The effects of metallicity on the evolution of protoplanetary disks may be studied in the outer Galaxy where the metallicity is lower than in the solar neighborhood. We present the VLT/KMOS integral ...field spectroscopy in the near-infrared of∼120 candidate young stellar objects (YSOs) in the CMa-ℓ224 star-forming region located at a Galactocentric distance of 9.1 kpc. We characterise the YSO accretion luminosities and accretion rates using the hydrogen Brγemission and find the median accretion luminosity of log (Lacc) =−0.82+0.80−0.82L⊙.Based on the measured accretion luminosities, we investigate the hypothesis of star formation history in the CMa-ℓ224. Their median values suggest that Cluster C, where most of YSO candidates have been identified, might be the most evolved part of the region. The accretion luminosities are similar to those observed toward low-mass YSOs in the Perseus and Orion molecular clouds, and do not reveal the impact of lower metallicity. Similar studies in other outer Galaxy clouds covering a wide range of metallicities are critical to gain a complete picture of star formation in the Galaxy
We have carried out a near-infrared imaging survey of luminous young stellar outflow candidates using the United Kingdom Infrared Telescope. Observations were obtained through the broad-band K (2.2 ...μm) and narrow-band filters at the wavelengths of H2 v= 1–0 S(1) (2.1218 μm) and Brγ (2.166 μm) lines. 50 regions were imaged with a field of view of 2.2 × 2.2 arcmin2. Several young embedded clusters are unveiled in our near-infrared images. 76 per cent of the objects exhibit H2 emission and 50 per cent or more of the objects exhibit aligned H2 emission features suggesting collimated outflows, many of which are new detections. These observations suggest that disc accretion is probably the leading mechanism in the formation of stars, at least up to late O spectral types. The young stellar objects (YSOs) responsible for many of these outflows are positively identified in our images based on their locations with respect to the outflow lobes, Two-Micron All-Sky Survey colours and association with Midcourse Space Experiment, Infrared Astronomical Satellite, millimetre and radio sources. The close association of molecular outflows detected in CO with the H2 emission features produced by shock excitation by jets from the YSOs suggests that the outflows from these objects are jet-driven. Towards strong radio emitting sources, H2 jets were either not detected or were weak when detected, implying that most of the accretion happens in the pre-ultracompact (pre-UC) H ii phase; accretion and outflows are probably weak when the YSO has advanced to its UC H ii stage.
Context.
The mass accretion rate (
Ṁ
acc
) is the fundamental parameter to understand the process of mass assembly that results in the formation of a low-mass star. This parameter has been largely ...studied in Classical T Tauri stars in star-forming regions with ages of ∼1 − 10 Myr. However, little is known about the accretion properties of young stellar objects (YSOs) in younger regions and early stages of star formation, such as in the Class 0/I phases.
Aims.
We present new near-infrared spectra of 17 Class I/Flat and 35 Class II sources located in the young (< 1 Myr) NGC 1333 cluster, acquired with the KMOS instrument at the Very Large Telescope. Our goal is to study whether the mass accretion rate evolves with age, as suggested by the widely adopted viscous evolution model, by comparing the properties of the NGC 1333 members with samples of older regions.
Methods.
For the Class II sources in our sample, we measured the stellar parameters (SpT,
A
V
, and
L
⋆
) through a comparison of the IR spectra with a grid of non-accreting Class III stellar templates. We then computed the accretion luminosity by using the known correlation between
L
acc
and the luminosity of HI lines (Pa
β
and Br
γ
). For the Class I sample, where the presence of a large IR excess makes it impossible to use the same spectral typing method, we applied a procedure that allowed us to measure the stellar and accretion luminosity in a self-consistent way. Mass accretion rates
Ṁ
acc
were then measured once masses and radii were estimated adopting suitable evolutionary tracks.
Results.
