71 research outputs found
Composition and chemical history of early solar system ices
Some models of protostellar collapse [1] indicate that all but the most volatile ices from the native envelope
survive the infall phase, at least at larger disk radii (10 AU). The compn. and chem. history of ices in
circumstellar envelopes, and sometimes in disks, can be traced with IR (2-30 μ) spectroscopy using ground and
space based telescopes. Surveys of large samples of Young Stellar Objects (YSOs) [2,3,4] and quiescent clouds
[5,6] have shown that the ice abundances and band profiles vary considerably as a function of environment. A
rather complex mixt. of simple species (CH3OH, CO2, H2O, NH3, CO) exists even before stars form, most likely
reflecting a history of chem. on cold grain surfaces. CH3OH abundance variations show that local phys.
conditions (CO freeze out) and time scales (CH3OH formation) are key factors in this early chem. Sublimation
and thermal processing of the ices are dominant processes during the YSO's evolution. But, contrary to present
day solar system ices, the evidence of processing of interstellar and circumstellar ices by photons and
energetic particles is weak. Lab. expts. are needed to further constrain the processes of mol. formation as well
as to det. the origin of several unidentified ice features. Such expts. will also be essential to interpret the
sensitive and high spectral resoln. observations that will become possible with new facilities (the SOFIA airplane
and JWST satellite)
The CO and CO Absorption Bands as Tracers of the Thermal History of Interstellar Icy Grain Mantles
Analyses of infrared signatures of CO in water dominated ices in the ISM
can give information on the physical state of CO in icy grains and on the
thermal history of the ices themselves. In many sources, CO was found in
the `pure' crystalline form, as signatured by the splitting in the bending mode
absorption profile. To a large extent, pure CO is likely to have formed
from segregation of CO from a CO:HO mixture during thermal
processing. Previous laboratory studies quantified the temperature dependence
of segregation, but no systematic measurement of the concentration dependence
of segregation is available. In this study, we measured both the temperature
dependence and concentration dependence of CO segregation in CO:HO
mixtures, and found that no pure crystalline CO forms if the CO:HO
ratio is less than 23%. Therefore the segregation of CO is not always a
good thermal tracer of the ice mantle. We found that the position and width of
the broad component of the asymmetric stretching vibrational mode of
CO change linearly with the temperature of CO:HO mixtures,
but are insensitive to the concentration of CO. We recommend using this
mode, which will be observable towards low mass protostellar envelopes and
dense clouds with the James Webb Space Telescope, to trace the thermal history
of the ice mantle, especially when segregated CO is unavailable. We used
the laboratory measured CO profile to analyze the ISO-SWS
observations of ice mantles towards Young Stellar Objects, and the
astrophysical implications are discussed.Comment: 11 pages, 12 figures, ApJ accepte
Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone
Previous radio observations revealed widespread gas-phase methanol (CH_3OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH_3OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF L’ imaging and 2–4 μm spectra. The star lies ~8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH_3OH ice abundance (17% ± 3% relative to H_2O ice) suggests that the 3.535 μm CH_3OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH_3OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH_3OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH_3OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO_2 ice mixed with CH_3OH
Properties of Protostars in the Elephant Trunk in the Globule IC 1396A
Extremely red objects, identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A and photometrically classified as Class I protostars and Class II T Tauri stars based on their mid-infrared (mid-IR) colors, were spectroscopically observed at 5.5-38 μm (Spitzer Infrared Spectrograph), at the 22 GHz water maser frequency (National Radio Astronomy Observatory Green Bank Telescope), and in the optical (Palomar Hale 5 m) to confirm their nature and further elucidate their properties. The sources photometrically identified as Class I, including IC 1396A:α, γ, δ, ε, and ζ, are confirmed as objects dominated by accretion luminosity from dense envelopes, with accretion rates 1-10 × 10^–6 M☉ yr^–1 and present stellar masses 0.1-2 M☉. The Class I sources have extremely red continua, still rising at 38 μm, with a deep silicate absorption at 9-11 μm, weaker silicate absorption around 18 μm, and weak ice features including CO2 at 15.2 μm and H2O at 6 μm. The ice/silicate absorption ratio in the envelope is exceptionally low for the IC 1396A protostars, compared to those in nearby star-forming regions, suggesting that the envelope chemistry is altered by the radiation field or globule pressure. Only one 22 GHz water maser was detected in IC 1396A; it is coincident with a faint mid-IR source, offset from near the luminous Class I protostar IC 1396A:γ. The maser source, IC 1396A:γb, has luminosity less than 0.1 L☉, the first H2O maser from such a low-luminosity object. Two near-infrared (NIR) H2 knots on opposite sides of IC 1396A:γ reveal a jet, with an axis clearly distinct from the H2O maser of IC 1396A:γb. The objects photometrically classified as Class II, including IC 1396A:β, θ, Two Micron All Sky Survey (2MASS)J 21364964+5722270, 2MASSJ 21362507+5727502, LkHα 349c, Tr 37 11-2146, and Tr 37 11-2037, are confirmed as stars with warm, luminous disks, with a silicate emission feature at 9-11 μm, and bright Hα emission; therefore, they are young, disk-bearing, classical T Tauri stars. The disk properties change significantly with source luminosity: low-mass (G-K) stars have prominent 9-11 emission features due to amorphous silicates while higher-mass (A-F) stars have weaker features requiring abundant crystalline silicates. A mineralogical model that fits the wide- and low-amplitude silicate feature of IC 1396A:θ requires small grains of crystalline olivine (11.3 μm peak) and another material to to explain its 9.1 μm peak; reasonable fits are obtained with a phyllosilicate, quartz, or relatively large (greater than 10 μm) amorphous olivine grains. The distribution of Class I sources is concentrated within the molecular globule, while the Class II sources are more widely scattered. Combined with the spectral results, this suggests two phases of star formation, the first (4 Myr ago) leading to the widespread Class II sources and the central O star of IC 1396 and the second (less than 1 Myr ago) occurring within the globule. The recent phase was likely triggered by the wind and radiation of the central O star of the IC 1396 H II region
