Ophiuchus Molecular Cloud Research Articles

Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud

The ρ Oph molecular cloud is undergoing intermediate-mass star formation. Ultraviolet radiation from its hottest young stars heats and dissociates exposed layers, but does not ionize hydrogen. Only faint radiation from the Rayleigh-Jeans tail of ∼10–100 K dust is expected at wavelengths longwards of ∼3 mm. Yet cosmic background imager (CBI) observations reveal that the ρ Oph W photodissociation region is surprisingly bright at centimetre wavelengths. We searched for interpretations consistent with the Wilkinson Microwave Anisotropy Probe radio spectrum, new Infrared Space Observatory-Long Wavelength Spectrograph (LWS) parallel mode images and archival Spitzer data. Dust-related emission mechanisms at 1 cm, as proposed by Draine & Lazarian, are a possibility. But a magnetic enhancement of the grain opacity at 1 cm is inconsistent with the morphology of the dust column maps Nd and the lack of detected polarization. Spinning dust, or electric-dipole radiation from spinning very small grains (VSGs), comfortably explains the radio spectrum, although not the conspicuous absence from the CBI data of the infrared circumstellar nebulae around the B-type stars S1 and SR3. Allowing for VSG depletion can marginally reconcile spinning dust with the data. As an alternative interpretation, we consider the continuum from residual charges in ρ Oph W, where most of carbon should be photoionized by the close binary HD 147889 (B2IV, B3IV). Electron densities of ∼102cm−3, or H-nucleus densities nH > 106cm−3, are required to interpret ρ Oph W as the C ii Strömgren sphere of HD 147889. However, the observed steep and positive low-frequency spectral index would then imply optically thick emission from an hitherto unobserved ensemble of dense clumps or sheets with a filling factor of ∼10−4 and nH∼ 107cm−3.

The structures of embedded clusters in the Perseus, Serpens and Ophiuchus molecular clouds

The young stellar population data of the Perseus, Ophiuchus and Serpens molecular clouds are obtained from the Spitzer Cores to Discs (c2d) legacy survey in order to investigate the spatial structure of embedded clusters using the nearest-neighbour (NN) and minimum-spanning tree method. We identify the embedded clusters in these clouds as density enhancements and analyse the clustering parameter Q with respect to source luminosity and evolutionary stage. This analysis shows that the older Class 2/3 objects are more centrally condensed than the younger Class 0/1 protostars, indicating that clusters evolve from an initial hierarchical configuration to a centrally condensed one. Only IC 348 and the Serpens core, the older clusters in the sample, show signs of mass segregation (indicated by the dependence of Q on the source magnitude), pointing to a significant effect of dynamical interactions after a few Myr. The structure of a cluster may also be linked to the turbulent energy in the natal cloud as the most centrally condensed cluster is found in the cloud with the lowest Mach number and vice versa. In general, these results agree well with theoretical scenarios of star cluster formation by gravoturbulent fragmentation.

Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid‐Infrared Surveys. II.

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the Cores to Disks (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these datasets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores - less than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion - including masses, star formation efficiencies, fraction of embedded sources etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC1333 and IC348) are found to have higher star formation efficiencies than small groups such as B1, L1455 and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.

PRELIMINARY ORBIT OF THE YOUNG BINARY HARO 1-14c

Using the Keck Interferometer, we spatially resolved the orbit of the pre-main sequence binary, Haro 1-14c, for the first time. We present these interferometric observations along with additional spectroscopic radial velocity measurements of the components. We performed a simultaneous orbit fit to the interferometric visibilities and the radial velocities of Haro 1-14c. Based on a statistical analysis of the possible orbital solutions that fit the data, we determined component masses of M_1 = 0.96 (+0.27/-0.08) Msun and M_2 = 0.33 (+0.09/-0.02) Msun for the primary and secondary, respectively, and a distance to the system of 111 (+19/-18) pc. The distance measurement is consistent with the close distance estimates of the Ophiuchus molecular cloud. Comparing our results with evolutionary tracks suggests an age of 3-4 Myr for Haro 1-14c. With additional interferometric measurements to improve the uncertainties in the masses and distance, we expect the low-mass secondary to provide important empirical data for calibrating the theoretical evolutionary tracks for pre-main sequence stars.

