Young Stellar Object NGC Research Articles

Constraining Free–Free Emission and Photoevaporative Mass-loss Rates for Known Proplyds and New VLA–identified Candidate Proplyds in NGC 1977

We present NSF's Karl G. Jansky Very Large Array observations covering the NGC 1977 region at 3.0, 6.4, and 15.0 GHz. We search for compact radio sources and detect continuum emission from 34 NGC 1977 cluster members and 37 background objects. Of the 34 radio-detected cluster members, 3 are associated with known proplyds in NGC 1977, 22 are associated with additional young stellar objects in NGC 1977, and 9 are newly identified cluster members. We examine the radio spectral energy distributions, circular polarization, and variability of the detected NGC 1977 sources and identify 10 new candidate proplyds whose radio fluxes are dominated by optically thin free–free emission. We use measurements of free–free emission to calculate the mass-loss rates of known proplyds and new candidate proplyds in NGC 1977, and find values ∼10−9 to 10−8 M ⊙ yr−1, which are lower than the mass-loss rates measured toward proplyds in the Orion Nebula Cluster but consistent with the mass-loss rates predicted by external photoevaporation models for spatially extended disks that are irradiated by the typical external ultraviolet (UV) fields encountered in NGC 1977. Finally, we show that photoevaporative disk winds in NGC 1977 may be illuminated by internal or external sources of ionization, depending on their positions within the cluster. This study provides new constraints on disk properties in a clustered star-forming region with a weaker UV environment than the Orion Nebula Cluster but a stronger UV environment than low-mass star-forming regions like Taurus. Such intermediate UV environments represent the typical conditions of Galactic star and planet formation.

The most variable VVV sources: eruptive protostars, dipping giants in the nuclear disc and others

ABSTRACT We have performed a comprehensive search of a VISTA Variables in the Via Lactea (VVV) data base of 9.5 yr light curves for variable sources with ΔKs ≥ 4 mag, aiming to provide a large sample of high amplitude eruptive young stellar objects (YSOs) and detect unusual or new types of infrared variable source. We find 222 variable or transient sources in the Galactic bulge and disc, most of which are new discoveries. The sample mainly comprises novae, YSOs, microlensing events, Long Period Variable stars (LPVs), and a few rare or unclassified sources. Additionally, we report the discovery of a significant population of aperiodic late-type giant stars suffering deep extinction events, strongly clustered in the Nuclear Disc of the Milky Way. We suggest that these are metal-rich stars in which radiatively driven mass loss has been enhanced by super-solar metallicity. Among the YSOs, 32/40 appear to be undergoing episodic accretion. Long-lasting YSO eruptions have a typical rise time of ∼2 yr, somewhat slower than the 6–12 month time-scale seen in the few historical events observed on the rise. The outburst durations are usually at least 5 yr, somewhat longer than many lower amplitude VVV events detected previously. The light curves are diverse in nature, suggesting that multiple types of disc instability may occur. Eight long-duration extinction events are seen wherein the YSO dims for a year or more, attributable to inner disc structure. One binary YSO in NGC 6530 displays periodic extinction events (P=59 d) similar to KH 15D.

The Internal Proper Motion Kinematics of NGC 346: Past Formation and Future Evolution

We investigate the internal kinematics of the young star-forming region NGC 346 in the Small Magellanic Cloud (SMC). We used two epochs of deep F555W and F814W Hubble Space Telescope Advanced Camera for Surveys observations with an 11 yr baseline to determine proper motions and study the kinematics of different populations, as identified by their color–magnitude diagram and spatial distribution characteristics. The proper motion field of the young stars shows a complex structure with spatially coherent patterns. NGC 346’s upper main sequence and pre-main sequence stars follow very similar motion patterns, with the outer parts of the cluster being characterized both by outflows and inflows. The proper motion field in the inner ∼10 pc shows a combination of rotation and inflow, indicative of inspiraling motion. The rotation velocity in this regions peaks at ∼3 km s−1, whereas the inflow velocity peaks at ∼1 km s−1. Subclusters and massive young stellar objects in NGC 346 are found at the interface of significant changes in the coherence of the proper motion field. This suggests that turbulence is the main star formation driver in this region. Similar kinematics observed in the metal-poor NGC 346 and in the Milky Way’s star-forming regions suggest that the differences in the cooling conditions due to different amounts of metallicity and dust density between the SMC and our galaxy are too small to alter significantly the process of star cluster assembly and growth. The main characteristics of our findings are consistent with various proposed star cluster formation models.

