Limits to Galactic Abundances based on Gas-Phase
Measurements in the Interstellar Medium
U. J. Sofia
Department of Astronomy, Whitman College, Walla Walla, WA 99362 USA
Abstract.
Gas-phase (including ions and molecules) interstellar measurements of some elements can place tight constraints on their Galactic, or at least their total (gas + dust) local interstellar abundances. The interstellar medium
within ISOOpc of the Sun seems to be well represented by a single standard reference abundance. Here we discuss the limits placed on that standard abundance by measurements of the elements O, C, N, Kr and Sn in the
interstellar medium. These are the elements for which we have some idea of their dust-phase compositions. The
abundance of Kr in the local (out to 1500 pc) interstellar medium is subsolar at about 60% of the meteoritic
value as compared to H. Oxygen in the same regions appears to have solar abundances. Tin, a primarily s-process
element, is supersolar in the local ISM, however we cannot determine to what extent. Nitrogen and carbon abundances are more difficult to ascertain, but they, like oxygen, appear to be solar within measurement errors.
1. INTRODUCTION
can do this because if we a priori understand the role of
an element in dust, and can measure its abundance in the
gas, then we will have a good determination of the total
ISM abundance.
In §2 we will discuss the elements for which we believe we can place some limit on in the local ISM composition (out to about 1500 pc from the Sun). We will
discuss how the limits compare to the Sun in §3. Our
conclusions are summarized in §4.
It is difficult to measure the total abundance of elements in the ISM because it is composed of both gas
and solid-grain material. We can, however, measure the
gas-phase abundances of many cosmically abundant elements in neutral (hydrogen) interstellar clouds if we have
access to the UV where most resonance lines of dominant species occur. Since the advent of spectroscopy
with the Hubble Space Telescope, particularly the GHRS
and STIS instruments, the measurement of interstellar
absorption features has become quite precise. This together with major improvements in the determination of
transition oscillator stregths, e.g. Bergeson and Lawler
[1], Cardelli and Savage [2], Fitzpatrick [3], Theodosiou
and Federman [4], has allowed for interstellar gas-phase
abundances to be determined often to an accuracy of 0.05
dex (about 10%) or less. These abundance determinations are very straight forward; they are not model dependent in any way so they are extremely reliable. This
leaves the dust-phase abundance of the elements as the
major uncertainty in the total (gas + dust) ISM composition.
The subject of neutral cloud abundances is usually
addressed by assuming a Galactic abundance and measuring a gas-phase abundance in order to infer the dust
abundance of an element. Some authors, however, have
instead tried to determine the Galactic composition by
combining gas-phase measurements with model constraints on dust abundances [5, 6, 7, 8, 9, 10, 11]. One
2. ELEMENTS
There is a very limited number of elements for which we
have reliable information concerning dust-phase abundances. For these elements, a reliable local total-ISM
(gas + dust) abundance can be well determined. Information (i.e. limits) for the total abundances of some other
elements can be inferred from those abundances, or from
their gas-phase abundances alone. The number of elements for which the total abundances or limits can be
determined is quite low, but some very important species
are among them.
Oxygen
Very good measurements of the gas-phase interstellar oxygen abundance exist for interstellar clouds. Meyer
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
221
500
400
300
200
ffi
1.1
o
0.9
0.7
^ 95
> 75
55
280
200
120
40
-6
-5
-4
-3
Log f(Ha)
-2
-1
FIGURE 1. The interstellar gas-phase abundances of oxygen [10], krypton [12], nitrogen [13] and carbon [8, 14] with respect to
hydrogen. The abundances are shown as number of atoms per 106 H nuclei except for Kr which is atoms per 109 H nuclei. The data
with their 1 a error bars are plotted as a function of the logarithmic fractional H2 abundance. The dotted lines show the weighted
average value for the measurements. Each of these elements apparently has a single abundance in the ISM out to about 1500 pc (the
range over which the observations were made).
et al. [10] report the O/H number ratios for 13 diffuse cloud, Ay < 1, sightlines within 1500 pc of the
Sun. They find that there are 319± 14 oxygen atoms
in the gas per million H nuclei along these sightlines.
The uniformity of the abundance with respect to hydrogen is amazing considering that the clouds have very
different fractions of their H nuclei in the form of H2,
/(H2)=2W(H2)/[Ar(HI) + 2JV(H2)]; from about 60% toward £ Oph to less than 1 part in 105 toward 8 Ori [15]
(see Figure 1). The large extremes in f(H2) likely represent very different physical conditions in the clouds
since this fraction indicates the equilibrium between the
molecule's formation on grain surfaces and its chemical
and/or photodestruction. The uniformity of the O/H values in diffuse clouds suggests that there is little exchange
of O between the gas and dust, and that the local totalISM O/H (gas + dust) abundance is probably constant.
