Phosphorus in the Diffuse Interstellar Medium

Abstract

We present FUSE and HST/STIS measurements of the PII column density toward Galactic stars. We analyzed PII through the profile fitting of the unsaturated $\lambda$1125 and $\lambda$1533 lines and derived column densities integrated along the sightlines as well as in individual resolved components. We find that phosphorus is not depleted along those sightlines sampling the diffuse neutral gas. We also investigate the correlation existing between PII and OI column densities and find that there is no differential depletion between these two specie. Furthermore, the ratio N(PII)/N(OI) is consistent with the solar P/O value, implying that PII and OI coexist in the same gaseous phase and are likely to evolve in parallel since the time they are produced in stars. We argue that phosphorus, as traced by PII, is an excellent neutral oxygen tracer in various physical environments, except when ionization corrections are a significant issue. Hence, PII lines (observable with FUSE, HST/STIS, or with VLT/UVES for the QSO sightlines) reveal particularly useful as a proxy for OI lines when these are saturated or blended. Comment: Accepted to A&A

Full text

Page 1

arXiv:astro-ph/0507404v1 18 Jul 2005
Astronomy & Astrophysics manuscript no. lebouteiller˙phosphorus
(DOI: will be inserted by hand later)
February 5, 2008
Phosphorus in the Diffuse Interstellar Medium
V. Lebouteiller1, Kuassivi2, & R. Ferlet1
1: Institut d’Astrophysique de Paris, UMR7095 CNRS, Universit´ e Pierre & Marie
Curie, 98 bis boulevard Arago, 75014 Paris
2: AZimov association, 14 rue Roger Moutte, 83270 St-Cyr sur Mer, France
e-mail: leboutei@iap.fr
Received; accepted 07/18/05
Abstract. We present FUSE and HST/STIS measurements of the Pii column
density toward Galactic stars. We analyzed Pii through the profile fitting of the
unsaturated λ1125 and λ1533 lines and derived column densities integrated along
the sightlines as well as in individual resolved components. We find that phospho-
rus is not depleted along those sightlines sampling the diffuse neutral gas. We also
investigate the correlation existing between Pii and Oi column densities and find
that there is no differential depletion between these two specie. Furthermore, the
ratio N(Pii)/N(Oi) is consistent with the solar P/O value, implying that Pii and
Oi coexist in the same gaseous phase and are likely to evolve in parallel since the
time they are produced in stars. We argue that phosphorus, as traced by Pii, is an
excellent neutral oxygen tracer in various physical environments, except when ion-
ization corrections are a significant issue. Hence, Pii lines (observable with FUSE,
HST/STIS, or with VLT/UVES for the QSO sightlines) reveal particularly useful
as a proxy for Oi lines when these are saturated or blended.
Key words. ISM: abundances, atoms, clouds, Galaxy: abundances, Ultraviolet:
ISM
1. Introduction
Phosphorus (31
31P) is an odd-Z element which is thought to be mainly produced in the same
massive stars that form α-elements (O, Ne, Mg, Si, S, Ar, ...). Although the nucleosyn-
thesis of odd-Z elements is still not well understood, phosphorus seems to be produced
during the carbon and neon burning in a hydrostatic shell (Arnett 1996). Woosley &
Weaver (1995) found that no significant amount is expected to be synthesized during the
Send offprint requests to: V. Lebouteiller

Page 2

2 Lebouteiller, Kuassivi & Ferlet: Phosphorus in the diffuse ISM
explosion phases and that the P yields should be metallicity dependent because of the
odd–even effect.
In the Galactic and extra–galactic diffuse interstellar medium (ISM), the phosphorus
gaseous phase abundance is largely unknown. Apart from the early investigations with
the Copernicus satellite within the solar neighborhood (Jenkins et al. 1986; Dufton et al.
1986), quite a few measurements are available in the distant ISM and almost none in the
extragalactic ISM. Nevertheless, the ISM phosphorus abundance provides an important
constraint for the dust/gas chemistry.
Dust grains are subject to dramatic chemical, morphological, and structural changes
during their life time, from the condensation of dusty cores within the outflows of evolved
stars to the final destruction in shocks or the formation of new generations of stars
and planetary systems. They undergo several growth (mantle accretion and molecules
adsorption) and erosion (photoprocessing, sputtering) periods as they move within dense
or diffuse media (Snow & Meyers 1979; Seab 1987; Turner et al. 1991; O’Donnell &
Mathis 1997). In that respect, refractory elements are expected to be the most depleted
in the gas phase. On the contrary, C, N, and O are less depleted onto dust grains because
of their lower condensation temperatures (Field 1974; Lodders 2003). Hence, one third
of the oxygen is bound to rocky elements at most (Cardelli et al. 1997). With respect to
its relatively high condensation temperature, phosphorus can be expected a priori to be
more heavily depleted.
Jenkins et al. (1986), in their ISM survey conducted with Copernicus toward about 80
stars, found through the analysis of the far-UV absorption lines of the dominant ion stage
Pii that phosphorus is not depleted along sightlines containing predominantly warm low
density neutral gas (?0.1 dex after updating the solar abundances from Asplund et al.
2004) and is depleted by ?0.5 dex in cooler and denser clouds. Using the same dataset
but with a different oscillator strength for the Pii λ1302 transition, Dufton et al. (1986)
derived phosphorus abundances systematically larger by ≈ 0.2 dex so that the previous
findings still hold. Finally, it must be added that in the cold diffuse interstellar cloud
toward ζ Oph, the depletion of phosphorus is 0.5±0.2 dex as compared with 0.4±0.1 dex
for oxygen, while in the warm diffuse cloud along the same sightline, P is depleted by
0.2 ± 0.1 dex as compared with 0.0 ± 0.3 dex for O (Savage & Sembach 1996).
The first detection of the PN molecule by Turner & Bally (1987) through the 140 GHz
(J = 3 − 2) and 234 GHz (J = 5 − 4) emission lines paved the way to extensive studies
of the abundance of phosphorus in molecular clouds. It was soon recognized that the
transfer of phosphorus from the gas to the solid and back to the gas phase was largely
involving carbon atoms via the HCP linear molecule (Turner et al. 1990). As a conse-
quence, phosphorus is believed to mainly reside in adsorbed HCP molecules which are
then released in the gas by photodesorption in warm media and readily photodissociated.
This scenario would account for the lack of depletion in the warm phase.