DETECTION OF LINEARLY POLARIZED 6.9 mm CONTINUUM EMISSION FROM THE CLASS 0 YOUNG STELLAR OBJECT NGC 1333 IRAS4A

ABSTRACT We report new Karl G. Jansky Very Large Array (JVLA), 0 .″ 5 angular resolution observations of linearly polarized continuum emission at 6.9 mm, toward the class 0 young stellar object NGC 1333 IRAS4A. This target source is a collapsing dense molecular core that was resolved at short wavelengths to have an hourglass shaped B-field configuration. We compare these 6.9 mm observations with previous polarization Submillimeter Array observations at 0.88 mm, which have a comparable angular resolution (∼ 0 .″ 7 ). We find that at the same resolution, the observed polarization position angles at 6.9 mm are slightly deviated from those observed at 0.88 mm. Due to the lower optical depth of the emission at 6.9 mm, and the potential effect of dust grain growth, the new JVLA observations are likely probing B-field alignments in regions interior to those sampled by the previous polarization observations at higher frequencies. Our understanding can be improved with more sensitive observations, and with observations for the more extended spatial scales.

CSI 2264: CHARACTERIZING YOUNG STARS IN NGC 2264 WITH SHORT-DURATION PERIODIC FLUX DIPS IN THEIR LIGHT CURVES

We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical {\em CoRoT} light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian co-rotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3<P<11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard "disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous {\em Spitzer} IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel-flow accretion columns arising near the inner disk wall.

Detection of glycolaldehyde toward the solar-type protostar NGC 1333 IRAS2A

Glycolaldehyde is a key molecule in the formation of biologically relevant molecules such as ribose. We report its detection with the Plateau de Bure interferometer toward the Class 0 young stellar object NGC 1333 IRAS2A, which is only the second solar-type protostar for which this prebiotic molecule is detected. Local thermodynamic equilibrium analyses of glycolaldehyde, ethylene glycol (the reduced alcohol of glycolaldehyde), and methyl formate (the most abundant isomer of glycolaldehyde) were carried out. The relative abundance of ethylene glycol to glycolaldehyde is found to be ~5 – higher than in the Class 0 source IRAS 16293-2422 (~1), but similar to the lower limits derived in comets (≥3–6). The different ethylene glycol-to-glycolaldehyde ratios in the two protostars might be related to different CH3OH:CO compositions of the icy grain mantles. In particular, a more efficient hydrogenation on the grains in NGC 1333 IRAS2A would favor the formation of both methanol and ethylene glycol. In conclusion, it is possible that like NGC 1333 IRAS2A, other low-mass protostars show high ethylene glycol-to-glycolaldehyde abundance ratios. The cometary ratios might consequently be inherited from earlier stages of star formation if the young Sun experienced conditions similar to NGC 1333 IRAS2A.

THE SPATIAL DISTRIBUTION OF ORGANICS TOWARD THE HIGH-MASS YSO NGC 7538 IRS9

Based on astrochemical theory, the complex molecular composition around high-mass YSOs should evolve from the outer envelope in toward the central hot region as a sequence of temperature dependent chemical pathways are activated in ices and in the gas-phase. The resulting complex molecules have been broadly classified into three generations dependent on the temperature (<25, >25, and >100 K) required for formation. We combine IRAM 30m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection, quantifying the emission and abundance profiles of key organic molecules as a function of distance from the central protostar. We find that emission from N-bearing organics and saturated O-bearing organics present large increases in emission around 8000 AU and R<3000 AU, while O-bearing molecules and hydrocarbons do not. The increase in flux from some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for some molecules at R<3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase chemistry drives the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used to constrain the chemical generation(s) to which different classes of molecule belong.

Mid-infrared interferometry of the massive young stellar object NGC 2264 IRS 1

Context. The optically invisible infrared-source NGC 2264 IRS 1 lying north of the Cone Nebula is thought to be a massive young stellar object (~10 M⊙). Although strong infrared excess clearly shows that the central object is surrounded by large amounts of circumstellar material, no information about the spatial distribution of this circumstellar material has been available until now.

