GSMT Science Use Case
Title: Mid-Infrared Disk Emission
Authors: Joan Najita (NOAO), John Carr (NRL), and Matt Richter (UC Davis)
Abstract: The difficulty of synthesizing complex biological compounds in the
environment of the young Earth has led to serious consideration of the possibility that
prebiotic molecules were delivered to Earth by asteroids, comets, and meteorites. To
what extent were biogenic molecules (or their molecular precursors) synthesized in the
solar nebula and in what abundance? Studies of the chemistry and molecular abundances
of present-day planet-forming disks offer the opportunity to address this important
question. A high resolution mid-infrared spectrograph on a GSMT will be a powerful
tool for this purpose. Mid-infrared spectroscopy of the emission from disks surrounding
nearby (d < 200pc), young (age ~ 1 Myr) stars will measure the physical properties
(temperature, density) and molecular content of the disks as a function of disk radius.
Such observations will (1) probe the chemical pathways by which disks synthesize
complex molecules of biological interest; (2) probe the importance of dynamical
processes such as radial mixing for disk chemistry; and (3) quantify the abundances of
important prebiotic species.
Summary Table:
Summarize the observations in terms of telescope, instruments, number of nights,
observing mode and instrument and AO requirements.
Telescope Instrument # Nights Mode  range(m) / AO Mode FOV
TMT
MIRES
24
7-14
105
Scientific Motivation:
What is the origin of the chemical building blocks of life?
The difficulty of synthesizing complex biological compounds in the environment of the
young Earth (Schlesinger & Miller 1983) has led to serious consideration of the
possibility that prebiotic molecules have an exogenous origin. In one hypothesis, cosmic
material that was initially chemically processed in molecular clouds underwent
gravitational collapse, rained down onto the nebular disk, and underwent further chemical
processing there. The chemically processed material was then incorporated into icy
bodies and delivered to Earth by asteroids, comets and meteorites (Figure 1).
Figure 1. Exogenous origin of prebiotic molecules. Material that is chemically processed in clouds
and circumstellar disks may be incorporated into asteroids and comets. These may bombard
planetary surfaces and seed them with the chemical building blocks of life.
Amino acids, nucleobases, and sugars have indeed been identified in some (but not all)
carbonaceous chondrites, a particular class of meteorites (e.g., Botta and Bada 2002).
The amino acids are thought to have formed on the parent body of the meteorite through
the aqueous alteration of molecular precursors that originated in the circumstellar disk
from which the meteorite formed. Such parent bodies, if they impacted the Earth under
the right conditions (e.g., with modest impact velocities and in sufficient abundance),
may have seeded the Earth with the chemical ingredients necessary for the development
of life (e.g., Botta 2004; Chyba et al. 1990).
GSMT can test several aspects of this scenario by probing the molecular abundances of
clouds, infalling gas, disks, and comets. These observations provide unique insights into
the extent to which clouds and disks can synthesize the precursor molecules. The
molecular abundances of clouds, infalling gas, and disks can be probed via absorption
line spectroscopy (see related proposal). The abundances of disks and comets can be
probed using emission line spectroscopy. This proposal focuses on studying disk
molecular abundances via emission line spectroscopy.
Probing Disk Chemical Synthesis
While more is known about cloud chemistry, circumstellar disks are essentially
unexplored. The theory of disk chemical synthesis is in its infancy (e.g., Markwick et al.
2002) and the theoretical building blocks of a more elaborate theory are available (e.g.,
Charnley 2001), but significant observational input is needed to develop a robust theory.
The needed observations include measurements of disk physical properties and
constraints on their dynamics (e.g., the extent of radial and vertical mixing) and
measurements of the abundances of detectable molecules. The latter probes the nature of
disk chemical synthesis processes (e.g., the efficiency of grain surface reactions, and the
interaction of its products with a warm gas phase chemistry). The needed measurements
can be obtained by using molecular emission to measure temperatures, densities, and
molecular abundances in the planet formation region of disks (e.g., Najita et al. 2007).
Of particular interest are the properties of disks at 1 to 5 AU where grains are expected to
be warm enough (> 100 K) that molecules have desorbed from grain surfaces, thereby
allowing us to probe the products of molecular synthesis on grains. By comparing
measured molecular abundances with the predictions of disk chemistry models and the
abundances of molecular clouds, it will be possible to constrain the roles of dynamical
processes such as radial mixing and chemical processes such as grain surface reactions in
the synthesis of complex molecules.
Disks Discovered to Possess a Rich MIR Emission Spectrum
Recent work carried out with the Spitzer Space Telescope demonstrates that circumstellar
disks commonly show a rich spectrum of emission lines in the mid-infrared (Carr &
Najita 2008; Salyk et al. 2008). As shown in Figure 2 for the case of a typical T Tauri
star, numerous individual emission lines and strong molecular bands are observed in the
wavelength region accessible to the high resolution mode of the Spitzer IRS (10-40
microns).
