Nearby OB Association Research Articles

A Radio-loud Semiregular Variable

As a byproduct of our search for Galactic stellar systems with gamma-ray emission, we have identified an unrelated cool and evolved star (IRC-10412) that attracted our attention due to its strong radio emission level with a spectral index matching, almost perfectly, the canonical +0.6 value expected from an ionized stellar wind. A follow-up observational analysis was undertaken given that these two properties are hard to reconcile as originating in the same stellar object. As a result, IRC-10412 has been classified as a new semiregular variable of SRb type in the asymptotic giant branch, and different but consistent estimates of its mass-loss parameter are reported. We propose that its unusually high radio emission arises from a ∼10−5 M ⊙ yr−1 stellar wind exposed to an external source of ionizing photons, possibly coming from nearby OB associations.

A Novel Survey for Young Substellar Objects with the W-band Filter. V. IC 348 and Barnard 5 in the Perseus Cloud

We report the discovery of substellar objects in the young star cluster IC 348 and the neighboring Barnard 5 dark cloud, both at the eastern end of the Perseus star-forming complex. The substellar candidates are selected using narrowband imaging, i.e., on and off photometric technique with a filter centered around the water absorption feature at 1.45 μm, a technique proven to be efficient in detecting water-bearing substellar objects. Our spectroscopic observations confirm three brown dwarfs in IC 348. In addition, the source WBIS 03492858+3258064, reported in this work, is the first confirmed brown dwarf discovered toward Barnard 5. Together with the young stellar population selected via near- and mid-infrared colors using the Two Micron All Sky Survey and the Wide-field Infrared Survey Explorer, we diagnose the relation between stellar versus substellar objects with the associated molecular clouds. Analyzed by Gaia EDR3 parallaxes and kinematics of the cloud members across the Perseus region, we propose the star formation scenario of the complex under influence of the nearby OB association.

Spectroscopic Line Modeling of the Fastest Rotating O-type Stars

We present a spectroscopic analysis of the most rapidly rotating stars currently known, VFTS 102 ( km s−1; O9: Vnnne+) and VFTS 285 ( km s−1; O7.5: Vnnn), both members of the 30 Dor complex in the Large Magellanic Cloud. This study is based on high-resolution ultraviolet spectra from Hubble Space Telescope/Cosmic Origins Spectrograph and optical spectra from the Very Large Telescope (VLT) X-shooter plus archival VLT GIRAFFE spectra. We utilize numerical simulations of their photospheres, rotationally distorted shape, and gravity darkening to calculate model spectral line profiles and predicted monochromatic absolute fluxes. We use a guided grid search to investigate parameters that yield best fits for the observed features and fluxes. These fits produce estimates of the physical parameters for these stars (plus a Galactic counterpart, ζ Oph) including the equatorial rotational velocity, inclination, radius, mass, gravity, temperature, and reddening. We find that both stars appear to be radial-velocity constant. VFTS 102 is rotating at critical velocity, has a modest He enrichment, and appears to share the motion of the nearby OB-association LH 99. These properties suggest that the star was spun up through a close binary merger. VFTS 285 is rotating at 95% of critical velocity, has a strong He enrichment, and is moving away from the R136 cluster at the center of 30 Dor. It is mostly likely a runaway star ejected by a supernova explosion that released the components of the natal binary system.

Carina High-contrast Imaging Project for massive Stars (CHIPS)

Context.Most massive stars belong to multiple systems, yet the formation process leading to such high multiplicity remains insufficiently understood. To help constrain the different formation scenarios that exist, insights into the low-mass end of the companion mass function of such stars is crucial. However, this is a challenging endeavour as (sub-)solar mass companions at angular separations (ρ) below 1″ (corresponding to 1000–3000 au in nearby young open clusters and OB associations) are difficult to detect due to the large brightness contrast with the central star.Aims.With the Carina High-contrast Imaging Project of massive Stars (CHIPS), we aim to obtain statistically significant constraints on the presence and properties of low-mass companions around massive stars in a previously unreachable observing window (Δmag ≳ 10 atρ ≲ 1″). In the second paper of the series, we focus on the Trumpler 14 cluster, which harbours some of the youngest and most massive O-type stars in the Milky Way.Methods.We obtained VLT-SPHERE observations of seven O-type objects in Trumpler 14 using IRDIFS_EXT mode. These provide us with a 12″ × 12″ field of view (approximately ((3 × 3)×104au) centred on each O star and allow us to search for companions at separations larger than 0″​​.15 (approx. 360 au) and down to magnitude contrast > 10 mag in the near-infrared. We used angular and spectral differential imaging along with Point Spread Function (PSF) fitting to detect sources and measure their flux relative to that of the central object. We then used grids of ATLAS9 and PHOENIX Local Thermodynamic Equilibrium (LTE) atmosphere models combined with (pre-)main-sequence evolutionary tracks to estimate the mass of the detected candidate companions.Results.We detected 211 sources with near-infrared magnitude contrast in the range of 2–12. Given the large surface number density of stars in Trumpler 14, one cannot reliably distinguish between cluster members and genuine companions for most of the detected sources. The closest companion, at only 0″​​.26, is characterised as a 1.4M⊙star with an age of 0.6 Myr, in excellent agreement with previous age estimates for Tr 14. The mass function peaks at about 0.4M⊙and presents a dearth of stars in the 0.5–0.8M⊙mass range compared to previous estimates of the initial mass function in Tr 14. While statistically significant, part of these differences may result from contamination of theK-band fluxes by circumstellar material.Conclusions.SPHERE is clearly suitable to probe the low-mass end of the mass function in the vicinity of massive stars. Follow-up SPHERE observations to obtain the full Y to K spectral energy distribution would allow for better constraints on the masses of the detected sources, and to confirm (or invalidate) the curious mass function that we derived for low-mass stars in the vicinity of the O-type objects in Trumpler 14.