Mid-infrared interferometry of the massive young stellar object NGC 3603 – IRS 9A

We present observations and models for one of these MYSO candidates, NGC3603 IRS 9A. Our goal is to investigate with infrared interferometry the structure of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O and B stars has blown away much of the obscuring foreground dust and gas. Observations in the N-band were carried out with the MIDI beam combiner attached to the VLTI. Additional interferometric observations which probe the structure of IRS 9A on larger scales were performed with an aperture mask installed in the T-ReCS instrument of Gemini South. The spectral energy distribution (SED) is constrained by the MIDI N-band spectrum and by data from the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS 9A range from simple geometrical models of the brightness distribution to one- and two-dimensional radiative transfer computations. The target is resolved by T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas (2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component was detected by MIDI which is possibly associated with the inner regions of a flattened dust distribution. Based on our interferometric data, no sign of multiplicity was found on scales between about 200AU and 700AU projected separation. A geometric model consisting of a warm (1000 K) ring (400 AU diameter) and a cool (140 K) large envelope provides a good fit to the data. No single model fitting all visibility and photometric data could be found, with disk models performing better than spherical models. While the data are clearly inconsistent with a spherical dust distribution they are insufficient to prove the existence of a disk but rather hint at a more complex dust distribution.

SPITZER OBSERVATIONS OF THE H II REGION NGC 2467: AN ANALYSIS OF TRIGGERED STAR FORMATION

We present new Spitzer Space Telescope observations of the region NGC 2467, and use these observations to determine how the environment of an H II region affects the process of star formation. Our observations comprise IRAC (3.6, 4.5, 5.8, and 8.0 μm) and MIPS (24 μm) maps of the region, covering approximately 400 arcmin2. The images show a region of ionized gas pushing out into the surrounding molecular cloud, powered by an O6V star and two clusters of massive stars in the region. We have identified as candidate young stellar objects (YSOs) 45 sources in NGC 2467 with infrared excesses in at least two mid-infrared colors. We have constructed color-color diagrams of these sources and have quantified their spatial distribution within the region. We find that the YSOs are not randomly distributed in NGC 2467; rather, over 75% of the sources are distributed at the edge of the H II region, along ionization fronts driven by the nearby massive stars. The high fraction of YSOs in NGC 2467 that are found in proximity to gas that has been compressed by ionization fronts supports the hypothesis that a significant fraction of the star formation in NGC 2467 is triggered by the massive stars and the expansion of the H II region. At the current rate of star formation, we estimate at least 25%-50% of the total population of YSOs formed by this process.

A Near‐Infrared Study of the NGC 7538 Star‐forming Region

We present subarcsecond (FWHM ~ 0farcs7), near-infrared (NIR) JHKs-band images and a high-sensitivity radio continuum image at 1280 MHz, using SIRIUS on the University of Hawaii 88 inch (2.2 m) telescope and the Giant Metrewave Radio Telescope (GMRT). The NIR survey covers an area of ~24 arcmin^2 with 10 σ limiting magnitudes of ~19.5, 18.4, and 17.3 in the J, H, and Ks bands, respectively. Our NIR images are deeper than any JHK surveys to date for the larger area of the NGC 7538 star-forming region. We construct JHK color-color and J - H/J and H - K/K color-magnitude diagrams to identify young stellar objects (YSOs) and to estimate their masses. Based on these color-color and color-magnitude diagrams, we identified a rich population of YSOs (Class I and Class II) associated with the NGC 7538 region. A large number of red sources (H - K > 2) have also been detected around NGC 7538. We argue that these red stars are most probably pre-main-sequence stars with intrinsic color excesses. Most of the YSOs in NGC 7538 are arranged from the northwest toward the southeast regions, forming a sequence in age: a diffuse H II region (northwest and oldest, where most of the Class II and Class I sources are detected), a compact IR core (center), and regions with an extensive IR reflection nebula and a cluster of red young stars (southeast and south). We find that the slope of the Ks-band luminosity function of NGC 7538 is lower than the typical values reported for young embedded clusters, although equally low values have also been reported in the W3 Main star-forming region. From the slope of the Ks-band luminosity function and the analysis by Megeath and coworkers, we infer that the embedded stellar population is composed of YSOs with an age of ~1 Myr. Based on the comparison of models of pre-main-sequence stars with the observed color-magnitude diagram, we find that the stellar population in NGC 7538 is primarily composed of low-mass pre-main-sequence stars similar to those observed in the W3 Main star-forming region. The radio continuum image from the GMRT observations at 1280 MHz shows an arc-shaped structure due to the interaction between the H II region and the adjacent molecular cloud. The ionization front at the interface between the H II region and the molecular cloud is clearly delineated by comparing the radio continuum, molecular line, and NIR images.