On the distance to the Ophiuchus star‐forming region

AbstractThe Ophiuchus molecular cloud complex has produced in Lynds 1688 the richest known embedded cluster within ∼300 pc of the Sun. Unfortunately, distance estimates to the Oph complex vary by nearly ∼40% (∼120–165 pc). Here I calculate a new independent distance estimate of 135±8 pc to this benchmark star‐forming region based on Hipparcos trigonometric parallaxes to stars illuminating reflection nebulosity in close proximity to Lynds 1688. Combining this value with recent distance estimates from reddening studies suggests a consensus distance of 139±6 pc (4% error), situating it within ∼11 pc of the centroid of the ∼5 Myr old Upper Sco OB subgroup of Sco OB2 (145 pc). The velocity vectors for Oph and Upper Sco are statistically indistinguishable within ∼1 km s–1 in each vector component. Both Oph and Upper Sco have negligible motion (<1 km s–1) in the Galactic vertical direction with respect to the Local Standard of Rest, which is inconsistent with the young stellar groups having formed via the high velocity cloud impact scenario. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The initial conditions of star formation in the Ophiuchus main cloud: Kinematics of the protocluster condensations

The earliest phases of clustered star formation and the origin of the stellar initial mass function (IMF) are currently much debated. In order to constrain the origin of the IMF, we investigated the internal and relative motions of starless condensations and protostars previously detected by us in the dust continuum at 1.2mm in the L1688 protocluster of the Ophiuchus molecular cloud complex. The starless condensations have a mass spectrum resembling the IMF and are therefore likely representative of the initial stages of star formation in the protocluster. We carried out detailed molecular line observations, including some N2H+(1-0) mapping, of the Ophiuchus protocluster condensations using the IRAM 30m telescope. We measured subsonic or at most transonic levels of internal turbulence within the condensations, implying virial masses which generally agree within a factor of ~ 2 with the masses derived from the 1.2mm dust continuum. This supports the notion that most of the L1688 starless condensations are gravitationally bound and prestellar in nature. We measured a global one-dimensional velocity dispersion of less than 0.4 km/s between condensations. This small relative velocity dispersion implies that, in general, the condensations do not have time to interact with one another before evolving into pre-main sequence objects. Our observations support the view that the IMF is partly determined by cloud fragmentation at the prestellar stage. Competitive accretion is unlikely to be the dominant mechanism at the protostellar stage in the Ophiuchus protocluster, but it may possibly govern the growth of starless, self-gravitating condensations initially produced by gravoturbulent fragmentation toward an IMF, Salpeter-like mass spectrum.

A Survey for Young Spectroscopic Binary K7–M4 Stars in Ophiuchus

This paper describes a high-resolution, infrared spectroscopic survey of young, low-mass stars designed to identify and characterize pre-main-sequence spectroscopic binaries. This is the first large infrared radial velocity survey of very young stars to date. The frequency and mass ratio distribution of the closest, low-mass binaries bear directly on models of stellar, brown dwarf, and planetary mass companion formation. Furthermore, spectroscopic binaries can provide mass ratios and ultimately masses, independent of assumptions, needed to calibrate models of young star evolution. I present the initial results from observations of a uniform sample of 33 T Tauri M stars in the Ophiuchus molecular cloud. The average mass of this sample is less than that of other young star radial velocity surveys of similar scope by a factor of ~2. Almost every star was observed at 3-4 epochs over 3 years with the 10 meter Keck II telescope and the facility infrared spectrometer NIRSPEC. An internal precision of 0.43 km/s was obtained with standard cross-correlation calibration techniques. Four of the targets are newly discovered spectroscopic binaries, one of which is located in a sub-arcsecond, hierarchical quadruple system. Three other sub-arcsecond visual binaries were also serendipitously identified during target acquisition. The spectroscopic multiplicity of the sample is comparable to that of earlier type, pre-main-sequence objects. Therefore, there is no dearth of young, low-mass spectroscopic binary stars, at least in the Ophiuchus region.