Figure 2. High resolution Spitzer IRS spectrum of a typical T Tauri star, AA Tau, located 140 pc
away in the Taurus star forming region (Carr & Najita 2008). Numerous emission lines of water
(unmarked features) and OH (diamonds) as well as bands of organic molecules (C 2H2, HCN, and
CO2) are detected. Atomic lines such as [NeII] are also detected.
By comparing the observed spectrum with a simple model for the emission (Figure 3), it
is clear that the spectrum includes emission lines of water and OH as well as bands of
organic molecules such as acetylene (C2H2), hydrogen cyanide (HCN), and carbon
dioxide (CO2). Emission lines of atomic species such as [NeII] and HI are also observed.
Work to date on a small sample of young stars surrounded by actively accreting disks
(classical T Tauri stars) demonstrates that a rich emission spectrum of this kind is
extremely common and characteristic of young planet-forming disks (Carr & Najita, in
preparation). The characteristic temperatures (300-600 K) and equivalent circular
emitting areas (radius of 1-2 AU) derived for each molecular species using the simple
model (Figure 3) implies that the molecular emission arises from the inner few AU of the
disk, i.e., in the main planet formation region of the disk. Thus the mid-infrared
diagnostics probe larger disk radii than are probed by near-infrared spectral line
diagnostics of disks (e.g., CO overtone and fundamental emission lines and emission
lines of water in the K-band).
One interesting result from the modeling is that the AA Tau abundances are enhanced in
organic molecules such as HCN relative to the abundances in cloud cores and comets,
suggesting that an active chemistry is operative in the inner disk. The abundances are,
moreover, found to vary from disk to disk. Understanding the origin of the chemical
diversity and the implications for the synthesis of complex molecules is an important
future goal.
Figure 3. A portion of the spectrum of AA Tau from Figure 2 (top) compared with a model of
emission from a circumstellar disk (bottom). The model, which includes emission bands of C 2H2,
HCN, and CO2 and emission from individual water lines (starred features), reproduces the structure
in the observed spectrum (Carr & Najita 2008).
We can more reliably determine the range of disk radii over which the emission
originates and characterize molecular abundances as a function of disk radius by
spectrally resolving individual emission lines. With current facilities, it is possible to
resolve bright emission lines in sources brighter than AA Tau. In the example shown in
Figure 4, the shape of the emission line, when interpreted under the assumption of
Keplerian rotation, confirms that the water emission arises from within a few AU of the
star. High resolution spectroscopy of this kind can be used to measure disk temperatures,
column densities, and molecular abundances as a function of disk radius (e.g., Najita et
al. 2007). This basic technique enables a wide range of studies of disk dynamics,
structure, and chemistry.
Figure 4. An individual water line in the 13 micron region of a source that is ~6 times brighter than
AA Tau at this wavelength. The spectrum, obtained with the high resolution (R=100,000) midinfrared spectrograph TEXES on the Gemini North telescope in ~1 hr of integration time (see Knez
et al. 2007), shows that an 8-m class telescope can resolve the bright emission lines in sources much
brighter than AA Tau.
Approach:
How can you use TMT and/or GMT and their candidate instruments to address this
problem? Describe the observing strategy, including target selection and the needed
measurements.
High resolution mid-infrared spectroscopy with a GSMT of molecular emission from
disks surrounding nearby (d < 200pc), young (age ~1 Myr) stars will measure the
physical properties (temperature, density) and molecular content of disks as a function of
disk radius. These observations will (1) probe the chemical pathways by which disks
synthesize complex molecules of biological interest; (2) probe the importance of
dynamical processes such as radial mixing for disk chemistry; and (3) quantify the
abundances of important prebiotic species.
A simple strategy is to focus on measuring the abundances of building block molecules
(e.g., NH3, HCN, HCOOH, etc), which are important chemical precursor species.
Molecules such as HCN, NH3, HNCO, and HC3N are known to react spontaneously in
liquid water to produce amino acids. Formic acid (HCOOH) is an important chemical
precursor of glycine, the simplest biologically important amino acid. Amino acids are the
building blocks of proteins and DNA. Thus, the abundances of these simple molecules
are of significant interest. Their abundances as a function of radius in the disk will
provide insights into the nature of disk chemistry and its ability to produce more complex
molecules. It may be possible to detect more complex species if their abundances are
high.
