Low-mass Young Stellar Objects Research Articles

YOUNG BROWN DWARFS IN THE CORE OF THE W3 MAIN STAR-FORMING REGION

We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR observations with the Subaru telescope, to search for very low mass young stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 region and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color (CC) and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low luminosity YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. Based on the comparison between theoretical evolutionary models of very low-mass PMS objects with the observed CM diagram, we find there exists a substantial substellar population in the observed region. The mass function (MF) does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the MF slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.

Detection of C3O in the Low‐Mass Protostar Elias 18

We have performed new laboratory experiments which gave us the possibility to obtain an estimate of the amount of carbon chain oxides (namely C3O2, C2O, and C3O) formed after irradiation (with 200 keV protons) of pure CO ice, at 16 K. The analysis of laboratory data indicates that in dense molecular clouds, when high CO depletion occurs, an amount of carbon chain oxides as high as 2-3x10^-3 with respect to gas phase carbon monoxide can be formed after ion irradiation of icy grain mantles. Then we have searched for gas phase C2O and C3O towards ten low-mass young stellar objects. Among these we have detected the C3O line at 38486.891 MHz towards the low-mass protostar Elias 18. On the basis of the laboratory results we suggest that in dense molecular clouds gas phase carbon chain oxides are formed in the solid phase after cosmic ion irradiation of CO-rich icy mantles and released to the gas phase after desorption of icy mantles. We expect that the Atacama Large Millimeter Array (ALMA), thanks to its high sensitivity and resolution, will increase the number of carbon chain oxides detected in dense molecular clouds.

LUMINOSITY AND MASS FUNCTIONS AT THE VERY LOW MASS SIDE IN NGC 1333

We present the results of a deep near-infrared (NIR) imaging survey searching for very low mass young stellar objects (YSOs) in the embedded cluster associated with the Perseus molecular cloud. Our observations cover an area of ~5'× 5' in the NGC 1333-S region at J-, H-, and Ks-bands. The 10σ limiting magnitudes exceed 18 mag in all three bands. Based on NIR color-color diagrams, embedded YSO candidates were identified using NIR excesses. The derived frequency of these YSO candidates with NIR excess emission among all the detected sources is 58+10 –13%. The higher frequency of YSOs with NIR excesses implies that NGC 1333 is an active and young star-forming region. Approximately half of the YSO candidates exhibit extremely low luminosity, indicating very low mass. Combining the reddening-corrected luminosity with theoretical evolutionary models, the low-luminosity YSO candidates are considered to be young substellar-mass objects. Furthermore, some sources could have planetary masses. In addition, we carried out CO molecular-line observations of the same cloud region. The results suggested that the YSO candidates are likely formed on the side of the parent molecular cloud and star-formation activity could be high. The K-band luminosity function of all the detected sources revealed a significant population of extremely low-luminosity sources in NGC 1333. The reddening-corrected J-band luminosity function of the YSO candidates does not clearly decline to the completeness limit and seems to be bimodal. We also argue that the fraction of substellar objects is larger than those in other young clusters and the mass function of the YSO candidates appears to be increasing toward the substellar-mass regime, similar to that of dust clumps. It implies that substellar-mass distributions may depend on the initial conditions of the molecular cloud.

An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry

Aims. We investigate the chemical relations between complex organics based on their spatial distributions and excitation conditions in the low-mass young stellar objects IRAS 16293-2422 “A” and “B”. Methods. Interferometric observations with the Submillimeter Array have been performed at 5 �� × 3 �� (800 × 500 AU) resolution revealing emission lines of HNCO, CH3CN, CH2CO, CH3CHO and C2H5OH. Rotational temperatures are determined from rotational diagrams when a sufficient number of lines are detected. Results. Compact emission is detected for all species studied here. For HNCO and CH3CN it mostly arises from source “A”, CH2CO and C2H5OH have comparable strength for both sources and CH3CHO arises exclusively from source “B”. HNCO, CH3CN and CH3CHO have rotational temperatures >200 K implying that they arise from hot gas. The (u ,v )-visibility data reveal that HNCO also has extended cold emission, which could not be previously determined through single dish data. Conclusions. The relative abundances of the molecules studied here are very similar within factors of a few to those found in highmass YSOs. This illustrates that the chemistry between high- and low-mass objects appears to be relatively similar and thus independent of luminosity and cloud mass. In contrast, bigger abundance differences are seen between the “A” and “B” source. For instance, the HNCO abundance relative to CH3OH is ∼4 times higher toward “A”, which may be due to a higher initial OCN − ice abundances in source “A” compared to “B”. Furthermore, not all oxygen-bearing species are co-existent, with CH3CHO/CH3OH an order of magnitude higher toward “B” than “A”. The different spatial behavior of CH2CO and C2H5OH compared with CH3CHO suggests that successive hydrogenation reactions on grain-surfaces are not sufficient to explain the observed gas phase abundance of the latter. Selective destruction of CH3CHO may result in the anti-coincidence of these species in source “A”. These results illustrate the power of interferometric compared with single dish data in terms of testing chemical models.