Deep Spitzer Spectroscopy of the "Flying Saucer" Edge-on Disk: Large Grains beyond 50 AU

We present deep Spitzer IRS low-resolution (λ/Δλ ~ 100) 5-35 μm spectroscopy of the edge-on disk the Flying Saucer (2MASS J16281370-2431391) in the Ophiuchus molecular cloud. The SED exhibits the characteristic two-peak shape predicted for a circumstellar disk viewed very close to edge-on. The short-wavelength peak is entirely due to photons scattered off the surface of the disk, while the long-wavelength peak is due to thermal emission from the disk itself. The Spitzer spectrum represents the first spectroscopic detection of scattered light out to 15 μm from a bona fide, isolated edge-on disk around a T Tauri star. The depth and the wavelength of the mid-infrared valley of the SED give direct constraints on the size distribution of large grains in the disk. Using a 2D continuum radiative transfer model, we find that a significant amount of 5-10 μm-sized grains is required in the surface layers of the disk at radii of 50-300 AU. The detection of relatively large grains in the upper layers implies that vertical mixing is effective, since grain growth models predict that the grains would otherwise settle deep in the disk on short timescales. Additionally, we tentatively detect the 9.66 μm S(3) line of H_2 and the 11.2 μm emission feature due to PAHs.

The Large‐ and Small‐Scale Structures of Dust in the Star‐forming Perseus Molecular Cloud

We present an analysis of ~3.5 deg2 of submillimeter continuum and extinction data of the Perseus molecular cloud. We identify 58 clumps in the submillimeter map, and we identify 39 structures ("cores") and 11 associations of structures ("super cores") in the extinction map. The cumulative mass distributions of the submillimeter clumps and extinction cores have steep slopes (α ~ 2 and 1.5-2, respectively), steeper than the Salpeter initial mass function (IMF; α = 1.35), while the distribution of extinction super cores has a shallow slope (α ~ 1). Most of the submillimeter clumps are well fit by stable Bonnor-Ebert spheres with 10 K < T < 19 K and 5.5 <(Pext/k) < 6.0. The clumps are found only in the highest column density regions (AV > 5-7 mag), although Bonnor-Ebert models suggest that we should have been able to detect them at lower column densities if they exist. These observations provide a stronger case for an extinction threshold than that found in analysis of less sensitive observations of the Ophiuchus molecular cloud (Johnstone et al.). The relationship between submillimeter clumps and their parent extinction core has been analyzed. The submillimeter clumps tend to lie offset from the larger extinction peaks, suggesting that the clumps formed via an external triggering event, consistent with previous observations.

Bolocam Survey for 1.1 mm Dust Continuum Emission in the c2d Legacy Clouds. II. Ophiuchus

We present a large-scale millimeter continuum map of the Ophiuchus molecular cloud. Nearly 11 deg2, including all of the area in the cloud with AV ≥ 3 mag, was mapped at 1.1 mm with Bolocam on the Caltech Submillimeter Observatory (CSO). By design, the map also covers the region mapped in the infrared with the Spitzer Space Telescope. We detect 44 definite sources, and a few likely sources are also seen along a filament in the eastern streamer. The map indicates that dense cores in Ophiuchus are very clustered and often found in filaments within the cloud. Most sources are round, as measured at the half-power point, but elongated when measured at lower contour levels, suggesting spherical sources lying within filaments. The masses, for an assumed dust temperature of 10 K, range from 0.24 to 3.9 M☉, with a mean value of 0.96 M☉. The total mass in distinct cores is 42 M☉, 0.5%-2% of the total cloud mass, and the total mass above 4 σ is about 80 M☉. The mean densities in the cores are quite high, with an average of 1.6 × 106 cm-3, suggesting short free-fall times. The core mass distribution can be fitted with a power law with slope α = 2.1 ± 0.3 for M > 0.5 M☉, similar to that found in other regions, but slightly shallower than that of some determinations of the local IMF. In agreement with previous studies, our survey shows that dense cores account for a very small fraction of the cloud volume and total mass. They are nearly all confined to regions with AV ≥ 9 mag, a lower threshold than found previously.

