46th Lunar and Planetary Science Conference (2015)
ISOTOPE ABUNDANCES. N. N. Monson1, M. R. Morris2 and E. D. Young1, 1Department of Earth, Planetary,
and Space Sciences, UCLA ([email protected]; [email protected]), 2Department of Physics and Astronomy,
UCLA ([email protected]).
Introduction: Stable isotope ratios provide important, long lasting tracers of the environment in
which the Solar System formed, and to this end isotopic abundance ratios of accessible Solar System materials are routinely measured to extreme degrees of
precision. However, with few observational constraints on the isotopic nature of the Galaxy, our place
within this greater Galactic context is not known. We
ask the question: was our Solar System formed from
typical material and by typical processes, or, was it
formed in some atypical environment and/or by unusual processes? In other words, are we normal?
While interstellar oxygen isotopes have been studied extensively [5] the same is not true of the other
light-element systems having three stable isotopes;
Mg and 28,29,30Si. Magnesium does not lend itself
to widespread interstellar observations, however silicon can be observed in molecular clouds at millimeter
and submillimeter wavelengths. The silicon isotope
system is largely analogous to that of oxygen, and galactic chemical evolution (henceforth GCE) necessitates that both systems evolve via the same mechanisms. This makes silicon a potentially valuable
benchmark for examining the methodology used to
measure Galactic oxygen isotope ratios.
Presolar silicon carbide grains found in meteorites
are thought to have condensed out of the winds of ancient AGB stars and therefore be representative of the
interstellar medium (henceforth ISM) as it existed
when those stars formed >> 5 Ga. The disparate rates
of nucleosynthesis between the primary nuclide 28Si
and secondary nuclides 29Si and 30Si leads to two predictions; that the primary to secondary isotope abundance ratio will rise linearly with time, and that the
abundance ratio of secondary isotopes will remain constant. Thus, to first order, GCE would dictate that Solar [28Si]/[29Si] and [28Si]/[30Si] ratios, being representative of the ISM when the sun formed (i.e. 4.5 Ga),
be smaller than the [28Si]/[29Si] and [28Si]/[30Si] ratios
found in presolar SiC grains, but this is not observed.
This deviation from the simple GCE prediction is not
well understood [1], and several hypotheses have been
put forth to explain it, including an incomplete understanding of the influence of winds from AGB stars on
the isotope ratios in the interstellar medium [2], and
pollution of the stellar birth environment by a nearby
Type-II supernova [3, 4].
The questions raised by the abundance trend of
presolar SiC grains can be put into a Galactic context
and perhaps resolved if the present Galactic isotope
ratios are determined as a function of Galactocentric
radius (a rough proxy for time), as ratios of stable isotopes in the Solar system can be placed in a Galactic
context only if the effects of time can be accounted for.
In effect, GCE must be considered in any comparison
between the Solar system and either the interstellar
medium or any extrasolar planetary systems. Here we
address this issue by reporting on the first new radio
astronomy measurements of silicon isotope ratios
across the Salaxy in nearly 30 years.
The Case for Silicon: A number of silicon species,
including SiC, SiS, SiCN, SiNC and SiH4, all have at
least one detection in a circumstellar envelope around
an AGB star, but local nucleosynthesis and hence the
potential for sample bias, makes these unsuitable proxies for the average interstellar abundances. This is not
true for silicon monoxide, which is the most commonly
observed silicon species in the ISM and is thought to
dominate the gaseous silicon budget [6]. Despite SiO
being an oxide, no knowledge of oxygen isotope ratios
within sources is required to extract silicon isotope
ratios from observations. For these reasons, SiO is well
suited for probing isotopic GCE, as the chance that
observational measurements are not representative of
the bulk silicon composition is minimized.
Figure 1: Uncorrected silicon isotope abundance
ratio data for the seven sources observed as part of
this survey. Mainstream SiC grain data are shown
for reference (open circles). The solid line is the
slope unity line predicted by GCE, and the dotted
line is a regression through the presolar SiC grains.
46th Lunar and Planetary Science Conference (2015)
Observations: Observations of the vibrational
ground-state, J = 1→0 pure-rotational transition of the
three silicon isotopologues of SiO were carried out at
the Robert C. Byrd Green Bank Telescope (GBT) between May 2013 and February 2014. Seven sources,
DR21(OH), W51m, NGC 7538, AFGL 51242, LDN
1157 GCM -0.13-0.08 and GCM 0.11-0.11 have been
observed to date. Extraction of isotope ratios from the
raw data was done via a novel vectorized calibration
routine and a series of line profile integrations performed by a suite of purpose built IDL and Fortran
programs written by the first author.
Discussion: Historically, SiO emission has been
assumed to be optically thin due to the modest brightness of the observed lines, but Penzias was quick to
demonstrate that SiO thermal emission often contravenes this assumption [8], and the same was found to
be true for this survey. But unlike in previous work,
modern high-resolution, low-noise receiver systems
are sensitive enough to determine variations in
[29SiO]/[28SiO] and [30Si]/[28SiO] with accuracy and
precision sufficient to address optical depth effects and
determine what, if any, silicon isotope gradient exists
in the Galaxy.
Optical depth was found to vary from source to
source. Emission lines from DR21(OH) and AFGL
5142 show no evidence of optical depth, with an estimated detection limit of τ ≈ 0.2. Other sources, including the two Galactic center sources, GCM -0.13-0.08
and GCM 0.11-0.11, show evidence of appreciable
optical depth in the main isotope emission line profiles.
Using a series of simple radiative transfer codes to
generate synthetic spectra, the optical depth at the center of the observed main isotope lines can be shown to
be of order unity for both sources. Correcting for the
effect of optical depth has a profound effect on silicon
isotope ratios. While our uncorrected data show evidence of a spread up and down the slope-one line in Si
three-isotope space (Figure 1), anchored by the two
Galactic center sources and LDN 1157 at Solar RGC.
Correcting for the effects of optical depth removes the
spread, clustering the data near the isotopically-heavy
end of the mainstream SiC presolar grain trend and
effectively destroying the apparent trend with RGC
(Figure 2). All seven datum lie well within error of one
another with respect to both δ29Si and δ30Si. One could
argue for a small, and poorly resolved, gradient remains in both isotopologues near the Solar circle,
which coupled with the depressed Galactic center may
suggest a trend similar to the [Fe/H] gradient reported
in young Cepheids in the thick disk and LBVs in the
Galactic core [9,10,11].
These results appear to support the argument that
optical depth was responsible for previous evidence of
Figure 2: Calculated silicon isotope abundance
ratios after correcting for optical depth effects.
a large Galactocentric silicon isotope gradient [7].
Desptie scant evidence of a gradient, Galactic silicon
isotopic abundances do not appear static, and are in a
state of temporal flux in so far as the modern Galaxy
appears to be isotopically heavy with respect to both
the Sun and the preponderance of presolar SiC grains
(the latter representing the ISM ≥ 4.6 Ga). However
the work is far from complete; our data for δ30Si suffers from poor signal to noise ratios in many sources,
and our survey lacks sources in the 2-5 kpc RGC region,
a shortcoming that we hope to correct with additional
observations scheduled to begin in February of 2015.
There are, however, other phenomena that could be
affecting the observed isotopic ratios; weak inversions
[12] or an enhanced radiation field within one of the
line profiles could is causing divergent excitation of
the three isotopologues, and effecting the extracted
isotope ratios in a manner largely indistinguishable
from optical depth. Efforts to constrain the excitation
temperature of all three species are still in progress.
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