Two Embedded Young Stellar Objects in NGC 2264 with FU Orionis Characteristics

We report the discovery of two young stars in the NGC 2264 star-forming molecular cloud whose physical and morphological characteristics are similar to FU Orionis objects. The objects form a close pair and exhibit a curved reflection nebula in the near-IR. The brighter of the two stars, AR 6A, is optically visible but considerably reddened, has a small 3 μm IR excess, shows pronounced 2.294 μm CO overtone band head absorption, and is the brightest near-IR object in the region. The fainter star, AR 6B, located 28 south, has a much larger thermal IR excess and shows considerably deeper CO band head absorption identical to that seen in several known FU Orionis stars (FUors). Both stars therefore resemble FUors in terms of thermal excess, near-IR spectral features, and morphology. Here we discuss the characteristics of AR 6A and AR 6B in relation to the FUor population and propose that both stars are likely additional members of this class of young stellar outburst objects. We speculate that the simultaneous appearance of two FUor–like objects in a binary system, which would be highly unlikely if unrelated, could be caused by orbital evolution in a newly formed hierarchical quadruple system.

Gemini Mid-Infrared Imaging of Massive Young Stellar Objects in NGC 3576

We present a mid-infrared (MIR) study of NGC 3576. The high-resolution images were taken at the Gemini South Observatory through narrow- and broadband filters centered between 7.9 and 18 μm. The nearly diffraction-limited images show IRS 1 resolved into four sources for the first time in the 10 μm band. The positions of the sources are coincident with massive young stellar objects (YSOs) detected previously in the near-infrared (NIR). The properties of each object, such as spectral energy distribution, silicate absorption feature, color temperature, and luminosities were obtained and are discussed. We also report observations of two other YSO candidates and the detection of a new diffuse MIR source without an NIR counterpart. We conclude that none of these sources contributes significantly to the ionization of the H II region. A possible location for the ionization source of NGC 3576 is suggested based on both radio and infrared data.

A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

We present the results of the first broadband 10.8 um survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry in order to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster, and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band magnitudes K < 10.5, we detected 35 (~59%) at 10 um. Combining these detections, and upper limits for the non-detections, with existing JHKL data, we identify 1 Class I, 6 flat spectrum, 28 Class II and 5 Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% +/- 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in NGC 2024 formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared excess region of the JHKL color-color diagram for the cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with disks. Of the 14 sources with K - L colors suggestive of protostellar objects, \~29% are protostellar in nature, while ~7% are true Class I YSOs. This may be due to extinction producing very red K - L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests caution must be applied when estimating the sizes and lifetimes of protostellar populations within star forming regions based on K - L colors alone. Finally, we calculate the luminosities of the Class II YSOs in NGC 2024, rho Oph and Taurus and discuss the results.

Detection of Interstellar H[TINF]2[/TINF]D[TSUP]+[/TSUP] Emission

We report the detection of the 110-111 ground-state transition of ortho-H2D+ at 372.421 GHz in emission from the young stellar object NGC 1333 IRAS 4A. Detailed excitation models with a power-law temperature and density structure yield a beam-averaged H2D+ abundance of 3 × 10-12 with an uncertainty of a factor of 2. The line was not detected toward W33A, GL 2591, and NGC 2264 IRS (in the latter source at a level that is 3-8 times lower than previous observations). The H2D+ data provide direct evidence in support of low-temperature chemical models in which H2D+ is enhanced by the reaction of H and HD. The H2D+ enhancement toward NGC 1333 IRAS 4A is also reflected in the high DCO+/HCO+ abundance ratio. Simultaneous observations of the N2H+ 4-3 line show that its abundance is about 50-100 times lower in NGC 1333 IRAS 4A than in the other sources, suggesting significant depletion of N2. The N2H+ data provide independent lower limits on the H abundance that are consistent with the abundances derived from H2D+. The corresponding limits on the H column density agree with recent near-infrared absorption measurements of H toward W33A and GL 2591.