A SCUBA survey of L1689 - the dog that didn't bark

We present submillimetre data for the L1689 cloud in the ρ Ophiuchi molecular cloud complex. We detect a number of starless and pre-stellar cores and protostellar envelopes. We also detect a number of filaments for the first time in the submillimetre continuum that are parallel both to each other, and to filaments observed in the neighbouring L1688 cloud. These filaments are also seen in the 13 CO observations of L1689. The filaments contain all of the star-formation activity in the cloud. L1689 lies next to the well-studied L1688 cloud that contains the ρ Oph-A core. L1688 has a much more active star-formation history than L1689 despite their apparent similarity in 13 CO data. Hence, we label L1689 as the dog that didn’t bark. We endeavour to explain this apparent anomaly by comparing the total mass of each cloud that is currently in the form of dense material such as pre-stellar cores. We note firstly that L1688 is more massive than L1689, but we also find that when normalized to the total mass of each cloud, the L1689 cloud has a much lower percentage of mass in dense cores than L1688. We attribute this to the hypothesis of Loren that the star formation in the ρ Ophiuchi complex is being affected and probably dominated by the external influence of the nearby Upper Scorpius OB association and predominantly by σ Sco. L1689 is further from σ Sco and is therefore less active. The influence of σ Sco appears none the less to have created the filaments that we observe in L1689.

The Dust‐scattered X‐Ray Halo aroundSwiftGRB 050724

This paper discusses the X-ray halo around the Swift gamma-ray burst GRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo, which forms a ring around the fading X-ray source, expands to a radius of 200 within 8 ks of the burst exactly as expected for small-angle X-ray scattering by Galactic dust along the line of sight to a cosmologically distant GRB. The expansion curve and radial profile of the halo constrain the scattering dust to be concentrated at a distance of D = 139 +/- 9 pc (from Earth) in a cloud/sheet of thickness delta-D 0.022; this is a lower limit to the true value because contribution from smaller grains, which scatter to larger angles, was not directly observed. The line-of-sight to the GRB passes close to the Ophiuchus molecular cloud complex, which provides a plausible site for the scattering dust.

An unbiased search for the signatures of protostars in the ρ Ophiuchi molecular cloud

The dense cores which conceive and cradle young stars can be explored through continuum emission from associated dust grains. We have performed a wide field survey for dust sources at 1.2 millimetres in the ρ Ophiuchi molecular cloud, covering more than 1 square degree in an unbiased fashion. We detect a number of previously unknown sources, ranging from extended cores over compact, starless cores to envelopes surrounding young stellar objects of Class 0, Class I, and Class II type. We analyse the mass distribution, spatial distribution and the potential equilibrium of the cores. For the inner regions, the survey results are consistent with the findings of previous narrower surveys. The core mass function resembles the stellar initial mass function, with the core mass function shifted by a factor of two to higher masses (for the chosen opacity and temperature). In addition, we find no statistical variation in the core mass function between the crowded inner regions and those in more isolated fields except for the absence of the most massive cores in the extended cloud. The inner region contains compacter cores. This is interpreted as due to a medium of higher mean pressure although strong pressure variations are evident in each region. The cores display a hierarchical spatial distribution with no preferred separation scale length. However, the frequency distribution of nearest neighbours displays two peaks, one of which at 5000 AU can be the result of core fragmentation. The orientations of the major axes of cores are consistent with an isotropic distribution. In contrast, the relative orientations of core pairs are preferentially in the NW-SE direction on all separation scales. These results are consistent with core production and evolution in a turbulent environment. Finally, we report the discovery of a new, low-mass Class 0 object candidate and its CO outflow.

Dust Properties in the Far‐Ultraviolet in Ophiuchus

We have derived the albedo (a) and phase function asymmetry factor (g) of interstellar dust grains at 1100 A using archival Voyager observations of diffuse radiation in Ophiuchus. We have found that the grains are highly forward-scattering, with g = 0.55 ± 0.25 and a = 0.40 ± 0.10. Even though most of the gas in this direction is in the Ophiuchus molecular cloud, the diffuse FUV radiation is almost entirely due to scattering in a relatively thin foreground cloud. This suggests that one cannot assume that the UV background is directly correlated with the total amount of gas in any direction.

Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multiobject spectrograph. The objects extend over a 1.3 deg2 area surrounding the main cloud of the ρ Oph complex. The objects were selected from narrowband images to have Hα in emission. Using the presence of strong Hα emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported X-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong Hα emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, X-ray-emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a median age for this population of 2.1 Myr, which is significantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low-mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper Centaurus subgroup.

