Oph Cloud Research Articles

The James Clerk Maxwell telescope Legacy Survey of the Gould Belt: a molecular line study of the Ophiuchus molecular cloud

CO, $^{13}$CO and C$^{18}$O ${\it J}$ = 3--2 observations are presented of the Ophiuchus molecular cloud. The $^{13}$CO and C$^{18}$O emission is dominated by the Oph A clump, and the Oph B1, B2, C, E, F and J regions. The optically thin(ner) C$^{18}$O line is used as a column density tracer, from which the gravitational binding energy is estimated to be $4.5 \times 10^{39}$ J (2282 $M_\odot$ km$^2$ s$^{-2}$). The turbulent kinetic energy is $6.3 \times 10^{38}$ J (320 $M_\odot$ km$^2$ s$^{-2}$), or 7 times less than this, and therefore the Oph cloud as a whole is gravitationally bound. Thirty protostars were searched for high velocity gas, with eight showing outflows, and twenty more having evidence of high velocity gas along their lines-of-sight. The total outflow kinetic energy is $1.3 \times 10^{38}$ J (67 $M_\odot$ km$^2$ s$^{-2}$), corresponding to 21$\%$ of the cloud's turbulent kinetic energy. Although turbulent injection by outflows is significant, but does ${\it not}$ appear to be the dominant source of turbulence in the cloud. 105 dense molecular clumplets were identified, which had radii $\sim$ 0.01--0.05 pc, virial masses $\sim$ 0.1--12 $M_\odot$, luminosities $\sim$ 0.001--0.1 K~km s$^{-1}$ pc$^{-2}$, and excitation temperatures $\sim$ 10--50K. These are consistent with the standard GMC based size-line width relationships, showing that the scaling laws extend down to size scales of hundredths of a parsec, and to sub solar-mass condensations. There is however no compelling evidence that the majority of clumplets are undergoing free-fall collapse, nor that they are pressure confined.

DEEP NEAR-INFRARED IMAGING OF THE ρ Oph CLOUD CORE: CLUES TO THE ORIGIN OF THE LOWEST-MASS BROWN DWARFS

A search for young substellar objects in the rho Oph cloud core region has been made using the deep-integration Combined Calibration Scan images of the 2MASS extended mission in J, H and Ks bands, and Spitzer IRAC images at 3.6, 4.5, 5.8 and 8.0 microns. The field of view of the combined observations was 1 deg x 9.3 arcmin, and the 5 sigma limiting magnitude at J was 20.5. Comparison of the observed SEDs with the predictions of the COND and DUSTY models, for an assumed age of 1 Myr, supports the identification of many of the sources with brown dwarfs, and enables the estimation of effective temperature, Teff. The cluster members are then readily distinguishable from background stars by their locations on a plot of flux density versus Teff. The range of estimated Teff extends down to ~ 750 K, suggesting the presence of objects of sub-Jupiter mass. The results also suggest that the mass function for the rho Oph cloud resembles that of the sigma Orionis cluster based on a recent study, with both rising towards lower masses. The other main result from our study is the apparent presence of a progressive blueward skew in the distribution of J-H and H-Ks colors, such that the blue end of the range becomes increasingly bluer with increasing magnitude. We suggest that this behavior might be understood in terms of the 'ejected stellar embryo' hypothesis, whereby some of the lowest-mass brown dwarfs could escape to locations close to the front edge of the cloud, and thereby be seen with less extinction.

A YOUNG PLANETARY-MASS OBJECT IN THE ρ OPH CLOUD CORE

We report the discovery of a young planetary-mass brown dwarf in the rho Oph cloud core. The object was identified as such with the aid of a 1.5-2.4 micron low-resolution spectrum obtained using the NIRC instrument on the Keck I telescope. Based on the COND model, the observed spectrum is consistent with a reddened (Av ~ 15-16) brown dwarf whose effective temperature is in the range 1200-1800 K. For an assumed age of 1 Myr, comparison with isochrones further constrains the temperature to ~ 1400 K and suggests a mass of ~ 2-3 Jupiter masses. The inferred temperature is suggestive of an early T spectral type, which is supported by spectral morphology consistent with weak methane absorption. Based on its inferred distance (~ 100 pc) and the presence of overlying visual absorption, it is very likely to be a rho Oph cluster member. In addition, given the estimated spectral type, it may be the youngest and least massive T dwarf found so far. Its existence suggests that the initial mass function for the rho Oph star-forming region extends well into the planetary-mass regime.