We should note that Cartledge et al. [16] have recently
found that the constancy of the interstellar gas-phase O/H
does not continue to denser regions such as translucent
222
clouds. In these clouds the abundance of oxygen sometimes, but not always, shows an enhanced level of depletion.
O is likely the most abundant element in dust since it is
so chemically reactive and abundant. The incorporation
of O into dust, however, is limited in diffuse clouds
since the grains do not have ice mantles, e.g. Savage
et al. [17]. Therefore, O incorporation into grains in these
sightlines is limited by the mineralogy of dust. Cardelli
et al. [8] estimate that no more than 180 O atoms can
be incorporated into dust per 106 H nuclei in a diffuse
cloud sightline. If we assume that the maximum number
of O atoms are in dust, then we conclude that the local
total ISM abundance of oxygen is 499 ± 14 O atoms per
million H nuclei. The uncertainty listed for this number
only includes the uncertainty in the diffuse cloud gasphase O/H; it does not include the error in the number of
O atoms incorporated into the dust.
Krypton
Cardelli and Meyer [12] report 11 measurements of
the gas-phase Kr/H in the diffuse neutral ISM and find
that, like O/H, the values are uniform over a large range
of f(H2) out to 1500 pc (see Figure 1). This again suggests a uniformity of the local interstellar abundances,
and that little exchange of Kr occurs between the gas and
dust phases of the ISM. This second point is not surprising because, as a noble gas, Kr is not likely to be incorporated into grains. Therefore, we believe that the dustphase characteristics of Kr are well understood and that
we can say that the Kr/H in the gas is a good representation of the total Kr/H abundance in the ISM. Cardelli and
Meyer [12] find that there are 0.96 ± 0.05 Kr atoms per
109 H nuclei in the local ISM.
Kr is the only noble gas whose total interstellar abundance can be found through UV absorption studies. The
only other candidate that has a measureable line of its
dominant ion in the neutral ISM is argon. Its abundance,
however, is greatly affected by ionization. lonization is
not an issue for the lines of sight with measured interstellar krypton because they sample substantially denser
clouds (and are therefore better shielded from photoionization) than the sightlines with measured interstellar argon [18].
pc. In order to use the gas-phase C abundance to find the
Galactic abundance one needs to know how much of it
is locked into grains. This is a difficult quantity to determine; current models vary greatly in the amount of C that
they incorporate into grains.
Another way to find the Galactic C abundance is to
assume that young stars are good representations of the
interstellar composition. The abundances in these stars,
however, do not agree and have C/O ratios that range
from about 0.5 in B stars [11] to 0.7 in young F and G
stars (and the Sun) [11,21]. We therefore, cannot place a
hard limit on the total Galactic C abundance. A Galactic
C-to-O ratio of 0.5 (the B star ratio) is a likely lower
limit to the possible C abundance. This would imply
a total (gas+dust) ISM abundance of 250 C atoms per
106 H nuclei (based on the O abundance of 499 per 106
H nuclei given above). This is a practical lower limit
because dust models have trouble reproducing extinction
with less than this total C abundance [5, 6, 9]. B stars
are bad representations of the ISM composition so their
C/O is probably not applicable [11]. A local Galactic C
abundance of 350 C atoms per 106 H nuclei (making the
Galactic C/O the same as the Sun's) is probably a more
realistic estimate [11].
Tin
Nitrogen
Like krypton, nitrogen is not expected to be incorporated into dust. It is blocked from grain core formation
because it is locked up as N2 molecules in stellar atmospheres [19], and it should not form grain mantles in its
atomic form [20]. Therefore, we can determine its total
interstellar abundance by making the reasonable assumption that it is the same as the interstellar gas-phase abundance. Meyer et al. [10] measured the N/H abundance in
7 diffuse interstellar clouds and found that they all agreed
within their errors at a value of 75 ± 4 N atoms per 106
H nuclei. This uniformity of the measurements again reinforces the notion that the total interstellar abundances
are similar to a distance of at least 1500 pc from the Sun,
the range over which the data are sampled (see Figure 1).
Tin is an interesting element because it is formed almost entirely by the s-process in moderate-to-low mass
AGB stars. Its total abundance in the ISM with respect
to the Sun may therefore tell us something about stellar
enrichment of the Galaxy in the past 4.5 Gyr. The gasphase interstellar tin-to-hydrogen ration has been measured in 14 diffuse sightlines by Sofia et al. [22]. They
found that a higher fraction of Sn was in the gas phase
in low as compared to high f(H2) sightlines (see Figure
2). This is not a surprising characteristic for an element
that is being exchanged between the gas and dust. In high
f(H2) sightlines the grains are better protected, as are the
H2 molecules, against destruction [23]. The Sn/H ratio
in the gas ranges from about 0.6 - 1.9 Sn atoms per 1010
H nuclei. We do not know enough about tin's incorporaton into dust to find a definitive total Sn/H for the diffuse
ISM.