An infrared-submillimeter study of starforming regions selected by the ISOSS 170 μm survey

Using the ISOPHOT Serendipity Survey (ISOSS) at 170um a sample of galactic star-forming regions exhibiting very cold dust temperatures (< 20 K) and high masses (> 100 M_sun) has been established. We characterise the star-forming content of five regions that were selected as potential sites for early stage high-mass star formation using SCUBA (JCMT) and Spitzer observations. In every region we identify one to four submillimeter clumps with projected sizes between 0.1 and 0.4 pc. The dust temperatures range from 11.6 to 21.3 K and the estimated clump masses are 2 to 166 M_sun. Towards the majority of submillimeter peaks we find point sources in the near- to mid-infrared. Most are interpreted as low-mass young stellar objects but we also detect very red sources. They probably represent very young and deeply embedded protostars that continue to accrete clump material and may reach higher masses. Several candidate intermediate-mass proto- or pre-main-sequence stars embedded in the clumps are identified. A subset of four clumps may be massive enough (> 100 M_sun) to form high-mass stars and accompanying clusters. The absence of stellar precursors with current masses in the high-mass regime leave the type of star formation occuring in the clumps unsettled. We confirm the presence of large fractions of cold material as derived from large-scale far-infrared measurements which dominates the emission of most clumps and suggests that the star-forming process will continue.

SEQUENTIAL STAR FORMATION IN A COMETARY GLOBULE (BRC37) OF IC1396

We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an H II region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slitless spectroscopy we detected ten Hα emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally toward an O9.5 star, but has evolved to align toward a younger O6 star when that formed. Near-IR and optical photometry suggests that four of these six stars are low-mass young stellar objects (YSOs) with masses of ~0.4 M ☉. Their estimated ages of ~1 Myr indicate that they were formed at the tip in advance of the formation of the IRAS source. Therefore, it is likely that sequential star formation has been taking place along the direction from the exciting stars toward the IRAS source, due to the UV impact of the exciting star(s). Interestingly, one faint, Hα emission star, which is the closest to the exciting star(s), seems to be a young brown dwarf that was formed by the UV impact in advance of the formation of other YSOs at the tip.

STAR FORMATION IN THE SOUTHERN DARK CLOUD DC 296.2-3.6

We report near- and mid-infrared (IR) images of the southern hemisphere dark cloud DC 296.2–3.6 associated with IRAS 11431 − 6516. The Ks and L' images show the presence of an IR nebulosity at the center of the dark cloud (DC). From the analysis of the near-IR color–color diagrams we have identified a young stellar population in the region. Five of these young stellar objects, here named A, B, C, D, and E, were also detected in the mid-IR. Sources B, D, and E are Class I–II T Tauri as suggested by the analysis of their spectral energy distributions. In addition, source E shows a long-term near-IR variability. The near-IR color–color diagrams indicate the presence of circumstellar dust envelope in sources A, B, D, and E, while the fit of SEDs of the intermediate- and low-mass objects A and B with a radiation transfer model including infalling envelope+disk+central source suggests circumstellar disks around these two objects. These results indicate that DC 296.2–3.6, located in the far Carina arm, is associated with an embedded cluster of low-mass young stellar objects.

The c2dSpitzerSpectroscopic 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. A total of 25 out of 52 ice sources in the "Cores to Disks" (c2d) Legacy program have an absorption feature at 7.7 μm, 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%. The latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2 × 1018 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 freezeout.

Precursors of complex organic molecules: NH3 and CH3OH in the ices surrounding low-mass protostars

AbstractNH3 and CH3OH are key molecules in the chemical networks leading to the formation of complex N- and O-bearing organic molecules. However, despite a number of recent studies, there is still a lot to learn about their abundances in the solid state and how they relate to those of other N/O-bearing organic molecules or to NH3 and CH3OH abundances in the gas phase. This is particularly true in the case of low-mass young stellar objects (YSOs), for which only the recent advent of the Spitzer Space Telescope has allowed high sensitivity observations of the ices in their enveloppes. We present a combined study of Spitzer data (obtained within the Legacy program “From Molecular Cores to Planet-Forming Disks”, c2d) and laboratory spectra, leading to the detections of NH3 and CH3OH in the ices of low-mass protostars. We investigate correlations with other ice features and conclude with prospects on further studies linking these two precursors of complex organic molecules with their gas-phase products.