Page 3

Lebouteiller, Kuassivi & Ferlet: Phosphorus in the diffuse ISM3
Since the advent of the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite (Moos
et al. 2000) and the STIS instrument onboard HST, it is now possible to probe denser
clouds and investigate longer sightlines. We thus revisit and extend previous works on
interstellar phosphorus abundance by presenting new Pii measurements toward Galactic
sightlines obtained from FUSE and HST/STIS data. Furthermore, in order to point out
the relative behavior of phosphorus as compared with α-elements and possibly reveal a
global trend toward the differential depletion under many different physical conditions,
we compare the phosphorus gaseous phase abundance with that of oxygen. Oxygen abun-
dance in the diffuse ISM has been extensively studied over the last years (see e.g., Jensen
et al. 2005, Cartledge et al. 2001, Cartledge et al. 2004, and Andr´ e et al. 2003 − hereafter
A2003) and is used here as a reference element. The observations and data analysis are
described in Sections 2 and 3. We present the results in Sect. 4. Final conclusions are
given in Sect. 5.
2. Observations
We have selected 10 sightlines toward Galactic stars with distances up to ≈ 5 kpc in order
to scan the distant ISM. Properties of the targets and the sightlines are summarized in
Table 1. Tables 2 and 3 provide the log of the FUSE and HST/STIS observations,
respectively. Given the Galactic latitudes and distances of the stars, all the sightlines
intersect clouds in the Galactic disk, except the sightline toward HD121968 which possibly
can intersect clouds in the halo.
All the FUSE spectra were obtained through the large 30′′×30′′(LWRS) aperture
which results in a resolving power R ≡ λ/∆λ ≈ 20,000 (or ∆v ≈ 15 km s−1, FWHM).
This spectral resolution depends on the co–addition procedure used to reconstruct the to-
tal exposure and varies with the wavelength and the detector. Hence, we did not attempt
to co–add different detectors, in order to minimize both the distortion of the resulting
Point Spread Function (PSF) and the propagation of the Fixed Pattern Noise (FPN)
proper to each detector. The detailed reduction, calibration and co–addition procedures
can be found in A2003 who reduced most of the present data in order to study the neutral
oxygen and hydrogen content along the sightlines.
The STIS observations were taken with the far-ultraviolet MAMA detector equipped
with the E140H grating. However, three different apertures were used. The 0.′′1 ×
0.′′03 aperture provides a resolving power R ≈ 200,000 (or ∆v ≈ 1.5 km s−1, FWHM).
The two others, 0.′′2 × 0.′′09 and 0.′′2 × 0.′′2, provide a spectral resolution of R ≈ 110,000
(velocity resolution of ∆v ≈ 2.7 km s−1, FWHM). Again, details about the data reduction
can be found in A2003.
Among the 10 targets we present, 8 were analyzed by A2003 with a particular concern
on the Oi column density. The other targets of their sample were not observed with STIS