A Study of the ρ Ophiuchi Cloud: Mapping the Distribution and the Motions of Interstellar Gas

Guest Investigator results from Hipparcos, combined with high-resolution optical spectra obtained with the Anglo-Australian Observatory, Kitt Peak National Observatory, and McDonald Observatory, are used to create a three-dimensional map of the stars and several interstellar gas components in the ? Oph cloud complex. Many principal gas components are interspersed among the bright stars, which can be subdivided into three main groupings on the basis of right ascension and declination. At least one of these main gas components may persist over about 10 pc laterally, and there is evidence for an overall bulk movement of the gas, as determined from the observed radial velocities. We find the diffuse portion of the cloud complex to be clumpy, scattered over a distance range of ~110-150 pc. We find the most likely distance to the dense molecular cloud to be -->122 ? 8 pc.

A survey for Fe 6.4 keV emission in young stellar objects in $\mathsf{\rho}$ Oph: The strong fluorescence from Elias 29

We report the results of a search for 6.4 keV Fe fluorescent emission in Young Stellar Objects (YSOs) with measured accretion luminosities in the ρ Oph cloud, using the existing Chandra and XMM-Newton observations of the region. A total of nine such YSOs have X-ray data with sufficiently high S /N for the 6.4 keV line to be potentially detected if present. A positive detection of the Fe 6.4 keV line is reported for one object, Elias 29, in both the XMM-Newton and the Chandra data. The 6.4 keV line is detected in Elias 29 both during quiescent and flaring emission, unlikely all previously reported detections of 6.4 keV Fe fluorescence in YSOs which were made during intense flaring. The observed equivalent width of the fluorescent line is large, Wα � 160 eV, ruling out fluorescence from diffuse circumstellar material. It is also larger than expected for simple reflection from a solar-composition photosphere or circumstellar disk, but it is compatible with being due to fluorescence from a centrally illuminated circumstellar disk. The X-ray spectrum of Elias 29 is also peculiar in terms of its high (ionized) Fe abundance, as evident from the very intense Fe  6.7 keV line emission; we speculate on the possible mechanism leading to the observed high abundance.

A Search for Close Binaries in the ρ Ophiuchi Star‐forming Region

We have carried out a new, near-infrared speckle imaging survey of 19 members of the young stellar popu- lation in the nearby (d ¼ 140 pc) � Oph cloud core. Results for four binary and one newly discovered triple system are reported. Data for all known multiple systems among the pre-main-sequence population ofOph are tabulated. We define a restricted binary fraction, Fb;r, and a restricted companion fraction, Fc;r, as counting only those systems most detectable in the present and previous high-resolution near-infrared imaging surveys having separations between 0> 1a nd� 1>1 and K-band magnitude differences DK < 3. Analysis of all the available multiplicity data results in updated values of Fb;r ¼ 24% � 11% and Fc;r ¼ 24% � 11% for the Ophiuchus pre-main-sequence population. These values are consistent with the values in the Taurus star- forming region, and Fc;r is in excess by a factor of 2 relative to the main sequence at the 1 � level. Subject headings: binaries: close — infrared: stars — stars: pre-main-sequence — techniques: high angular resolution

Structure of the Mid‐Infrared–emitting Disk around WL 16

WL16 is a unique member of the embedded young stellar population in the nearby rho Ophiuchi cloud core: its extended (900 AU diameter), high surface brightness disk is visible only at mid-infrared wavelengths. We present diffraction-limited (~ 0.3 arcsec) images, from 7.9 to 24.5 microns, of WL16 acquired at the Keck II telescope. We find the central star to be a 250 Lsun, 4 Msun, Herbig Ae star, seen through foreground material of the rho Oph cloud core that provides an extinction of Av=31+/-1 magnitudes. WL16's disk is detected through all nine observed passbands. We confirm, therefore, that the emitting particles are composed of both polycyclic aromatic hydrocarbons (PAHs) and very small (5--100 Angstroms) graphitic grains (VSGs). We confirm a resolved (1.5 diameter) core component at 7.9 and 8.8 microns, due to emission from positively charged PAHs. An enhancement in the emission at 12.5 microns at the disk's edges is found for the first time, and signals the presence of larger (>= 50-80 carbon atoms) PAHs and/or more hydrogenated PAHs than those found in the bulk of the disk. We find a disk asymmetry, observed at all nine mid-infrared wavelengths, at projected radii 1-2.5 (corresponding to 125 AU <= r <= 300 AU) from the central source.

Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar

We present high-resolution (R is approximately equal to 18,000), high signal-to-noise 2 micron spectra of two luminous, X-ray flaring Class I protostars in the rho Ophiuchi cloud acquired with the NIRSPEC (near infrared spectrograph) of the Keck II telescope. We present the first spectrum of a highly veiled, strongly accreting protostar which shows photospheric absorption features and demonstrates the stellar nature of its central core. We find the spectrum of the luminous (L (sub bol) = 10 solar luminosity) protostellar source, YLW 15, to be stellar-like with numerous atomic and molecular absorption features, indicative of a K5 IV/V spectral type and a continuum veiling r(sub k) = 3.0. Its derived stellar luminosity (3 stellar luminosity) and stellar radius (3.1 solar radius) are consistent with those of a 0.5 solar mass pre-main-sequence star. However, 70% of its bolometric luminosity is due to mass accretion, whose rate we estimate to be 1.7 x 10(exp -6) solar masses yr(exp -1). We determine that excess infrared emission produced by the circumstellar accretion disk, the inner infalling envelope, and accretion shocks at the surface of the stellar core of YLW 15 all contribute significantly to its near-IR (infrared) continuum veiling. Its rotational velocity v sin i = 50 km s(exp -1) is comparable to those of flat-spectrum protostars but considerably higher than those of classical T Tauri stars in the rho Oph cloud. The protostar may be magnetically coupled to its circumstellar disk at a radius of 2 - 3 R(sub *). It is also plausible that this protostar can shed over half its angular momentum and evolve into a more slowly rotating classical T Tauri star by remaining coupled to its circumstellar disk (at increasing radius) as its accretion rate drops by an order of magnitude during the rapid transition between the Class I and Class II phases of evolution. The spectrum of WL 6 does not show any photospheric absorption features, and we estimate that its continuum veiling is r(sub k) is greater than or equal to 4.6. Its low luminosity (2 solar masses) and high veiling dictate that its central protostar is very low mass, M is approx. 0.1 solar masses. We also evaluate multi-epoch X ray data along with these spectra and conclude that the X ray variabilities of these sources are not directly related to their protostellar rotation velocities.

Statistical study of C$\mathsf{^{18}}$O dense cloud cores and star formation

Dense molecular cloud cores are studied statistically in nearby (d 200 pc) star-forming regions (SFRs) that show various modes of star formation. As a result of the C 18 O survey of NANTEN and the 4 m radio telescopes of Nagoya University, 179 cores have been collected in the SFRs of Taurus, the Oph cloud, the Ophiuchus north region, the Lupus clouds, L1333, the Corona Australis cloud, Southern Coalsack, and the Pipe nebula, and their physical properties investigated. According to their star-formation activities, the cores are divided into 3 categories as 136 starless, 36 star-forming, and 7 cluster-forming cores. It is found that cores with active star formation tend to have larger N(H2), n(H2), and M. The mass function of the cores does not appear to follow a single power-law function, but the power-law index is subject to change with the mass range. The average star-formation eciency (SFE) of the cores is roughly10%, and the expected stellar mass function from the SFE approximates the stellar initial-mass function (IMF). Virial analysis shows that the star-forming cores are gravitationally more bound, with smaller virial ratios than the starless cores, while cluster-forming cores are marginally bound with moderate virial ratios. We found that turbulent decay is indicated by diminishing V from the starless to the star-forming cores. It is suggested that the turbulent decay is necessary for star formation, while formed star clusters provide the turbulence and make the cores unbound. Molecular clouds associated with the clusters tend to have head-tail structures and the cluster formation takes place at the head. This implies that the clouds are aected by external shocks, which have triggered cluster formation. We suggest that star and cluster formation are strongly controlled by the initial amount of internal turbulence and the interaction with the external shocks.

Herbig-Haro Objects in the Off-Core Regions of the ρ Ophiuchi Dark Cloud

A wide-field survey for Herbig-Haro (HH) objects has been carried out in the ρ Ophiuchi dark cloud, which extends the previous HH-surveyed area in this cloud from 1 deg2 to 11 deg2. Besides the confirmation of all the 10 known HH objects in the ρ Oph cloud core (Reipurth, published in 1999), seven groups of HH objects, including HH 548, 549A–C, 550, 551, 552, 553A–E, and 554, are newly discovered in the off-core regions. The newly found HH objects concentrate in three regions, which are 2–3 pc away from the ρ Oph dark cloud core. Among these, the three brightest objects, HH 549, 550, and 551, show characteristic HH morphologies of shock or knot with tail. HH 553 and 554 are located in the L1689N region, but they are unlikely driven by the well-known protostar IRAS 16293-2422. HH 550 and 551 are located more than 1 pc away from the cloud and possibly form a parsec-scale HH flow in Ophiuchus.

