a unified representation of gas- phase element depletions in the interstellar medium a consolidation...

A Unified Representation of Gas-Phase Element Depletions

in the Interstellar Medium

A Unified Representation of Gas-Phase Element Depletions

in the Interstellar Medium

A Consolidation of Findings from 30 Years of Investigation of Ultraviolet Absorption Lines

A Consolidation of Findings from 30 Years of Investigation of Ultraviolet Absorption Lines

Edward B. Jenkins

Princeton University Observatory

Edward B. Jenkins

Princeton University Observatory

Interstellar Dust

Mellinger 2009 arXiv 0908.4360

Interstellar Dust

NGC 4103

Average Density of Dispersed Solids

2aN g Qe

Number of grains per unit area in a column of length d

Radius of each grain

Optical depth Extinction

efficiency factor

0 2

22

2

14

m

madQe

1

2

4 2

2

330

m

m

a

dN g

Refractive index at low freq.

sg

g

a

d

N

3

4 3

sg m

m

d

d

1

2

3

12

20

2

An application of the Kramers-Kronig dispersion relation by Purcell (1969, ApJ, 158, 433)

Average Density of Dispersed Solids

sg m

m

d

d

1

2

3

12

20

2

Combine AV = 1.8 mag kpc-1

with relative extinctions from 1000 Å to 20 μm

≈ 2 for most substances ≈ 3

H

326 006.0cm g108.1 g

Compare Compare with Z/X in with Z/X in

the Sunthe Sun

0.0180.018

Absorption Lines vs. Energy

May 1973May 1973

The Dark AgesDate: Fri, 13 Aug 2004 16:22:40 -0400 (EDT)From: [email protected]: [email protected]: STIS Update

Dear HST User,

On Tuesday, August 3, STIS entered a "suspend" state in response to the loss of 5-volt power in the Side 2 electronics. The Side 1 electronics suffered a short circuit in May 2001 and are currently not working. Failures in the two redundant Sides make STIS unusable. While it is possible that further investigation will point out a way to restore STIS to a useful state, we believe it unlikely that STIS can be revived without physical servicing. Fortunately, all other science instruments and the observatory itself continue to function normally.

The observing programs that use the unique capabilities of STIS will be suspended. We expect to notify all STIS observers about the status of their programs within the next several weeks.

Date: Fri, 13 Aug 2004

The RenaissanceThe RenaissanceThe RenaissanceThe Renaissance

Fundamental Goals of Abundance Studies


1. Galactic Disk: Assume a total abundance, based on stellar abundances, and then determine what proportions of the atoms are in the gas and solid (dust) phases.

We characterize the loss of some element X from the gas phase by the depletion [X/H], defined by the relationship

stellarobs H

X

H

XHX

loglog]/[

stellarobs H

X

H

XHX

loglog]/[

Reference AbundanceSometimes called D(X)



1. Galactic Disk: Assume a total abundance, based on stellar abundances, and then determine what proportions of the atoms are in the gas and solid (dust) phases.

2. Systems outside the disk of the Galaxy: Measure the relative abundances of different elements to understand better a system’s element production history.

3. Often, for either of the two options, we must recognize the possible influence of one on the other, i.e., extragalactic systems have dust and fundamental abundances (gas + dust) may vary in the Galaxy

What has been Known for What has been Known for some Timesome Time

• Depletions vary Depletions vary from one location from one location to the nextto the next– Sightlines with low Sightlines with low

average density average density N(H)/d have less N(H)/d have less depletiondepletion

– Gas at high Gas at high velocity velocity displacements have displacements have less depletionless depletion

• Depletions vary Depletions vary from one element from one element to the nextto the next– Depletion Depletion

strengths are strengths are greater for greater for elements that can elements that can form refractory form refractory compounds and compounds and are small for those are small for those that can only form that can only form volatile compoundsvolatile compounds

Classical Assumptions and some of their weaknesses

Classical Assumptions and some of their weaknesses

• Reference standard for total element abundance ratios:– Solar &

Meteoritic?– B stars?

Why is it that the heavy element abundances in the Sun (4.5 Gyr old) are generally higher than those in young B stars?

( needed to infer dust composition once the abundances of free atoms are determined)

0

100

200

300

400

500

600

O C N Mg Si Fe

B stars

Sun

Compilation by Asplund, Grevesse, Sauval & Scott, 2009, ARAA, 47, 481

Abu

nd R

el.

to H

× 1

06 Kilian-Montenbruck, Gehren & Nissen “The galactic distribution of chemical elements as derived from B-stars in open clusters …” 1994: Astr. Ap., 291, 757:

A Study of ISM Gas-Phase Abundance Results in the

Literature Objective: Organize the information so that we can learn more aboutGrain compositionHow to correct for dust depletions in distant

absorption line systemsSurvey scope:

17 different elements243 sight linesAll results have been renormalized to a single

compilation of f-values (Morton 2003).This represents a massive bookkeeping and

quality control effort.