Page 4

4 Lebouteiller, Kuassivi & Ferlet: Phosphorus in the diffuse ISM
at the wavelength of the Pii λ1533 line (see next section). Furthermore, we analyzed STIS
spectra of two additional targets, HD24534 and HD121868.
Finally, we also compiled published Pii and Oi measurements in the Milky Way and
along a few extragalactic sightlines. These are listed in Table 6 and discussed in Sect. 4.3.
3. Data analysis
Because of the ionization potential of Oi (13.62 eV as compared with 13.60 eV for
Hi) and the efficient charge exchange between Oii and Hi, Oi is expected to be the
dominant ionization state of oxygen in the diffuse neutral gas, and thus a good tracer
of the neutral gas. Observations of A2003 confirm this finding. On the other hand, the
ionization potentials of Pi and Pii (resp. 10.49 eV and 19.72 eV) suggest a priori that
Pii should be the dominant state of phosphorus in this gaseous phase. However a fraction
of Pii atoms could actually reside in a potential ionized gaseous phase where oxygen is
into Oii and hydrogen into Hii. The fraction is unknown and depends on the ionizing
radiations illuminating the diffuse clouds. The present study will help to identify this
possible correction.
Absorption lines were analyzed assuming Voigt profiles, by using the profile fitting
program Owens. This fortran code, developed by M. Lemoine and the FUSE French
team, is particularly suitable for simultaneous fits of far-UV spectra (Lemoine et al.
2002). A great advantage of this routine is the ability to fit different spectral domains
and various species in a single run. It was then possible to analyze simultaneously the
Pii and Oi absorption lines, together with Cli, Ci, and Si lines. The use of these species
in a simultaneous fit allowed us to check and constrain the radial velocity structure of
the sightlines when uncertain. The Pii and Oi lines we analyzed being unsaturated, the
column density determination does not depend on the b-parameter. The errors on the
column densities are calculated using the ∆χ2method described in H´ ebrard et al. (2002)
and include the uncertainties on all the free parameters such as the continuum shape and
position. All the errors we report are within 1 σ.
Numerous Oi lines are observed in the HST/STIS + FUSE spectral ranges, with
oscillator strengths (f) spanning several orders of magnitude. However, the main con-
straint on the Oi column density consists in using the weak 1355.5977˚ A intersystem
transition (f = 0.116×10−5). The total Oi column densities toward most of the present
sightlines were derived by A2003 using this line. However, we have updated these val-
ues by deriving column densities of each cloud along the sightlines and by performing a
simultaneous analysis of Oi and Pii lines.
A total of seven Pii lines are available in the far-UV spectral domain. Thanks to
the combination of datasets, the phosphorus atoms content is readily explored through
transitions spanning more than 2 orders of magnitude in oscillator strengths, always

Page 5

Lebouteiller, Kuassivi & Ferlet: Phosphorus in the diffuse ISM5
allowing the choice of adequat lines for a particular study. Wavelengths and f-values are
from the revised compilation of atomic data by Morton (1991; 2003). One must be aware
that oscillator strengths of Pii lines could be relatively uncertain. Indeed, phosphorus
atomic data have been poorly investigated and no laboratory experiments exist.
In the present study, the three Pii lines at 961.0412 ˚ A (f = 0.349 × 100),
963.8005˚ A (f = 0.146 × 101) and 972.7791˚ A (f = 0.210 × 10−1), observable with
FUSE, are located in a region overcrowded with strong absorption lines − mainly Hi
lines from the Lyman serie and H2 lines. We thus rejected these transitions because
blended. In addition, the two first lines are heavily saturated, thus preventing reliable
Pii column determinations.
Similarly, we avoided the Pii lines at 1152.8180˚ A (f = 0.245× 100, observable with
FUSE), and at 1301.8743˚ A (f = 0.127 × 10−1, observable with STIS) since they both
present most of the time saturation effects. It must be added that the later is most
often blended with the extremely strong Oi line at 1302.1685˚ A, even at the highest
STIS resolution.
The Pii line at 1124.9452˚ A (f = 0.248×10−2, observable with FUSE) is the weakest
transition available and is always found to lie on the linear part of the curve of growth.
When detected, it can provide an accurate measurement of the total phosphorus column
density along a given sightline but with little information on the velocity structure. One
should note that analyzing this line requires high–quality data and a good knowledge of
FPN for the FUSE detectors. Indeed, duplicate detectors make more easy to discriminate
FPN and absorption features in most cases. Unfortunately, in some cases this line appears
slightly blended with the broad Feiii* λ1124 stellar line.
Slightly stronger is the Pii line at 1532.5330˚ A (f = 0.303 × 10−2, observable with
STIS). Thanks to the STIS higher resolution, this line allows the investigation of the de-
tailed velocity distribution of sightlines and the derivation of phosphorus column densities
in individual clouds. We thus used this line to derive the Pii column densities (column
density of each individual clouds and integrated column density over the sightline) and
compare with the Oi values. The other great and main advantage of this line is that its
optical depth is systematically found to be similar to the optical depth of the Oi λ1356
line (see Fig. 1). This is due to the combination of the Oi column density, about 2000
times larger than the Pii one, and the oscillator strength, 2000 times lower than for the
Pii λ1533 line. Hence, the simultaneous analysis of these two lines minimizes possible
systematic errors due to possible saturation and/or unresolved components. This com-
bination further allows to investigate individual cloud column densities since these two
lines are observed in the same high-resolution dataset.
Another possibility consists in comparing the integrated column density as derived
from the unresolved profile of the Pii λ1125 line with FUSE, with the sum of the Pii col-
umn densities of individual clouds along the sightlines as derived with the STIS resolved