 ±0.15(-0.12) O I 14.84 (-0.19) 14.94 ± 0.2(-0.38) 15.32(-0.11) 15.45 ± 0.1(-0.15) Ar I <12.44(<-0.42) 12.70 ± 0.1(-0.45) 12.8 ± 0.1(-0.43) 13.06 ± 0.1(-0.37)
The values in the paranthesis is the logarithimic amount the abundances relative to HI deviate from their respective values from B stars.
For Ar, the result may be interpreted as :
1)The deficiency of Ar I for all four stars is of order -0.4 dex.This is not because Ar, depleting onto the surfaces of dust grains.But the photoionization cross section of Ar is 10 times more than that of H over a broad range of energies.
The Argon deficiency is not just because they are depleting on the dust grains, rather being inert in nature they form only Vanderwaals weak bonding with dust grains.It has been calculated by Sofia and Jenkins (1998) that the mean time interval for the Dust grains to escape is much smaller that the e-folding time for depletion.
For Nitrogen, in G191-B2B and WD 23312-475 stars , the result may be interpreted as,
1)N I photoionization cross-section > H I. 2)Ionization fraction of N coupled to that of H via Resonant Charge Exchange Reaction. The charge Exchange Rate = 2 × Recombination Rate. 3)For ne > > nH , N behaves like Ar.
f 0=n (neutral)/n (total) of N, O, and Ar, compared to those of H
Shielding depths [scaled to N(H I)] within a cloud at a uniform pressure P/k = 1.5×103 cm-3 K and temperature T = 7000 K.
The graph shows:Ionization rate is stronger at the cloud edge.
For Nitrogen , in GD 394 and WD 22112-495;
1)Conduction front is not the cause for ionization.
2)40-80 eV from recombination radiation to He II may be the cause.
3)N I is more easily ionized than Ar I if recombination is important than charge exchange over this energy range.
Over this energy range N I is more easily ionized than Ar. (calculated)
Ar abundance is -0.4 dex for all stars.If this 40-80 ev is the cause of ionization and looking at the Ar deficiency, this may be the cause of steady state ionization of He.
[edit] Future Directions:
1)The calculation that can confirm this conjecture.
2)Verification of this result by taking some other sources rather than white dwarfs.
3)Proposals for different other models for the verification of Oxygen deficiency.
Questions…??
Why the ionization rate of Ar is less than that of N, He etc. in the energy range 40 to 80 eV?
If the incoming photon of energy 40-80 eV is ionizing both Ar and N then why there is a discrepancy in the deficiency of N in all four stars under consideration.
Why the deficiency in Oxygen is like this?
So now we must expand these sections to see what to do next. have you looked at this paper
[edit] FUSE SURVEY OF THE 'LIM' WITHIN 200 PARSECS
N. Lehner,E. B. Jenkins,C. Gry, H. W. Moos,P. Chayer,and S. Lacour
The Astrophysical Journal, 595:858–879, 2003
[edit] MOTIVATION
To probe the physical conditions within the LISM,towards the sight lines within 200 pc.
[edit] LINE OF SIGHT
1)FUSE observations of the interstellar gas toward 30 WDs and one subdwarf (SdO) star.
2)Sight lines probe the Local Bubble (LB) and the (LISM) near the LB.
[edit] FINDINGS
1)There is no evidence from calculations that H2 exists well inside the perimeter of the LB.
2)There are different gas phases in the LISM, as the kinematical temp for H2 is less than the usual temp observed in the local interstellar clouds.
3)The relative abundance ratios of SiII,PII, and FeII give insight about the dust content.The variation in these ratio are similar to the depletion patterns observed in warm and halo diffuse clouds in more distant sight lines in the Galaxy.
4)Photoionization is the main ionization mechanism in the LISM and do not support the existence of a highly ionized condition in the past.
[edit] INTRODUCTION
Aspects to study the charactristics of LISM:
1)Ground-based telescopes.
2)The Copernicus, International Ultraviolet Explorer (IUE).
3)Hubble Space Telescope (HST) satellites.
(e.g., McClintock et al. 1975; Bruhweiler & Kondo 1982; Frisch & York 1983; Linsky et al. 1993; Lallement et al. 1995; Redfield et al. 2002).
Why UV studies:
Ground-based observations, via mainly the CaII K and NaI D absorption lines of very accurate absolute wavelength and very high spectral resolution provide precise information on the local gasdynamics and the complex velocity structure of nearby gas. (e.g., Vallerga et al. 1993;Ferlet 1999 and references therein).
UV resonance absorption lines are much stronger than the available optical absorption lines (which are not in the dominant ionization stages) and are therefore more sensitive probes of the warm,low-density LISM.
Physical properties of LIC:
Density (nH~0.1 cm^-3), Temp (T~ 7000 K) and is partially ionized (e.g., Frisch & York 1983; Lallement 1996;Linsky 1996).
Near the LIC, there are similar clouds with different velocities. (Linsky et al. 2000). All these clouds constitute collectively the LISM, and the clouds within roughly 100 pc are in a region called the Local Bubble (LB;the cavity has a radius of about 100 pc in almost every direction,except toward one very low column density direction [ / Beta / , CMa] where it extends to ~200 pc. ( Sfeir et al. 1999).
IONIZATION STRUCTURE OF LISM:
1)(Dupuis et al. 1995). Helium is therefore more ionized than H.As the neutral helium–to–neutral hydrogen ratio is about 0.07, which is somewhat less than the cosmic abundance of He to H of 0.1 .
2)(Wolff, Koester, & Lallement 1999) He shows less variability in its degree of ionization from one region to the next .
PROPOSALS
(Frisch & Slavin 1996; Lyu & Bruhweiler 1996). One possible explanation for the high fractional ionization of He in the LISM is that there has not been enough time for the ionized He to recombine to an equilibrium concentration from a more highly ionized condition in the past, perhaps from the influence of radiation from supernova or its shock wave less than a few times 106 ears ago.
A different alternative is that the LISM is currently exposed to a strong steady flux of photons with E > 24:6 eV (e.g., a conductive interface between the LISM and a surrounding medium at T ~10^6 [Slavin & Frisch 2002], or by recombination radiation from highly ionized but cool gases. [Breitschwerdt & Schmutzler 1994]).
A way to distinguish between these possibilities is to study the lightly depleted elements Ar, N, and O (Sofia & Jenkins 1998). In particular,the ionization of Ar (and to a lesser extent of N) can help us to differentiate between photoionization equilibrium and nonequilibrium cooling models.
Jenkins et al. (2000) A deficiency of ArI toward four white dwarf (WD) stars, thus favoring photoionization equilibrium.
OBSERVATIONS
Following Jenkins et al.,31 WDs within 200pc has been considered.
Why WDs ?.
They are nearer to the solar system.
They have relatively simple stellar continua.
They are often nearly featureless.
Why FUSE Observation ?.
Access to the wavelength interval between 905 and 1187 A ˚
This spectral region containing features of some important neutral species (N I, O I, and ArI), many ionized species (CII, C III, N II, P II, Fe II, and Fe III), and H2.
FUSE has a moderate resolution of R ~ 20000 and hence, only the properties of the dominant cloud or the average properties of the clouds can be derived.