Astrometry of H$_2$O Masers in Nearby Star-Forming Regions with VERA I. IRAS 16293$-$2422 in $\rho$ Oph East

Abstract We report on results of multi-epoch VLBI observations of H$_2$O masers associated with a low-mass young stellar object, IRAS 16293$-$2422 in $\rho$ Oph East, and a fringe-phase and position reference source, ICRF J162546.8$-$252738, using the VLBI Exploration of Radio Astrometry (VERA) for high-precision astrometry. We obtained an annual parallax of a maser feature to be $\pi=$ 5.6$^{+1.5}_{-0.5}$ mas, corresponding to a distance of $D=178^{+18}_{-37}$ pc. We also found 10 relative proper motions of maser features with respect to the maser feature mentioned above. The motion of the accompanying young stellar object (YSO) has already been found in thermal continuum emission previously observed with the Very Large Array. The intrinsic motions of masers have been estimated from the relative proper motions after the YSO’s motion is subtracted from, and a systemic secular motion of the position reference feature is added to the proper motions originally measured. The intrinsic maser kinematical structure may trace a bipolar outflow.

ASTE Submillimeter Observations of a Young Stellar Object Condensation in Cederblad 110

We present results of submillimeter observations of a low-mass young stellar objects (YSOs) condensation in the Cederblad 110 region of the Chamaeleon I dark cloud with Atacama Submillimeter Telescope Experiment. Our HCO+(J=4-3) map reveals a dense molecular gas with an extent of ~ 0.1 pc, which is a complex of two envelopes associated with class I sources Ced110 IRS4 and IRS11 and a very young object Cha-MMS1. The other two class I sources in this region, IRS6 and NIR89, are located outside the clump and have no extended HCO+ emission. HCO+ abundance is calculated to be 2.6 x 10^{-10} for MMS1 and 3.4 x 10^{-9} for IRS4, which are comparable to the reported value for other young sources. Bipolar outflows from IRS4 and IRS6 are detected in our 12CO(J=3-2) map. The outflow from IRS4 seems to collide with Cha-MMS1. The outflow has enough momentum to affect gas motion in MMS1, although no sign has been detected to indicate that a triggered star formation has occurred.

Tracing high energy radiation with molecular lines near deeply embedded protostars

Aims. The aim is to probe high energy radiation emitted by deeply embedded protostars. Methods. Submillimeter lines of CN, NO, CO + and SO + , and upper limits on SH + and N 2 O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models. Results. Constant fractional abundances derived from radiative transfer modeling of the line strengths are $x({\rm CN}) \approx$ a few $\times$10 -11 –10 -8 , $x({\rm NO}) \approx 10^{-9}$–10 -8 and $x({\rm CO^+}) \approx 10^{-12}$–10 -10 . SO + has abundances of a few $\times 10^{-11}$ in the high-mass objects and upper limits of ≈ 10 -12 –10 -11 in the low-mass sources. All abundances are up to 1–2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region ( ≲ 1000 AU) of the envelope. For high-mass sources, the CN, SO + and CO + abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of $T \gtrsim 300$ K are much higher than found in models, so that an X-ray enhanced region close to the protostar ($r \lesssim 500$ AU) is more plausible. Gas-phase chemical models produce more NO than observed, suggesting an additional reduction mechanism not included in current models. Conclusions. The observed CN, CO + and SO + abundances can be explained with either enhanced X-rays or FUV fields from the central source. High-mass sources likely have low opacity regions that allow the FUV photons to reach large distances from the central source. X-rays are suggested to be more effective than FUV fields in the low-mass sources. The observed abundances imply X-ray fluxes for the Class 0 objects of $L_{\rm X} \approx 10^{29}$–10 31 erg s -1 , comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.

A Search for Circumstellar Disks in Low Mass Young Stellar Objects and Young Brown Dwarfs

I present a key project relative to the determination of the physical characteristics of the circumstellar disks around low mass young stellar objects (YSOs) and young brown dwarfs, that could be developed from the Antarctic Plateau at Dome Concordia station with a 2–3 m class telescope equipped with a mid-infrared camera.