The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4
CH4 is proposed to be the starting point of a rich organic chemistry. Solid
CH4 abundances have previously been determined mostly toward high mass star
forming regions. Spitzer/IRS now provides a unique opportunity to probe solid
CH4 toward low mass star forming regions as well. Infrared spectra from the
Spitzer Space Telescope are presented to determine the solid CH4 abundance
toward a large sample of low mass young stellar objects. 25 out of 52 ice
sources in the (cores to disks) legacy have an absorption feature at 7.7
um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances
are 2-8%, except for three sources with abundances as high as 11-13%. These
latter sources have relatively large uncertainties due to small total ice
column densities. Toward sources with H2O column densities above 2E18 cm-2, the
CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation
plots with solid H2O, CH3OH, CO2 and CO column densities and abundances
relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than
with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with
models where CH4 is formed through sequential hydrogenation of C on grain
surfaces. Finally the equal or higher abundances toward low mass young stellar
objects compared with high mass objects and the correlation studies support
this formation pathway as well, but not the two competing theories: formation
from CH3OH and formation in gas phase with subsequent freeze-out.Comment: 27 pages, 7 figures, accepted by Ap
Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes
To study the demographics of interstellar ices in the Central Molecular Zone
(CMZ) of the Milky Way, we obtain near-infrared spectra of red point
sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and
mid-infrared photometry, including objects in the previous paper of this
series, to ensure that these sources trace a large amount of absorption through
clouds in each line of sight. We find that most of the sample ( objects)
show CO band-head absorption at m, tagging them as red (super-)
giants. Despite the photospheric signature, however, a fraction of the sample
with -band spectra () exhibit large HO ice column densities
(), and six of them also reveal CHOH ice
absorption. As one of such objects is identified as a young stellar object
(YSO) in our previous work, these ice-rich sight lines are likely associated
with background stars in projection to an extended envelope of a YSO or a dense
cloud core. The low frequency of such objects in the early stage of stellar
evolution implies a low star-formation rate ( yr),
reinforcing the previous claim on the suppressed star-formation activity in the
CMZ. Our data also indicate that the strong "shoulder" CO ice absorption at
m observed in YSO candidates in the previous paper arises from
CHOH-rich ice grains having a large CO concentration [].Comment: 28 pages, 12 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star
We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star
VV Ser. We combine the Spitzer data with interferometric and spectroscopic data
from the literature covering UV to submillimeter wavelengths. The full set of
data are modeled by a two-dimensional axisymmetric Monte Carlo radiative
transfer code. The model is used to test the prediction of (Dullemond et al.
2003) that disks around UX Orionis stars must have a self-shadowed shape, and
that these disks are seen nearly edge-on, looking just over the edge of a
puffed-up inner rim, formed roughly at the dust sublimation radius. We find
that a single, relatively simple model is consistent with all the available
observational constraints spanning 4 orders of magnitude in wavelength and
spatial scales, providing strong support for this interpretation of UX Orionis
stars. The grains in the upper layers of the puffed-up inner rim must be small
(0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events,
while the shape and strength of the mid-infrared silicate emission features
indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0
micron). From the model fit, the location of the puffed-up inner rim is
estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small
grains. This is almost twice the rim radius estimated from near-infrared
interferometry. A best fitting model for the inner rim in which large grains in
the disk mid-plane reach to within 0.25 AU of the star, while small grains in
the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to
reproduce all the data, including the near-infrared visibilities. [Abstract
abridged]Comment: 12 pages, accepted for publication in Ap
Spitzer Space Telescope Spectroscopy of Ices toward Low-Mass Embedded Protostars
Sensitive 5-38 μm Spitzer Space Telescope and ground-based 3-5 μm spectra of the embedded low-mass protostars B5 IRS1 and HH 46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO_2 bending mode at 15 μm is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN^- bands, however. The observed CO2 profile indicates an intimate mixture with H_(2)O, but not necessarily with CH_(3)OH, in contrast to some high-mass protostars. This is consistent with the low CH_(3)OH abundance derived from the ground-based L-band spectra. The CO_2 : H_(2)O column density ratios are high in both B5 IRS1 and HH 46 IRS (~35%). Clearly, the Spitzer spectra are essential for studying ice evolution in low-mass protostellar environments and for eventually determining the relation between interstellar and solar system ices
- …