Carbon
The measured gas-phase C/H ratios in diffuse interstellar clouds with very different f(H2) values are constant at 140 ± 20 C atoms per 106 H nuclei (see Figure 1). This suggests little exchange between the gas and
dust phases of C in the ISM, and once again supports the
idea of a uniform ISM abundance out to at least 1500
223
3. COMPARISON OF THE ISM
ABUNBDANCES TO THE SUN
The argument has been made for years that the Sun has
unusual abundances. The interstellar community was a
great advocate for this because problems occurred when
the dust-phase composition was inferred from the solar
-9,6
-9,8
-10,0
Log f(Hg)
FIGURE 2. The logarithmic interstellar gas-phase abundance of tin with respect to hydrogen as a function of logarithmic
fractional H2 abundance. Unlike the elements in Figure 1, Sn shows enhanced depletions in denser diffuse-cloud regions. The
lowest f(H2) sightlines have an average abundance that is slightly supersolar. The solar abundance is shown by the dotted line.
standard [5], A particular problem was the large dustphase oxygen abundance. There was simply no place to
chemically attach the number of oxygen atoms inferred
to be in the dust. This led to the suggestion that the
Sun is not a reasonable proxy to interstellar abundances
and drove the search for a better representation of the
interstellar standard. The reported solar abundance of
oxygen, however, has been slowly dropping over the past
several years [24, 25, 26, 27], A recent photospheric
measurement reported in these proceedings now places
the oxygen abundance at log(O/H) = -3.264 ±0.078
[21]f significantly down from the Anders and Grevesse
[24] value of -3.07 from 13 years earlier. This change
brings the O/H number ratio in the Sun down to 545 ± 99
from 851 oxygen atoms per million H nuclei. Within the
reported errors, this solar abundance now agrees with the
interstellar abundance proposed by Cardelli et al, [8] and
Meyeretal. [10].
The Kr/H value found for the ISM is approximately
60% below the meteoritic value [24]. Since the abundance of noble gases are so difficult to determine, the
interstellar value may be a more reasonable approximation for the solar abundance.
The nitrogen abundance given above for the ISM is
slightly lower than the solar photospheric value reported
by Holweger [21]; 85 ± 22 N atoms per 106 H nuclei.
This could mean that some N is incorporated into dust.
The large uncertainty in the solar value, however, does
not preclude it from agreeing with the total ISM abundance.
The C/H abundance is likely close to the solar value.
The Sun and young F and G disk stars, which seem to
well represent the local interstellar standard [11], have
224
the same ratios of C/O. Since the total-interstellar O
abundance now appears to agree with the solar value, this
would imply that the total C/H ratio in the local ISM is
also approximately solar.
Ususally an element such as tin with variable depletion
levels tells us little about the total ISM abundance. Tin is
different, though, because in the lowest f(H2) sightlines,
Le, the sightlines with the most Sn in the gas, the measured gas-phase Sn/H is supersolar (by about 0.07 ± 0.04
dex). A characteristic of elements that show variable depletions is that even in the least depleted sightlines some
of faQ eiement remains in dust. If this is the case for Sn,
then we cannot say exactly how overabundant it is (because we do not know how much is in the dust in those
regions). Sn is in the same column of the periodic table as
Si, so they are chemically similar. We might expect, then,
that they will be similarly incorporated into dust. In the
lowest f(H2) sightlines, Si is depleted by about 0.2 dex
with respect to the Sun. This provides a possible limit of
about 0.3 dex for the ISM overabundance of Sn/H. In any
case, we can be confident that Sn is enriched in the ISM
as compared to the Sun. This is the only element that has
a measured gas-phase abundance that is supersolar.
4. CONCLUSIONS
Out to 1500pc, the interstellar gas-phase abundances of
C/H, O/H and Kr/H are stable in diffuse clouds with
different physical conditions as measured by the fraction
of H in the form of Ha. The weighted average logarithmic
gas-phase element-to-H abundances are —3.85 ± 0.06
[14], -3.50 ±0.02 [10] and -9.02 ±0.02 [12] for C,
O, and Kr. The RMS scatter values for these abundances
are relatively low at 0.07, 0.05, and 0.06, respectively.
This uniformity suggest that there is a standard local ISM
abundance.
Interstellar measurements can be a useful tool for determining that standard abundance for those elements
whose dust incorporation is well understood. From
gas-phase measurements we find that the local totalinterstellar abundances of O, N and probably C agree
well with the solar photospheric measurements of Holweger [21]. The abundance of krypton appears to be low
compared to the solar system value. This may simply be
the result of the difficulty of measuring Kr in meteroites.
Sn is overabundant with respect to the Sun, however its
incorporation into dust grains does not allow us to determine by how much.
ACKNOWLEDGMENTS
This work was supported in part by a grant from Whitman College, and by the NASA grant NAG5-8249.
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