Literature• Conventional Approach in Past

Investigations:– Measure the depletion of a specific

element and then characterize it in terms of some property of the sight line

• Average density: N(H)/d• Fraction of hydrogen in molecular form


Literature• New Tactic:

– Ignore the sightline properties and characterize depletions of elements with respect to each other, recognizing that the severity of depletions differ from one element to the next and from one region of space to the next.

Characteristics of the Survey

Characteristics of the Survey

Used in the analysis

Not used in the analysis, but shows up in various plots

The Effects of Ionization

From Meiring et al. (2009) arXiv:0905.4473

Underlying Strategy

• Basic premise: All elements deplete

together in some systematic fashion, but by differing degrees that change from one region to another and from one element to the next.

Propose a single

parameter, F*, that expresses a general level of depletion along a sight line.

F*

F*(F* is much

like <n(H)> that has been used in the past.)

Underlying Strategy

• Scale for F* is arbitrary;

I chose a calibration as follows:F* = 0 corresponds to

lowest depletions seen (but subject to the restriction that N(H I) > 1019.5 cm-2).

F* = 1 corresponds to the

depletion pattern observed toward ζ Oph

F*

F*

Underlying Strategy

• Another basic premise:Differences in how

the elements respond to changes in F* are

represented by other parameters specific to each element.

F*

F*

Basic Equation

)(]H/[ * XXXgas zFABX For element X:

zx

BX

Slope: AX

Depletion factor

The Buildup of Dust GrainsConventional

Formula:)101((X/H)H)/( ]H/[ gasX

dustX

Differential Element Contributions:

*)H/()10(ln

H)/(

*FgasX

dust XAdF

Xd

What about …

… an “undepleted element?”

Well, no, not exactly

But most people refer to Zn as a “relatively undepleted element” (i.e., compared to Fe or Cr), so perhaps we have not really been misled much.

Behavior for N(H I) < 1019.5 cm-2

Indicates that N(H I) < 1019.5 cm-2

Indicates that N(H I) > 1019.5 cm-2

Sightlines

Trends of Overall Depletion Strengths

Elements

Depletion Trends against Condensation Temperatures

[Xga

s/H] 0

Depletion Trends against Condensation Temperatures

Distant Absorption Line Systems

Basic Questions:1. What is the pattern of

intrinsic element abundances?

2. How does depletion onto dust grains affect this pattern?

3. The above two considerations lead to the measured column densities.

Background Quasar

Galaxy

Missing Information

1. What happens when I can’t measure N(H I) from the Lα feature of the system?– i.e., suppose I’m stuck with just a ground-based

telescope and the system has z < 1.5. • How do I correct for dust depletion?

• Can I estimate N(H I) modulo the metallicity?

2. OR … Suppose I can observe Lα, but I wish to determine both the effects of dust depletion and the overall metallicity of the system?

Recipe: Start with Depletion Data

Elem. X

Log (X/H)

+12 AX BX zX

)(]H/[ * XXXgas zFABX Recall the basic equation:

Rearrange Terms in the Original Formulae

Recall that for any element X, its depletion is given by

[Xgas/H]=Log N(X)– Log N(H)

–Log (X/H)

=BX + AXF*– AXzXLog N(X) –Log (X/H)

BX

–

AXzX

+

=Log N(H)

+ AXF* + AX(F*–zX)

+ F*

AXy a b x= +Measured quantity

Other stuff: see tabulated values

See tabulated values

Then derive the coefficients of a least squares best-fit equation

But this correct only if the gas system has an overall metallicity that is solar, i.e., [M/H] = 0. If it’s subsolar, then the true N(H I) is higher.

But this correct only if the gas system has an overall metallicity that is solar, i.e., [M/H] = 0. If it’s subsolar, then the true N(H I) is higher.

Alternate Form [to use if you know N(H I) independently]Recall that for any element X, its depletion is given by

[Xgas/H]=Log N(X)– Log N(H)

–Log (X/H)

=BX + AXF*– AXzXLog N(X) –Log (X/H)

BX

– +

=Log N(H)

+ AXF* + AX(F*–zX)

y a b x= +Measured quantity

Other stuff: see tabulated values

See tabulated values

Then derive the coefficients of a least squares best-fit equation

[Xgas/H] BX AXzX [M/H]

= Log N(H)pred. from metals – LogN(H)obs.

+ F*

AX

x ( = AX)

y =

Log N

(X)

+ s

tuff

-2 -1 019

20

21

Ti

Cr

Mg

Zn

Slope: F * =

0.69

y intercept: Log N(H) = 21.1

Now suppose that you can measure the Lα feature, and you find that Log N(H I) = 21.4. What is the sytem’s metallicity?

Now suppose that you can measure the Lα feature, and you find that Log N(H I) = 21.4. What is the sytem’s metallicity?

This is an oversimplification. In reality, one must consider errors in both x and y

This is an oversimplification. In reality, one must consider errors in both x and y

Also, an important requirement is that the values of AX must span a large enough range to define a good fit line.

Also, an important requirement is that the values of AX must span a large enough range to define a good fit line.