A multiplicity survey of the ρ Ophiuchi molecular clouds

We present a volume-limited multiplicity survey with magnitude cutoff () of 158 young stellar objects located within or in the vicinity of the ρ Ophiuchi Dark Cloud. With exception of eleven already well observed objects, all sources have been observed by us in the K-band with 3.5 m telescopes by using speckle techniques. The separation range covered by our survey is , where the lower limit is given by the diffraction limit of the telescopes and the upper limit by confusion with background stars. The multiplicity survey is complete for flux ratios 0.1 () at the diffraction limit. After taking the background density into account the degree of multiplicity is and thus only marginally higher than the value derived for the given separation range for the main-sequence solar-like stars in the solar neighbourhood (Duquennoy & Mayor 1991). We discuss the implications of these findings.

Large‐Scale CO and [Ci] Emission in the ρ Ophiuchi Molecular Cloud

We present a comprehensive study of the ρ Ophiuchi molecular cloud that addresses aspects of the physical structure and condition of the molecular cloud and its photodissociation region (PDR) by combining far-infrared and submillimeter-wave observations with a wide range of angular scale and resolution. We present 1600 arcmin2 maps (2.3 pc2) with 0.1 pc resolution in submillimeter CO (4 → 3) and [C I] (3P1 → 3P0) line emission from the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) and pointed observations in the CO (7 → 6) and [C I] ( 3P2 → 3P1) lines. Within the large-scale maps, smaller spectral line maps of 3000 AU resolution over ~90 arcmin2 (0.2 pc2) of the cloud in CO, CS, HCO+, and their rare isotopomers are made at the Heinrich Hertz Telescope (HHT) in Arizona. Comparison of CO, HCO+, and [C I] maps with far-infrared observations of atomic and ionic species from the Infrared Space Observatory (ISO) far-infrared and submillimeter continuum emission and near-infrared H2 emission allows clearer determination of the physical and chemical structure of the ρ Oph PDR, since each species probes a different physical region of the cloud structure. Although a homogeneous plane-parallel PDR model can reproduce many of the observations described here, the excitation conditions needed to produce the observed HCO+ and [O I] emission imply inhomogeneous structure. Strong chemical gradients are observed in HCO+ and CS; the former is ascribed to a local enhancement in the H2 ionization rate, and the latter is principally due to shocks. Under the assumption of a simple two-component gas model for the cloud, we find that [C II] and [C I] emission predominantly arises from the lower density envelopes (103-104 cm-3) that surround denser cloud condensations, or clumps. The distribution of [C I] is very similar to that of C18O and is generally consistent with illumination from the far side of the cloud. A notable exception is found at the western edge of the cloud, where UV photons create a PDR viewed edge-on. The abundance of atomic carbon is accurately modeled using a radiation field that decreases with increasing projected distance from the exciting star HD 147889 and a total gas column density that follows that of C18O, decreasing toward the edges of the cloud. In contrast to the conclusions of other studies, we find that no nonequilibrium chemistry is needed to enhance the [C I] abundance. Each spectral line is traced to a particular physical component of the cloud and PDR. Although CO rotational line emission originates from both dense condensations and diffuse envelopes, the millimeter-wave transitions mostly find their origins in envelope material, whereas the high-J submillimeter lines stem more from the dense clumps. Submillimeter HCO+ and infrared [O I] and [C II] emission indicate clump surface temperatures of 50-200 K, an ultraviolet radiation field with IUV ≈ 10-90, densities of 105-106 cm-3, and interior temperatures of ≤20 K. This study highlights the value of large-scale infrared and submillimeter mapping for the interpretation of molecular cloud physical and chemical structure, and important future observations are highlighted.