Fine structure in the luminosity function in young stellar populations with Gaia DR2

Context. A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at MV ≃ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H− opacity in a certain range of low-mass stars. Aims. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. Methods. We analyzed the astrometric properties of stars in the solar neighborhood (< 20 pc) and in various relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB associations, and compare the features in the luminosity function with those generated by PARSEC models. Results. The Wielen dip is confirmed in the LF of all the populations, including the solar neighborhood, at MG ≃ 7 mag. The H-peak is present in the LF of the field stars in the solar neighborhood. It likely signals that the population is mixed with a significant number of stars younger than 100 Myr. The H-peak is found in the LF of young open clusters and OB associations, and its location varies with age. The PARSEC evolutionary models predict that the H-peak moves from ∼ − 1 mag towards ∼ + 6 mag in MG for populations with ages increasing from 1 to ∼70 Myr. Our observations with Gaia DR2 confirm the evolution of the H-peak from ∼5 Myr up to ∼47 Myr. We provide a calibration function between MG and age that works in the age domain between 1 and 30 Myr. Conclusions. The fine structure in the luminosity function in young stellar populations can be used to estimate their age.

The dynamics of the γ Vel cluster and nearby Vela OB2 association

ABSTRACT The kinematics of low-mass stars in nearby OB associations can provide clues about their origins and evolution. Combining the precise positions, proper motions, and parallaxes given in the second Gaia Data Release with radial-velocity measurements obtained with the Hermes spectrograph at the Anglo-Australian Telescope, we have an opportunity to study in detail the kinematics of low-mass stars belonging to the nearby γ Vel cluster and the Vela OB2 association it is projected against. The presence of lithium is used to confirm the youth of our targets. We separate our sample into the cluster and association populations based on the Gaia-ESO Survey membership probabilities their parallaxes, and kinematics. We find strong evidence for expansion in the OB association population with at least 4σ significance along all three axes, though the expansion is notably anisotropic. We discuss these results in the context of cluster and association dispersal theories.

Triggered star formation in a cometary atomic/molecular cloud in the Cep OB3 association

ABSTRACT We present a study of a small atomic/molecular cometary cloud associated with the infrared source IRAS 23153+6938. The cloud is located 70 pc from the massive O-type stars in the Cepheus OB3 association, and is very likely an excellent example of triggered star formation via radiation-driven implosion (RDI). The cloud was studied using $\rm{H\,\small{I}}$ and 12CO data from the Canadian Galactic Plane Survey (CGPS) and infrared observations from the Wide-field Infrared Survey Explorer (WISE) telescope. The molecular mass is approximately MH2 = 350 ± 45 M$\odot$, and we find that the single IRAS source is actually the centre of a small cluster of class I and class II young stellar objects (YSOs). To compare with theory, we make reasonable estimates for the cometary cloud’s initial conditions and find that the cloud is located within the correct theoretical phase space for RDI to occur. In addition, both the morphology of the cloud and the location of different YSO classes relative to the cloud match what would be expected for RDI. We conclude that RDI is the most likely explanation for star formation within the cloud, and we suggest that similar studies of molecular clouds associated with nearby OB associations may be able to identify comparable examples.