How Well Does this Work?Compare the values of Log

N(H I)Compare the values of F*

Log N(H I) > 19.5Log N(H I) < 19.5

Log N(H I) from Lα

Der

ived

Syn

thet

ic L

og N

(H I

)

Original Value of F*

F*

Fro

m B

est-

fit S

lope

Warnings, Disclaimers, Caveats, and all that …

• Suppose we start with an assumption that the general pattern of element abundances does not differ appreciably from that of our Galaxy, aside from a difference in the ratios relative to hydrogen.

• Suppose we start with an assumption that the general pattern of element abundances does not differ appreciably from that of our Galaxy, aside from a difference in the ratios relative to hydrogen.

• After the best-fit line has been plotted, we see if there are any deviations that are significant (and make sense from a chemical evolution standpoint).

• If there are, then we need to pause and think a little bit.

• After the best-fit line has been plotted, we see if there are any deviations that are significant (and make sense from a chemical evolution standpoint).

• If there are, then we need to pause and think a little bit.


• Here are some ideas:– From the earlier

discussion of grain growth, one may think of AX representing element X’s proclivity to attach to dust grains and form compounds

– Thus, AX is sort of a rate constant.

• Here are some ideas:– From the earlier

discussion of grain growth, one may think of AX representing element X’s proclivity to attach to dust grains and form compounds

– Thus, AX is sort of a rate constant.

– Starting with some different initial (intrinsic) element abundance pattern, one might be able to solve an integral equation that tracks the depletions of elements as the grains grow.

– Starting with some different initial (intrinsic) element abundance pattern, one might be able to solve an integral equation that tracks the depletions of elements as the grains grow.


Retention of atoms on grains after initial sticking may depend on the underlying composition of the seed material in the grain

Retention of atoms on grains after initial sticking may depend on the underlying composition of the seed material in the grain

Most elements probably depend on the presence of others to form chemically stable compounds that are durable enough to remain in solid form for long periods of time.

Most elements probably depend on the presence of others to form chemically stable compounds that are durable enough to remain in solid form for long periods of time.


Example:1. Most favorable

means of depleting Ni & Ge uses Fe as a host element to form an alloy.

Example:1. Most favorable

means of depleting Ni & Ge uses Fe as a host element to form an alloy.

Another Example:2. Refractory

silicate compounds that sequester Zn and Mn need Mg, Si & O to form the original host minerals.

Another Example:2. Refractory

silicate compounds that sequester Zn and Mn need Mg, Si & O to form the original host minerals.

The mix of primitive grains ejected from different kinds of sources (evolved stars, Type II and Ia supernovae, etc.) may differ from one galaxy to the next, depending on the IMF and star formation history. This could alter the pattern of compositions of the grain cores..

The mix of primitive grains ejected from different kinds of sources (evolved stars, Type II and Ia supernovae, etc.) may differ from one galaxy to the next, depending on the IMF and star formation history. This could alter the pattern of compositions of the grain cores..


Calibration to Categories Defined by Sembach & Savage (1996)

Halo:F* = –0.28

Disk + Halo:F* = –0.08

Warm Disk:F* = 0.12

Cool Disk:F* = 0.90

N(H I) not measured because Lα could not be observed from the ground. But we do know this

Application Examples

x

y

Circumburst medium for GRB 020813 at z = 1.255 (Savaglio & Fall 2004)

Ti

Fe

Mn

ZnNi

Cr

Si

Mg

Disk + Halo:

F* = –0.08

This one of two strong components, separated by Δv = 12 km s-1; N(H I) not known for individual cases, so metallicity can not be determined


x

y

z = 2.08679 system toward Q1444+014 (Ledoux, Petitjean & Srianand 2003)

Ti

Fe

ZnNi

Cr

Si

Mg

NP

Halo:

F* = –0.28

But once again,


x

y


Fe

Zn

P

Si

N

Ignore

“This is r

eminiscent o

f warm

and cold Galactic

disc cloud dust-

depletion patte

rns …” (

for this

one particular v

elocity component)

“This is r

eminiscent o

f warm

and cold Galactic

disc cloud dust-

depletion patte

rns …” (

for this

one particular v

elocity component)

This is the other velocity component

Warm Disk:

F* = 0.12

Cool Disk:

F* = 0.90


x

y


Fe

Zn

P

Si

N

Ignore

Cool Disk:

F* = 0.90

This is the other velocity component


x

y

Sub-damped Lα systems at z < 1.5 (Meiring et al. 2009)

Ti Cr

Fe

Si

Mn

Zn

Halo:

F* = –

0.28

N(H I) was measured, so metallicity can be determined


x

y

SDSS J0256+0110 (Peroux et al 2006)

Ti

Cr

FeMn

Zn

Disk + Halo:

F* = –0.08


x

y

z = 2.626 system toward FJ081240.6+320808 (Prochaska, Howk & Wolfe 2003)

Ti Ni CrFe

SiMg Cu

Ge

Zn

O

Kr

Mn

P

ClN

Warm Disk:

F* = 0.12But this is only from Cl I, not Cl II !

a unified representation of gas- phase element depletions in the interstellar medium a consolidation...

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total abundance

gas phase

heavy element abundances

total element abundance

stellar abundances

stis unusable

stis observers

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