An unbiased search for the signatures of protostars in the ρ Ophiuchi A molecular cloud

We present an unbiased search for molecular hydrogen emission in the L1688 cloud within the ρ Ophiuchi molecular cloud complex. Our near-infrared survey covers a connected region of extent 35´ $\times$ 35´. We detect several new H 2 flows but the total number of detected outflows is low and is consistent with the paucity of Class 0 and Class 1 sources in the molecular cloud. From the spatial distribution, their collimation and the individual shapes of the bow shocks, we suggest possible candidates for the outflow sources. Most of the candidate driving sources are deeply embedded in dense cores of the molecular cloud. A very young outflow arises from the newly discovered Class 0 source MMS 126. Two major outflows in the NE–SW direction arise from the YLW 15 and YLW 16 Class I sources. Three additional outflows, which both extend over several arcminutes, arise from the Class I sources YLW 31 and YLW 52. Flow directions are generally NE–SW, perpendicular to the elongation directions of the cloud filaments. The apparent extents of molecular flows are related to either the widths of cloud filaments or to the separation between filaments. The estimated jet power needed to continuously drive and excite the detected portions of the shocked H 2 outflows lies in the range 0.02-0.2 $L_\odot$. Given the critical dependence on the environment, however, the total sizes and powers of the outflows may be considerably larger.

On the Relative Abundance of C18O and C17O in the Taurus Molecular Cloud

We present measurements of the C18O/C17O abundance ratio based on observations of J = 1 → 0 lines of these isotopomers toward star-forming cores in the Taurus molecular cloud. Our data set includes measurements along 648 lines of sight through these clouds, covering both low and high column density regions. We compare the integrated intensity ratio for each line of sight with a simple model of emission from a dense cloud to determine this abundance ratio. Using this model, we find that a C18O/C17O abundance ratio of 4.0 ± 0.5 is consistent with the data. However, at low column densities, it appears that a higher abundance ratio may be more appropriate. We examine ways in which the abundance ratio might be changed in the outer parts of molecular clouds and conclude that selective photodissociation of C17O by external ultraviolet light can increase the abundance ratio. A two-phase model, incorporating a C17O-free of cloud material surrounding a self-shielded inner cloud region, is fitted to the data. Using this model and an assumed sheath H2 column density of 4 × 1021 cm-2, we find a lower abundance ratio of 2.8 ± 0.4 for the material in the shielded inner cloud. This new result is consistent with recent results from ultraviolet absorption spectroscopy through translucent clouds and measurements of the 13C18O/13C17O ratio in the Ophiuchus molecular cloud.

A Spectroscopic Technique for Measuring Stellar Properties of Pre-Main-Sequence Stars

We describe a technique for deriving effective temperatures, surface gravities, rotation velocities, and radial velocities from high-resolution near-IR spectra. The technique matches the observed near-IR spectra to spectra synthesized from model atmospheres. Our analysis is geared toward characterizing heavily reddened pre–main-sequence stars, but the technique also has potential applications in characterizing main-sequence and post–main-sequence stars when these lie behind thick clouds of interstellar dust. For the pre–main-sequence stars, we use the same matching process to measure the amount of excess near-IR emission (which may arise in the protostellar disks) in addition to the other stellar parameters. The information derived from high-resolution spectra comes from line shapes and the relative line strengths of closely spaced lines. The values for the stellar parameters we derive are therefore independent of those derived from low-resolution spectroscopy and photometry. The new method offers the promise of improved accuracy in placing young stellar objects on evolutionary model tracks. Tests with an artificial noisy spectrum with typical stellar parameters and a signal-to-noise ratio of 50 indicate a 1 σ error of 100 K in Teff, 2 km s-1 in v sin i, and 0.13 in continuum veiling for an input veiling of 1. If the flux ratio between the sum of the Na, Sc, and Si lines at 2.2 μm and the (2–0) 12CO band head at 2.3 μm is known to an accuracy of 10%, the errors in our best-fit value for log g will be Δ log g = 0.1–0.2. We discuss the possible systematic effects on our determination of the stellar parameters and evaluate the accuracy of the results derivable from high-resolution spectra. In the context of this evaluation, we quantitatively explore the degeneracy between temperature and gravity that has bedeviled efforts to type young stellar objects using low-resolution spectra. The analysis of high-resolution near-IR spectra of MK standards shows that the technique yields very accurate values for the effective temperature. The greatest uncertainty in comparing our results with optical spectral typing of MK standards is in the spectral type–to–effective temperature conversion for the standards themselves. Even including this uncertainty, the 1 σ difference between the optical and infrared temperatures for dwarfs at 3000–5800 K is only 140 K. In a companion paper, we present an analysis of heavily extincted young stellar objects in the ρ Ophiuchi molecular cloud.