Cosmography of OB stars in the solar neighbourhood

We construct a 3D map of the spatial density of OB stars within 500 pc from the Sun using the Hipparcos catalogue and find three large-scale stream-like structures that allow a new view on the solar neighbourhood. The spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius to Canis Majoris stream, covering 350 pc and 65 Myr of star formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr of star formation history; and 3) the Orion stream, including not only the well-known Orion OB1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the Sun. The map also reveals a remarkable and previously unknown nearby OB association, between the Orion stream and the Taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. This new association also appears to be the birthplace of Betelgeuse, as indicated by the proximity and velocity of the red giant. If this is confirmed, it would solve the long-standing puzzle of the origin of Betelgeuse. The well-known nearby star-forming low-mass clouds, including the nearby T and R associations Lupus, Cha, Oph, CrA, Taurus, Vela R1, and various low-mass cometary clouds in Vela and Orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. We also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. Finally, we find no evidence of an elliptical structure such as the Gould belt, a structure we suggest is a 2D projection effect, and not a physical ring.

HerschelCHESS discovery of the fossil cloud that gave birth to the Trapezium and Orion KL

As part of the Herschel Guaranteed Time Key Programme CHESS, we present the discovery of a diffuse gas component in the foreground of the intermediate-mass protostar OMC-2 FIR 4, located in the Orion A region. Making use of the full HIFI spectrum of OMC-2 FIR 4 obtained in CHESS, we detected several ground-state lines from OH+, H2O+, HF, and CH+, all of them seen in absorption against the dust continuum emission of the protostar's envelope. The lines peak at a velocity of 9 km/s, which is blue-shifted by 2 km/s with respect to the systemic velocity of OMC-2 FIR 4 (V = 11.4 km/s). We derived column densities for each species, as well as an upper limit to the column density of the undetected H3O+. In order to model and characterise the foreground cloud, we used the Meudon PDR code to run a homogeneous grid of models that spans a reasonable range of densities, visual extinctions, cosmic ray ionisation rates and far-ultraviolet (FUV) radiation fields. The results of our modelling indicate that the foreground cloud is composed of predominantly neutral diffuse gas (n(H) = 100 cm-3) and is heavily irradiated by an external source of FUV that most likely arises from the nearby Trapezium OB association. The cloud is 6 pc thick and bears many similarities with the so-called C+ interface between Orion-KL and the Trapezium cluster, 2 pc south of OMC-2 FIR 4. We conclude that the foreground cloud we detected is an extension of the C+ interface seen in the direction of Orion KL, and interpret it to be the remains of the parental cloud of OMC-1, which extends from OMC-1 up to OMC-2.

THESPITZERSURVEY OF INTERSTELLAR CLOUDS IN THE GOULD BELT. V. OPHIUCHUS NORTH OBSERVED WITH IRAC AND MIPS

We present Spitzer IRAC (2.1 deg2) and MIPS (6.5 deg2) observations of star formation in the Ophiuchus North (Oph N) molecular clouds. This fragmentary cloud complex lies on the edge of the Sco–Cen OB association, several degrees to the north of the well-known ρ Oph star-forming region, at an approximate distance of 130 pc. The Oph N clouds were mapped as part of the Spitzer Gould Belt project under the working name "Scorpius." In the regions mapped, selected to encompass all the cloud with visual extinction AV > 3, 11 young stellar object (YSO) candidates are identified, 8 from IRAC/MIPS color-based selection and 3 from Two Micron All Sky Survey (2MASS) KS/MIPS colors. Adding to one source previously identified in L43, this increases the number of YSOcs identified in Oph N to 12. During the selection process, four color-based YSOcs were rejected as probable asymptotic giant branch stars and one as a known galaxy. The sources span the full range of YSOc classifications from Class 0/1 to Class III, and starless cores are also present. Twelve high extinction (AV > 10) cores are identified with a total mass of ∼100 M☉. These results confirm that there is little ongoing star formation in this region (instantaneous star formation efficiency <0.34%) and that the bottleneck lies in the formation of dense cores. The influence of the nearby Upper Sco OB association, including the 09V star ζ Oph, is seen in dynamical interactions and raised dust temperatures but has not enhanced levels of star formation in Oph N.

Atomic hydrogen produced in M33 photodissociation regions

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-UV photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess far-UV emission that reaches the GMC from the OB association, and the excess 21-cm radio HI emission produced after these far-UV photons convert H2 into HI on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs, and is especially sensitive to a range of density well below the critical density for CO(1-0) emission. We describe our "PDR method" in more detail and apply it using GALEX far-UV and VLA 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80 pc resolution. The cloud densities we derive range from 1 to 500 cm-3, with no clear dependence on galactocentric radius; these results are generally similar to those obtained earlier in M81, M83, and M101 using the same method.