High resolution spectroscopy is critical, not only to measure quantities as a function of
disk radius, but also to enable the detection of weak emission features from rare
molecular species. While the water emission from circumstellar disks is extremely
interesting in its own right, from the perspective of trying to detect rare molecular
species, it is a contaminant that complicates or precludes the detection of weaker
emission features from rare species, particularly at lower spectral resolution. Given the
expected degree of line crowding, a resolution of R > 20,000 is needed to resolve blends
and measure individual lines. Even higher spectral resolution (R~100,000) is needed to
resolve individual lines and measure the properties of disks as a function of radius out to
<~ 5 AU.
With the combination of the high angular resolution (diffraction limited or near
diffraction limited) and the high spectral resolution of GSMT (R~100,000) in the midinfrared, we can also use spectro-astrometry to measure directly the average emitting
radius of a given spectral line without making the assumption of Keplerian rotation.
Recent work that demonstrates the power of spectro-astrometry in the mid-infrared (e.g.,
Pontoppidan et al. 2008) confirms that Keplerian rotation is a good assumption.
Given the known chemical diversity of disks, it will be important to survey a significant
sample of objects. Continuum fluxes of ~0.3 Jy at 12 microns are also typical among
Myr old disks (e.g., a typical T Tauri star such as AA Tau; Figure 2). For fixed stellar
mass, the brightest disks are typically the least evolutionarily evolved. So it will be
important to study the fainter disks in order examine the role of evolutionary age on the
extent of chemical processing in disks.
The high sensitivity of GSMT will be critical to studying the abundances in typical disk
systems such as AA Tau. With TEXES on Gemini one can now study the brightest lines
in bright sources ( continuum fluxes of ~1-3 Jy at 12 microns). The study of fainter,
more typical, or evolutionarily older systems will require the sensitivity of a GSMT. In
particular, the GSMT will be needed to study faint emission lines from rare molecular
species.
Broad wavelength coverage is also needed, because we will need to study multiple lines
from a given species in order to measure temperatures and column densities. We will
also need to study multiple species in order to measure relative molecular abundances.
Limiting Factors and the Current State of the Art:
What are the limiting factors for this problem (e.g. sensitivity, spatial resolution, time
resolution)? Why hasn’t this problem been solved with current facilities?
High sensitivity at high spectral resolution is the critical limiting factor to date. Thus the
large collecting area of a GSMT will enable critical breakthroughs. Important pathfinding work can be carried out in advance of GSMT with high resolution mid-infrared
spectrographs such as TEXES on Gemini or CRIRES on the VLT. Because of the lower
sensitivity of these facilities, studies with these facilities will be restricted to primarily
bright emission lines (~10-17 W/m2; e.g., lines several times the strength of the brightest
lines in Fig. 2) from bright sources (~1-3 Jy continuum at 12 microns).
Weaker lines will be beyond the reach of 8-m class facilities. Thus, it will not be
possible to search for faint emission lines from rare molecular species even in the
brightest sources. More importantly, it will be difficult to characterize the bright water
emission from typical disk systems at 1 Myr age (~0.3 Jy at 12 microns), much less the
emission from rare molecular species in these sources. These sensitivity considerations
strongly limit the number of sources that can be studied as well as the range of molecular
species that can be probed.
Sensitive spectroscopy in the mid-infrared is of course also possible currently with the
Spitzer Space Telescope. However, the low spectral resolution of the Spitzer IRS
(R=600) severely restricts the detectability of rare molecules in part because of sensitivity
constraints, but also because of severe line crowding from brighter water emission lines.
Much higher spectral resolution (>20,000) is needed to resolve blended emission lines to
enable the detection of weak features.
Technical Details:
How would you actually carry out this program? Justify the sensitivities, exposure times,
number of fields, total cost in terms of telescope hours or nights. Mode of observation,
queue, classical, TOO, synoptic etc.
The above program requires measuring the line profiles of multiple lines of multiple
molecular species, including those that have previously been detected in low-resolution
mid-infrared spectra. The targets for the study would be carefully selected, based on
Spitzer and JWST spectroscopy of classical T Tauri stars, to cover the observed range of
disk spectral characteristics (e.g., known diversity of abundant molecular species) and
other system parameters (e.g., disk masses, evolutionary age). A sample size of ~ 30
would be an appropriate minimum initial sample size. Given the need for high signal-tonoise spectra (in order to detect weak emission features) the selected targets will be
located in nearby star forming regions, within ~160 pc.