Near-Infrared Polarization Images of the Orion Nebula

Wide-field (~8' x 8') and deep near-infrared (JHKs bands) polarization images of the Orion nebulae (IRNe) around young stellar objects (YSOs), both massive and low-mass. We found the IRNe around both IRc2 and BN to be very extensive, suggesting that there might be two extended (>0.7 pc) bipolar/monopolar IRNe in these sources. We discovered at least 13 smaller-scale (~0.01-0.1 pc) IRNe around less-massive YSOs including the famous source theta^2 Ori C. We also suggest the presence of many unresolved (<690 AU) systems around low-mass YSOs and young brown dwarfs showing possible intrinsic polarizations. Wide-field infrared polarimetry is thus demonstrated to be a powerful technique in revealing IRNe and hence potential disk/outflow systems among high-mass to substellar YSOs.

A Sensitive Survey for Water Maser Emission Toward Bok Globules Using the Robledo 70 m Antenna

We report on the most sensitive water maser survey toward Bok globules to date, performed using NASA's 70 m antenna at Robledo de Chavela (Spain). We observed 207 positions within the Clemens and Barvainis catalog with a higher probability of harboring a young star, using as selection criteria the presence of radio continuum emission (from submillimeter to centimeter wavelengths), geometric centers of molecular outflows, peaks in maps of high-density gas tracers (NH3 or CS), and IRAS point sources. We have obtained seven maser detections, six of which (in CB 34, CB 54, CB 65, CB 101, CB 199, and CB 232) are reported for the first time here. Most of the water masers we detected are likely to be associated with young stellar objects (YSOs), except for CB 101 (probably an evolved object) and CB 65 (uncertain nature). The water maser in CB 199 shows a relatively high shift (30 km s-1) of its velocity centroid with respect to the cloud velocity, which is unusual for low-mass YSOs. We speculate that high-velocity masers in this kind of object could be related to episodes of energetic mass loss in close binaries. Alternatively, the maser in CB 199 could be pumped by a protoplanetary or a young planetary nebula. CB 232 is the smallest Bok globule (0.6 pc) known to be associated with water maser emission, although it would be superseded by the cases of CB 65 (0.3 pc) and CB 199 (0.5 pc) if their association with YSOs is confirmed. All our selection criteria have statistically compatible detection rates, except for IRAS sources, which tend to be somewhat worse predictors for the presence of maser emission.

Photometric and spectroscopic studies of very low mass YSOs and young Brown Dwarfs in S106

Young brown dwarfs have been identified in a significant population in various star forming regions. Some deep surveys have yielded less massive objects with planetary-mass (e.g., Oasa et al. 1999; Lucas &amp; Roche 2000). Nevertheless, it is not yet clear how abundant these very low-mass objects are formed. S106 is one of the nearest massive star-forming regions associated with prominent bipolar nebulae and an HII region. We have conducted near-infrared photometric and spectroscopic observations of very low-mass young stellar objects (YSOs) in the S106 region.

Infrared spectroscopy of solid CO–CO2 mixtures and layers

The spectra of pure, mixed and layered CO and CO2 ices have been studied systematically under laboratory conditions using Transmission-Absorption Fourier Transform infrared spectroscopy. This work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and asymmetric stretching mode, as well as the CO stretching mode, extending the existing Leiden databasea of laboratory spectra to match the spectral resolution reached by modern telescopes and to support the interpretation of the most recent data from the Spitzer Space Telescope. It is shown that mixed and layered CO and CO2 ices exhibit very different spectral characteristics, which depend critically on thermal annealing and can be used to distinguish between mixed, layered and thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in mixed ices below 50 K under the current experimental conditions, where it exhibits a single asymmetric band profile in intimate mixtures. In all other ice morphologies the CO2 bending mode shows a double peaked profile, similar to that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in mixed ices, and 2140–2146 cm-1 in layered ices. As such, the CO2 bending mode puts clear constraints on the ice morphology below 50 K, whereas beyond this temperature the CO2 stretching vibration can distinguish between initially mixed and layered ices. This is illustrated for the low-mass young stellar object HH 46, where the laboratory spectra are used to analyse the observed CO and CO2 band profiles and try to constrain the formation scenarios of CO2.