Massive runaway stars in the Large Magellanic Cloud

The origin of massive field stars in the Large Magellanic Cloud (LMC) has long been an enigma. The recent measurements of large offsets (~100 km/s) between the heliocentric radial velocities of some very massive (O2-type) field stars and the systemic LMC velocity provides a possible explanation of this enigma and suggests that the field stars are runaway stars ejected from their birth places at the very beginning of their parent cluster's dynamical evolution. A straightforward way to prove this explanation is to measure the proper motions of the field stars and to show that they are moving away from one of the nearby star clusters or OB associations. This approach however is complicated by the large distance to the LMC, which makes accurate proper motion measurements difficult. We use an alternative approach for solving the problem, based on the search for bow shocks produced by runaway stars. The geometry of detected bow shocks would allow us to infer the direction of stellar motion and thereby to determine their possible parent clusters. In this paper we present the results of a search for bow shocks around six massive field stars which were suggested in the literature as candidate runaway stars. Using archival (Spitzer Space Telescope) data, we found a bow shock associated with one of our program stars, the O2 V((f*)) star BI 237, which is the first-ever detection of bow shocks in the LMC. Orientation of the bow shock suggests that BI 237 was ejected from the OB association LH 82 (located at ~120 pc in projection from the star). A by-product of our search is the detection of bow shocks generated by four OB stars in the field of the LMC and an arc-like structure attached to the candidate luminous blue variable R81 (HD 269128). The geometry of two of these bow shocks is consistent with the possibility that their associated stars were ejected from the 30 Doradus star forming complex.

Fundamental parameters of “normal” B stars in the solar neighborhood

AbstractUnderstanding phenomena of activity in stars, like pulsations or magnetism, benefits from systematic comparisons of some key physical parameters of active with those of “normal” stars. Here we concentrate on a careful derivation of fundamental parameters of a well selected sample of 27 “normal” B stars in nearby OB associations and in the field. A quantitative spectral analysis methodology based on hybrid non-LTE techniques is applied to high-resolution and high-S/N spectra. Results derived from the pure spectroscopic analysis are compared to other data/indicators of stellar parameters in order to prove the reliability of the method. Very good agreement is obtained among all of them. Besides the fundamental parameters, the chemical composition of the stars is also determined at high precision, turning out to be highly homogeneous. A comparative study of the present results with those of well known active massive stars will help to improve our understanding of the driving mechanisms of activity.

PROTOPLANETARY DISK EVOLUTION AROUND THE TRIGGERED STAR-FORMING REGION CEPHEUS B

The Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the Infrared Array Camera detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main-sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, ~95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample that is almost complete down to 0.5 M ? outside of the cloud, and somewhat above 1 M ? in the cloud. Analyses of the nearly disk-unbiased combined Chandra and Spitzer-selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core toward the primary ionizing O star HD 217086. This strongly supports the radiation-driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of ~1 km s?1 is very similar to theoretical models of RDI in shocked molecular clouds. The initial mass function (IMF) of the lightly obscured triggered population exhibits a standard Galactic field IMF shape. The unusually high apparent value of 70% star formation efficiency inferred from the ratio of star mass to current molecular gas mass indicates that most of the Cep B molecular cloud has been already ablated or transformed to stars. Contrary to the current RDI simulations, our findings indicate that star formation triggering by H II region shocks is not restricted to a single episode but can continue for millions of years. Other results include: (1) agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; (2) confirmation of the youthfulness of the embedded Cep B cluster; (3) confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.

Proper motions of thermally emitting isolated neutron stars measured with Chandra

The remarkable astrometric capabilities of the Chandra Observatory offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0−5022, RX J0806.4−4123 and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as a reference the background of extragalactic or remote Galactic X-ray sources. We derive 2σ upper limits of 123 mas yr −1 and 86 mas yr −1 on the proper motion of RX J0420.0−5022 and RX J0806.4−4123, respectively. RX J1308.6+2127 exhibits a very significant displacement (∼9σ) yielding μ = 220 ± 25 mas yr −1 , the second fastest measured among all ROSAT-discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ∼740 (d/700 pc) km s −1 might be the largest of all ROSAT INSs and its corresponding spatial velocity lies among the fastest recorded for neutron stars. RX J1308.6+2127 is thus a middle-aged (age ∼1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. Overall, the distribution in transverse velocity of the ROSAT INSs is not statistically different from that of normal radio pulsars.