Possible Infall around the Intermediate-Mass Young Stellar Object NGC 2264 IRS

Analysis of multitransitional CS and CO observations toward the class I intermediate-mass young stellar object NGC 2264 IRS indicates that the outflow associated with this source dominates the kinematic structure of the CS J = 2 → 1, J = 3 → 2, and J = 5 → 4 spectra. For example, toward the source, centroid velocities (the first moments) of these line profiles increase with increasing optical depth; i.e., the emission becomes increasingly redshifted, which is expected for outflow from a central source. In contrast, the CS J = 7 → 6 spectrum toward NGC 2264 IRS is asymmetric, having a single blueshifted peak with a red shoulder, which suggests the possible presence of infall toward NGC 2264 IRS. In addition, the centroid velocities for the CS J = 7 → 6 line become more negative (lines are blueshifted) as one approaches the source perpendicular to the outflow. Furthermore, the CS J = 7 → 6 line is blueshifted relative to the CS J = 2 → 1, J = 3 → 2, and J = 5 → 4 lines toward NGC 2264 IRS. Integrated intensity contours of the CS J = 5 → 4 emission are extended in the direction of the outflow; however, the CS J = 7 → 6 emission is extended in the direction perpendicular to the outflow. We describe a preliminary attempt to model the CS J = 7 → 6 spectra with a Monte Carlo radiative transfer code (Choi et al.) using an inside-out collapse scenario (Shu).

Millimeter Interferometric Polarization Imaging of the Young Stellar Object NGC 1333/IRAS 4A

We present a 3.4 mm polarization image of the dust emission associated with the young stellar object NGC 1333/IRAS 4A made with the Owens Valley Millimeter Array with 5'' resolution. The integrated linear polarization of the dust continuum is 4% of the total intensity. The polarization is produced by magnetically aligned dust grains and arises from very dense gas (n > 108 cm-3), which indicates that the dust alignment process remains viable in the dense protostellar envelope. The magnetic field directions inferred from our observations are aligned with features seen in the high-velocity outflow emanating from IRAS 4A. The magnetic field directions are not aligned with the field on much larger scales as measured by optical and infrared selective extinction, which suggests significant field structure in the cloud core. The peak of the polarized emission is offset from the total intensity peak, which perhaps indicates considerable unresolved structure in the magnetic field.

Images of star-forming regions. III - Sources in the NGC 7538 molecular cloud complex

view Abstract Citations (34) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Images of Star-Forming Regions. III. Sources in the NGC 7538 Molecular Cloud Complex Campbell, Bel ; Persson, S. E. Abstract Deep optical CCD images of the embedded infrared sources associated with NGC 7538 are presented. A reflected image of IRS 1, the most luminous infrared source in the region, is shown to be positionally offset from the VLA radio source in a direction that is consistent with that of the blueshifted portion of the bipolar molecular outflow toward the north. Maps of the IRS 1 region at 1.65 and 2.2 microns, made with a prototype infrared camera, confirm the positional displacement. The H-K color of IRS 1 is extremely red (greater than 5 mag), which shows that the radio source is indeed totally obscured at wavelengths less than 1.5 micron. This is also consistent with other estimates of the extinction to IRS 1. New VLA radio images of IRS 2 and IRS 3 are also presented. The peculiar object IRS 9, a source that displays deep absorption due to CO molecules in the solid phase, is associated with a diffuse optical nebula that corresponds with the infrared reflection nebula found by Werner et al. (1979). The young stellar objects in NGC 7538 represent a broad evolutionary range in high-mass star formation, from a fully evolved blister-type H II region to much younger objects which have yet to break through their protostellar cocoons. Publication: The Astronomical Journal Pub Date: April 1988 DOI: 10.1086/114714 Bibcode: 1988AJ.....95.1185C Keywords: Infrared Sources (Astronomy); Molecular Clouds; Stellar Evolution; Very Large Array (Vla); Carbon Monoxide; Charge Coupled Devices; Millimeter Waves; Radio Emission; Stellar Envelopes; Astrophysics; STARS: PRE-MAIN-SEQUENCE; INTERSTELLAR: MATTER; INFRARED: SOURCES full text sources ADS | data products SIMBAD (18) Related Materials (2) Part 1: 1986ApJ...305..336C Part 2: 1988AJ.....95.1173C