Infrared Properties of Weak Radio Sources in the ρ Ophiuchi Molecular Cloud

We report mid-infrared ISOCAM observations for a sample of radio continuum sources in the ρ Ophiuchi molecular cloud core with unknown or poorly studied infrared counterparts. These data are combined with existing infrared photometry, including recently published ISOCAM data, for previously studied radio sources to investigate the evolutionary states of 35 radio-emitting young stars in the ρ Oph cloud core. About 50% of the radio stars are found to have class I, flat, or class II spectral energy distributions with near-infrared excesses arising from circumstellar disks. Their radio emission is most likely thermal emission from gas ionized by stellar winds. The remaining radio emitters are young stars with class III spectral energy distributions that lack infrared excesses and circumstellar disks. Their radio emission likely is nonthermal emission from magnetic surface activity. The lack of young stars with weak infrared excesses supports earlier suggestions that disk dissipation is rapid. Class III sources are twice as common as class II sources in this radio-selected sample in comparison with extinction-limited samples, which underscores the importance of radio surveys in obtaining a complete census of young stellar objects. The concentration of diskless class III objects in the high column-density molecular core, and their median age of 0.3 Myr derived from their positions in a Hertzsprung-Russell diagram, indicate they are contemporaries of class II objects which include the classical T Tauri stars. It appears that these class III objects have shorter disk survival times than class II objects in the cloud.

Rotation and X‐Ray Emission from Protostars

The ASCA satellite has recently detected variable hard X-ray emission from two class I protostars in the o Oph cloud, YLW 15 (IRS 43) and WL 6, with a characteristic timescale of D20 hr. In YLW 15, the X-ray emission is in the form of quasi-periodic energetic Nares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The Nare modeling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows us to calculate the rotation parameters of the star and the disk. In WL 6, X-ray Naring is rotationally modulated and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. On the basis of these observations, we Ðnd that YLW 15 is a fast rotator (near break-up), while WL 6 rotates with a signiÐcantly longer period. We thus use X-ray Naring as a clock II to measure the rota- tion of protostars. With the help of the mass-radius relation on the stellar birthline,II we derive masses of and for the central stars of YLW 15 and WL 6, respectively. YLW 15 thus M * D 2 M _ (0.4 M _ appears to be a future A star. In the long term, the magnetic interactions between the star and the disk result in magnetic braking and angular momentum loss of the star. A comparison of the rotation behav- ior of YLW 15 and WL 6 conÐrms that for solar-mass stars magnetic braking takes place on timescales few ) 105 yr, i.e., of the same order as the estimated duration of the class I protostar stage. The t br D a main parameter determining turns out to be the stellar mass, so that close to the birthline there must t br be a mass-rotation relation, such that stars with are fast rotators, while t br D P M * , M * Z 1E2 M _ their lower mass counterparts have had the time to spin down and reach synchronous rotation with the inner surrounding accretion disk. The rapid rotation and strong star-disk magnetic interactions of YLW 15 also naturally explain the observation of superNares II of X-ray luminosities as high as L X Z ergs s~1 during a few hours, while at the WL 6 stage the lower X-ray luminosities are likely 1033E1034 to be of purely stellar origin. The mass-rotation relation through magnetic braking may also explain why so few class I protostars have been detected in X-rays to date, and why they all lie in clusters. In the case of YLW 15, and perhaps also in other protostars, a hot coronal wind (T D 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the Ðrst clues to the magnetic properties of protostars, which govern their rotation status and evolution. Subject headings : stars : Nare E stars : individual (YLW 15, WL 6) E stars : magnetic Ðelds E stars : preEmain-sequence E stars : rotation E X-rays : stars