A Cosmic Abundance Standard: Chemical Homogeneity of the Solar Neighborhood and the ISM Dust-Phase Composition

A representative sample of unevolved early B-type stars in nearby OB associations and the field is analyzed to unprecedented precision using NLTE techniques. The resulting chemical composition is found to be more metal-rich and much more homogeneous than indicated by previous work. A rms scatter of ~10% in abundances is found for the six stars (and confirmed by six evolved stars), the same as reported for ISM gas-phase abundances. A cosmic abundance standard for the present-day solar neighborhood is proposed, implying mass fractions for hydrogen, helium, and metals of -->X = 0.715, -->Y = 0.271, and -->Z = 0.014. Good agreement with solar photospheric abundances as reported from recent 3D radiative-hydrodynamical simulations of the solar atmosphere is obtained. As a first application we use the cosmic abundance standard as a proxy for the determination of the local ISM dust-phase composition, putting tight observational constraints on dust models.

Origin of the Local Bubble

We present a new unbiased search for OB associations in the Solar neighbourhood which have hosted the progenitor stars of the core collapse supernovae responsible for the Local Bubble in the interstellar gas. For this purpose we have analyzed a volume complete set (with a diameter of 400 pc) of B stars drawn from the \(\mathsf{Hipparcos}\) catalogue and the \(\mathsf{Arivel}\) data base, from which candidate members were selected by a kinematical criterion. After careful dereddening the star colours we have constructed a colour-magnitude diagram and confirmed that the Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux subgroups of the Sco OB2 association are the youngest nearby OB associations. We dated their ages with theoretical isochrones in the range of 20–30 Myr, in agreement with previous work. We have traced backwards in time the paths of the stars and found that they entered the volume of the present bubble at 10 to 15 Myr ago. We argue that the Local Bubble began to form then and estimate that 14 to 20 supernovae have exploded since. The implied energy input into the ambient medium can be shown to be sufficient to excavate a bubble of the presently observed size.

Mapping the Shores of the Brown Dwarf Desert. I. Upper Scorpius

We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture mask interferometry to achieve typical contrast limits of ΔK ~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered seven directly resolved companions. The overall frequency of binary companions (~35 +5 -4% at separations of 6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among lower mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate lognormal parameterization of the mass function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called brown dwarf desert that has been observed by radial velocity planet searches. Finally, our survey’s deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (≳99.5%), but we have identified four candidate companions at lower confidence (≳97.5%) that merit additional follow-up to confirm or disprove their existence.

Suzaku Observations of the North Polar Spur: Evidence for Nitrogen Enhancement

Abstract We present observations of the North Polar Spur (NPS) using the X-ray Imaging Spectrometer (XIS) aboard the Suzaku X-ray satellite. The NPS is a large region of enhanced soft X-ray and radio emission projected above the plane of the Galaxy, likely produced by a series of supernovae and stellar winds from the nearby Sco-Cen OB association. The exceptional sensitivity and spectral resolution of the XIS below 1keV allow unprecedented probing of low-energy spectral lines, including CVI (0.37keV) and NVII (0.50keV), and we have detected highly-ionized nitrogen toward the NPS for the first time. For this single pointing toward the brightest 3/4keV emission ($l =$ 26°.8, $b =$$+$22°.0), the best-fit NPS emission model implies a hot ($kT \approx 0.3$keV), collisional ionization equilibrium (CIE) plasma with depleted C, O, Ne, Mg, and Fe abundances of less than 0.5solar, but an enhanced N abundance, with N$/$O $=$$(4.0^{+0.4}_{-0.5})$ times solar. The temperature and total thermal energy of the gas suggest heating by one or more supernovae, while the enhanced nitrogen abundance is best explained by enrichment from stellar material that has been processed by the CNO cycle. Due to the time required to develop AGB stars, we conclude that this N$/$O enhancement cannot be caused by the Sco-Cen OB association, but may result from a previous enrichment episode in the solar neighborhood.

USco J1606‐1935: An Unusually Wide Low‐Mass Triple System?

We present photometric, astrometric, and spectroscopic observations of USco J160611.9-193532 AB, a candidate ultrawide (~1600 AU), low-mass (M_(tot) ~ 0.4 M_⊙) multiple system in the nearby OB association Upper Scorpius. We conclude that both components are young, comoving members of the association; we also present high-resolution observations that show that the primary is itself a close binary system. If the Aab and B components are gravitationally bound, the system would fall into the small class of young multiple systems that have unusually wide separations as compared to field systems of similar mass. However, we demonstrate that physical association cannot be assumed purely on probabilistic grounds for any individual candidate system in this separation range. Analysis of the association's two-point correlation function shows that there is a significant probability (25%) that at least one pair of low-mass association members will be separated in projection by ≾ 15, so analysis of the wide binary population in Upper Sco will require a systematic search for all wide systems; the detection of another such pair would represent an excess at the 98% confidence level.