Ammonia cores in high mass star formation regions

We observed a sample of 35 water masers not coincident with known HII regions and/or low mass young stellar objects (YSOs) with the Effelsberg 100 m telescope in the NH3(J,K) = (1,1), (2,2), (3,3) and (4,4) transitions. Sixteen sources were detected in the NH3 emission. The detection rate is 46%. All these sixteen sources have NH3 (1,1) and (2,2) emission, among which four sources have NH3 (3,3) emission. Comparing with the IRAS and the 2MASS data, we analyzed the relationship between the detection rate and the infrared color, the dust temperature and the source distance. All the detected sources were mapped and 17 cores were obtained (one source IRAS 20215+3725 has two cores). From the detected sources five cores do not coincide with radio continuum or IRAS and MSX point sources. Excluding one core that has no MSX data available, the remaining eleven cores are coincident with IRAS or MSX point sources. The typical size and mass of the cores are 1.6 pc and 1.5 x 10(3) M-circle dot, respectively. The average line widths of the NH3 (1,1) and (2,2) are 1.54 and 1.73 km s(-1). The average kinetic temperature of the gas is about 19 K. These values are much larger than those of low mass cores. The NH3 cores that coincide with IRAS sources (referred to as Group I) have slightly larger line widths (1.65 and 1.75 km s(-1) for the (1,1) and (2,2) lines, respectively) and larger masses (1.8 x 10(3) M-circle dot) than the mean values of the sample. For this type of core the kinetic temperature correlates with the line width. The line width appears to correlate with the bolometric luminosity and the core size. Despite the average luminosity of 2.9 x 10(4) L-circle dot, there is no detectable 6 cm emission. These are candidates for high mass protostars or precursors of UC HII regions. The NH3 cores with peaks onset from infrared sources (referred to as Group II) have an average size of 1.7 pc and an average line width of 1.50 km s(-1) for the (1,1) line. The line width of the (1,1) emission is smaller than that of the group I. The average mass is 9.4 x 10(2) M-circle dot. One possible explanation for the deviation is that the NH3 peak and the infrared source correspond to different clumps. These cores are potential high mass star formation sites and may be at an earlier evolutionary stage than those with IRAS point sources. This type of core is seen in mapping observations, and can be easily missed by single-spectrum observations toward the IRAS position.

Very Low Luminosity Young Cluster and the Luminosity and Mass Functions in S106

We present the results of deep near-infrared observations searching for very low mass young stellar objects (YSOs) in the massive star-forming region of S106 taken with the Subaru Telescope. The survey, whose limiting magnitude exceeds 20 mag in the JHK' bands, is sensitive enough to provide unprecedented details in the two nebular lobes. In addition, it reveals a census of the stellar population down to objects below the deuterium-burning limit, a fiducial boundary between brown dwarfs and planetary-mass objects. Based on color-color diagrams, nearly 600 embedded YSO candidates with near-infrared excesses have been identified in an area of ∼5' × 5' that are not uniformly distributed but centrally concentrated. Combining the reddening-corrected luminosity of the YSO candidates with the theoretical evolutionary models, we suggest that there exists a substantial substellar population. If we divide the cluster into three subgroups based on stellar and cloud properties, both their luminosity functions and mass functions across the stellar-substellar boundary demonstrate local variation on a subparsec scale. Nevertheless, all the mass functions have distinct characteristics in common: they do not have any turnover and do not decline down to the completeness limits. Furthermore, the mass function for S106 appears to be more abundant in young substellar objects compared to those obtained for other young clusters, such as Trapezium and IC 348. This implies that the substellar mass function for young clusters may have a variation.

Hot Organic Molecules toward a Young Low-Mass Star: A Look at Inner Disk Chemistry

Spitzer Space Telescopespectra of the low-mass young stellar object (YSO) IRS 46 ( ) in Ophiuchus L ≈ 0.6 L bol , reveal strong vibration-rotation absorption bands of gaseous C 2H2, HCN, and CO2. This is the only source out of a sample of ∼100 YSOs that shows these features, and this is the first time that they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra reveals excitation temperatures of 350 K and abundances of 10 6 to 10 5 with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN line is barely detected with the J p 4–3 James Clerk Maxwell Telescope (JCMT), indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limit the mass and temperature of any remnant collapsing envelope to less than 0.01 M, and 100 K, respectively. This excludes a hot-core–type region as found in high-mass YSOs. The most plausible origin of this hot gas rich in organic molecules is in the inner (!6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edgeon two-dimensional disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe the organic chemistry, gas temperatures, and gas kinematics in the planet-forming zones close to a young star. Subject headings: infrared: ISM — ISM: individual (IRS 46) — ISM: jets and outflows — ISM: molecules — planetary systems: protoplanetary disks — stars: formation