The sensitivity of a MIR echelle on a 30-m telescope has been previously estimated (e.g.,
the MIRES instrument study for the TMT). These results predict that we will be able to
obtain S/N = 300 in 2 hr on a 0.3 Jy continuum, which is equivalent to a NELF (1-, 1
sec) of 1.0 x 10-19 W m-2. Based on Spitzer IRS spectroscopy, individual emission lines
of H2O, HCN or C2H2 in a typical T Tauri star spectrum (e.g., Fig. 2) have a line flux on
the order of 2 x 10-18 W/m2. For such a line flux and an assumed line width of 60 km s-1,
one could obtain a S/N ~ 20 per resolution element on the line profile in 0.5 hr. This
would produce a high-quality line profile. For much weaker lines (e.g., from rarer
molecular species) that are ~0.1 the strength of HCN, a 2.0 hr integration would yield
S/N ~ 4 per resolution element, which is sufficient to (1) determine whether the emission
is centered on the stellar velocity, as expected for emission from a disk; (2) measure the
width of the line and thereby estimate the average disk radius from which the emission
arises; and (3) determine the relative abundance of the molecule.
Assuming a wavelength coverage of 1.5-2%, observing two settings with an integration
time of 0.5 hr each would measure multiple HCN, C2H2 and H2O lines that span a range
of excitation potential. These measurements would allow us to derive the excitation
temperature and column density of the emitting gas as a function of radius. We estimate
that about 3 deeper settings (of 2.0 hr each) could be used to probe additional species
whose emission lines are anticipated to be much weaker (e.g., HNCO, NH3, HCOOH,
CH3OH). For a sufficiently large radial velocity, CH4 can be observed in one setting,
with an integration time of 1.0 hr. Hence, about 8 hours of spectroscopy in the 7-14
micron spectral region would be required to study a variety of molecular species.
Observations of 30 targets would therefore require 24 nights.
Such a project is best carried out in a queue scheduled observing mode in order to
optimize observations of molecular species that require low water vapor or a particular
observed target radial velocity.
Preparatory, Supporting, and Followup Observations:
What data are needed in advance of, or in support of these projects? If these require
observing time on 4-10m class telescopes estimate the amount of time needed. What
followup observations are needed?
We will select targets from ongoing Spitzer spectroscopy of young stars in nearby (d ~
150 pc) molecular clouds.
Anticipated Results:
What would you expect to get from the observations? Describe simulated data and results
where appropriate.
Discussed above.
Requirements and Goals Beyond the GMT and TMT Baseline Instrument Designs:
Are there capabilities needed for this science that are not in the TMT and GMT telescope,
AO system and baseline instrument configurations? If so, what is the flow-down from the
high level goals to the instrument requirements?
Describe the need for specific observing conditions or operations mode(s) (needed image
quality; atmospheric transmission; need for ‘interrupt-driven’ observations)
Water vapor is an important consideration for some spectral features (e.g., H2O lines, the
C2H2 Q-branch). In addition, an appropriate radial velocity shift is often needed to avoid
telluric absorption lines in order to enable the detection of emission features. The needed
radial velocity will be different for different spectral features, depending on whether we
are trying to avoid telluric lines of the same species that we are observing or lines of a
different species. Queue scheduling would enable a match to water vapor as well as the
needed radial velocity shifts.
Describe the potential of the resulting database for ‘mining’ in service of carrying out
complementary scientific programs; planning future programs
Describe the potential role of other ground- and space- based facilities in carrying out
the proposed investigation (e.g. JWST; ALMA; LST)
An important discovery component of this program (the search for rare molecular
species) relies on detecting faint emission lines in a region that is spectrally crowded with
other emission lines. With its combination of high sensitivity and high spectral
resolution, GSMT will make a huge contribution. In comparison, the lower resolution of
MIRI on JWST (R = 3000), will severely limit its ability to address this problem. MIRI
on JWST would be better suited to assessing the impact of generic disk chemistry
processes (mixing, irradiation, grain settling) on the observed abundances of abundant
molecules, e.g., through spectrally unresolved surveys of large samples of young stars.
Our interpretation of the observations carried out with a GSMT would build on the
insights obtained from such a program. ALMA will be a powerful tool with which to
address the properties of outer (> 10 AU) planet-forming disks. It will typically not have
the sensitivity to detect emission lines from the inner (< 10 AU) disk (e.g., Semenov et al.
2008).
Summary:
The high sensitivity of a high resolution mid-infrared spectroscopy on a GSMT will open
a new window on our understanding chemical processing in the planet formation regions
of disks and the origin of the chemical building blocks of life. Recent low resolution
spectroscopy carried out with the Spitzer Space Telescope shows that disks surrounding
nearby (d < 200 pc), young (age ~ 1 Myr) stars possess a mid-infrared spectrum that is
rich in molecular emission lines of simple molecules. GSMT observations that detect and
measure line profiles for emission lines from those and more complex molecular species
will (1) probe the chemical pathways by which disks synthesize complex molecules of
biological interest; (2) probe the importance of dynamical processes such as radial mixing
for disk chemistry; and (3) quantify the abundances of important prebiotic species.