C18O Observations of the Dense Cloud Cores and Star Formation in Ophiuchus

In order to reveal the distribution of the dense gas of ≥104 cm3, C18O (J = 1-0) observations have been made toward the molecular clouds in the Ophiuchus region of ~6.4 deg2 with the two 4 m telescopes of Nagoya University. Forty dense cores have been identified, providing the first complete sample of such dense cores in Ophiuchus. The C18O dense cores are distributed not only in the active star-forming region, ρ Oph cloud core, but also in the North region where star formation is less active. The typical core mass, MLTE, radius, R, and average number density, n(H2), of the cores are 90 M☉, 0.24 pc, and 1.7 × 104 cm-3 in the ρ Oph region, respectively, and 14 M☉, 0.19 pc, and 7.6 × 103 cm-3 in the North region, respectively. Nine of the 40 cores are associated with young stellar objects, and most of the C18O cores are starless. An analysis of the physical parameters of the C18O cores show that star-forming cores tend to have larger N(H2) than the rest by a factor of ~3, although there is no significant trend in the other physical parameters between star forming and starless cores. We have compared the present C18O data with the 13CO data (Nozawa et al.) and with the associated YSOs, in order to understand better the condensing process from molecular gas with density of ~103 cm-3 to protostars. It is found that 55% of the 13CO cores are associated with C18O cores and that the C18O cores are typically less massive, smaller and denser by ~34%, ~32%, and a factor of ~3, respectively, than the 13CO cores. It is also found that the C18O cores have steeper density profiles than the 13CO cores; when we fit the density profile by a power law as ρ ∝ r-β, the values of β for C18O and 13CO are estimated as ~1.5 and 1.2, respectively. This suggests that the C18O cores are gravitationally more relaxed than the 13CO cores. In order to investigate the energetics of the cores, the virial mass, MVIR, has been calculated for each core. It is found that most of the 13CO cores have MVIR larger than MLTE. On the other hand, 22 of the 40 C18O cores have MVIR smaller than MLTE, suggesting that the C18O cores are more deeply gravitationally bound than the 13CO cores. Further, we have found a correlation between the ratio MVIR/MLTE and star formation activity: (1) For 13CO cores, the fraction of the 13CO cores associated with the C18O cores tends to increase with decreasing MVIR/MLTE, and (2) for the C18O cores, the fraction of the C18O cores associated with stars tends to increase with decreasing of MVIR/MLTE. We interpret this to indicate that the gradual dissipation of the internal turbulence leads to formation of denser cores and subsequent star formation. Through the evolution from the 13CO cores to the C18O cores, they should lose the turbulence energy of ~1044 ergs. The supersonic gas motion with the magnetic fields produces shocks, and the radiation from the small shocked region may significantly contribute to the cooling. We suggest that the cores have continuous collisions between turbulent eddies to produce the C-shocks. Also, the Alfvenic energy loss may be viable as the dissipation mechanism.

A Search for Radio Emission at the Bottom of the Main Sequence and Beyond

We have used the VLA to conduct a deep search for 3.6 cm radio emission from nearby very low mass stars and brown dwarfs. The G?del-Benz relation is used to predict radio luminosities for some very low mass stars and candidate brown dwarfs with measured X-ray fluxes. The predicted radio fluxes are quite small, whereas the measured radio flux from the brown dwarf candidate GY 31 in the ? Oph cloud is relatively strong. In light of our new observations, this object remains an anomaly. We present upper limits for our measured radio fluxes at 3.6 cm for our targets.

X-rays from protostars

Very recently, ROSAT and ASCA have been able to detect X-rays from protostars typically 10 times younger than T Tauri stars (i.e., having ages ∼ 10 5 yrs), in spite of their heavy extinction (A V ∼ 30 – 60). The intrinsic luminosities, either quiescent or in the form of flares, are ∼ 10 times higher than T Tauri stars or more. In one particular case (a well-documented protostar in the nearby ϱ Oph cloud, YLW15), the L x / L bol ratio has been > 1 during a “superflare”. This high value implies not a stellar origin, but rather (for the first time) star-disk and/or star-envelope magnetic interactions, not unlike those invoked for compact objects. The present status of X-ray emission by protostars, as seen with ROSAT and ASCA, is summarized.

HERBIG-HARO OBJECTS IN THE \\RHO OPHIUCHI CLOUD

ABSTRACT Using deep, narrow-band images of the main ρ Oph dark cloud centered on the wavelengths of Hα and [S~II], we present evidence for three new Herbig-Haro objects. This increases the total number in the cloud to five. In addition, positions for 5 candidate Herbig-Haro objects are given. Relatively high [S~II]/Hα ratios indicate low excitation conditions for all of these nebulae. We list potential exciting stars for each Herbig-Haro object and candidate by identifying nearby young stellar objects with strong infrared excesses and/or millimeter continuum emission. The location of most of these nebulae near the cloud edges underscores the important role that extinction by dust plays in the ρ Oph cloud in masking the presence of Herbig-Haro objects. Among the newly identified Herbig-Haro objects is a jet-like string of emission nebulae emanating from the classical T Tauri star SR~4. [S~II] emission is also found coincident with a knot of strong molecular hydrogen emission associated with the highly-collimated VLA~1623 molecular outflow. Indeed, the low excitation nature of all of the objects in our study, coupled with the high extinction of the cloud, suggests that a deep survey for shocked molecular hydrogen at λ=2.122 μm would be the best way to search for evidence of strong winds from the large population of young stellar objects in the cloud.

The Magnetic Fields in the Ophiuchus and Taurus Molecular Clouds

We discuss the significance of previously published OH Zeeman effect measurements of the magnetic field in the ρ Oph cloud (L1688) and also a new OH Zeeman effect measurement for the dark cloud core TMC-1C, the latter obtained with the Arecibo telescope. Results for both of these clouds, like previously published OH Zeeman results for other dark clouds, show that the line-of-sight field strength |B||| ≤ 10 μG. The ρ Oph cloud is significantly higher in column density than other clouds studied, and recent H I Zeeman effect observations have revealed B|| ≈ +10 μG in the H I self-absorption region in front of the cloud. The low values of B derived from the OH Zeeman effect may indicate that OH poorly samples the dense gas in the ρ Oph cloud, or else that the cloud is in a state of supercritical collapse. The TMC-1C cloud core may have formed via ambipolar diffusion in a manner similar to recent modeling of dark cloud Barnard 1.

Revised chemical models of ? Oph based on HST data

Hubble Space Telescope observations by Savage, Cardelli and Sofia (1992) and Cardelliet al. (1993) have led to improved gas phase abundances for many elements in the diffuse cloud towards the star ζ Oph. Most remarkably, it was found that oxygen is much more strongly depleted than previousCopernicus observations indicated. As a consequence, chemical models of the ζ Oph cloud are severely affected by the drop in the observed oxygen abundance by almost a factor of two; some previous model calculations for the ζ Oph cloud failed to reproduce-even approximately-the observed high CO column density. The model calculations for the ζ Oph cloud developed by Wagenblast (1992) in which the abundance of all observed neutral molecules could be reproduced have been revised and it is found that oxygen and nitrogen hydrides are required to be formed efficiently on the surface of grains; further, there are indications for a high cosmic ray ionization rate of the order 10−16 s−1.

The stellar populations within the? Oph cloud cores

The stellar populations within the? Oph cloud cores

X-ray sources in regions of star formation. 5: The low mass stars of the Upper Scorpius association

We report followup investigations of Einstein x-ray observations of the Upper Scorpius OB association. We identify 28 low mass pre-main sequence stars as counterparts of x-ray sources in the approximately = 7 square degrees of the OB association observed. Based on kinematics and lithium abundances, these stars are low mass members of the OB association. We use optical spectra and optical and near-IR photometry to determine the stellar luminosities, effective temperatures, masses, and ages. We show that the bolometric corrections and effective temperatures of the G and K stars are consistent with those of subgiants. The low mass stars have isochronal ages of 1-2 Myr, depending on the choice of evolutionary models, with very small dispersion (sigma approximately = 1 Myr). This age is significantly younger than the 5-6 Myr found for the more massive B stars. The small dispersion in stellar ages, less than 10% the sound-crossing time of the association, suggests that star formation was triggered. We present two scenarios for star formation in this association. In the two-episode scenario, formation of the low mass stars was triggered by a supernova explosion, and the low mass stars form quickly, with high efficiency. Alternatively, high and low mass star formation was all initiated at the same time, some 5-6 Myr ago, and the apparent systematic age difference is an artifact of how the isochrones are dated. The effect of the supernova is to terminate mass accretion and yield an apparently coeval population. We show that the incompleteness in the x-ray sampling is about 65%, and is strongly dependent on stellar mass. After correction for incompleteness, we estimate there are about 2000 low mass members (stellar mass less than 2 solar mass) of this association. The mass function in this association is indistinguishable from that of the field. The ratio of naked to classical T Tauri stars is much larger than in Tau-Aur, and may be attributable to the local environment. We also present observations of eight ROX sources associated with the rho Oph cloud, and observations of non-pre-main sequence (PMS) stars in our fields.