The NGC 1333 Class II sources of our sample have
L
acc
∼ 10
−4
− 1
L
⊙
and
Ṁ
acc
∼ 10
−11
− 10
−7
M
⊙
yr
−1
. We find a correlation between accretion and stellar luminosity in the form of log
L
acc
= (1.5 ± 0.2)log
L
⋆
+ ( − 1.0 ± 0.1), and a correlation between the mass accretion rate and stellar mass in the form of log
Ṁ
acc
= (2.6 ± 0.9) log
M
⋆
+ (−7.3 ± 0.7). Both correlations are compatible within the errors with the older Lupus star-forming region, while only the latter is consistent with results from Chamaeleon I. The Class I sample shows larger accretion luminosities (∼10
−2
− 10
2
L
⊙
) and mass accretion rates (∼10
−9
− 10
−6
M
⊙
yr
−1
) with respect to the Class II stars of the same cloud. However, the derived mass accretion rates are not sufficiently high to build up the inferred stellar masses, assuming steady accretion during the Class I lifetime. This suggests that the sources are not in their main accretion phase and that most of their mass has already been accumulated during a previous stage and/or that the accretion is an episodic phenomenon. We show that some of the targets originally classified as Class I through
Spitzer
photometry are in fact evolved or low accreting objects. This evidence can have implications for the estimated protostellar phase lifetimes.
Conclusions.
The accretion rates of our sample are larger in more embedded and early stage YSOs. Further observations of larger samples in young star-forming regions are needed to determine if this is a general result. In addition, we highlight the importance of spectroscopic surveys of YSOs to confirm their classification and perform a more correct estimate of their lifetime.
Westerlund 2 (Wd2) is the central ionizing star cluster of the H ii region RCW 49 and the second most massive young star cluster ( ) in the Milky Way. Its young age (∼2 Myr) and close proximity to ...the Sun (∼4 kpc) makes it a perfect target to study stars emerging from their parental gas cloud, the large number of OB-stars and their feedback onto the gas, and the gas dynamics. We combine high-resolution multi-band photometry obtained in the optical and near-infrared with the Hubble Space Telescope, and Very Large Telescope/MUSE integral field spectroscopy to study the gas, the stars, and their interactions, simultaneously. In this paper we focus on a small, 64 × 64 arcsec2 region north of the main cluster center, which we call the Northern Bubble (NB), a circular cavity carved into the gas of the cluster region. Using MUSE data, we determined the spectral types of 17 stars in the NB from G9III to O7.5. With the estimation of these spectral types, we add 2 O and 5 B-type stars to the previously published census of 37 OB-stars in Wd2. To measure radial velocities we extracted 72 stellar spectra throughout Wd2, including the 17 of the NB, and show that the cluster-member stars follow a bimodal velocity distribution centered around (8.10 1.53) km s−1 and (25.41 1.57) km s−1 with a dispersion of (4.52 1.78) km s−1 and (3.46 1.29) km s−1, respectively. These are in agreement with CO(J = 1-2) studies of RCW 49 leaving cloud-cloud collision as a viable option for the formation scenario of Wd2. The bimodal distribution is also detected in the Gaia DR2 proper motions.
ABSTRACT
Protostellar jets and outflows are signposts of active star formation. In H ii regions, molecular tracers like CO only reveal embedded portions of the outflow. Outside the natal cloud, ...outflows are dissociated, ionized, and eventually completely ablated, leaving behind only the high-density jet core. Before this process is complete, there should be a phase where the outflow is partially molecular and partially ionized. In this paper, we capture the HH 900 outflow while this process is in action. New observations from the Enhanced Resolution Imager and Spectrograph/SPIFFIER near-infrared (IR) integral field unit spectrograph using the K-middle filter (λ = 2.06–2.34 μm) reveal H2 emission from the dissociating outflow and Br-γ tracing its ionized skin. Both lines trace the wide-angle outflow morphology but H2 only extends ∼5000 au into the H ii region while Br-γ extends the full length of the outflow (∼12 650 au), indicating rapid dissociation of the molecules. H2 has higher velocities further from the driving source, consistent with a jet-driven outflow. Diagnostic line ratios indicate that photoexcitation, not just shocks, contributes to the excitation in the outflow. We argue that HH 900 is the first clear example of an evaporating molecular outflow and predict that a large column of neutral material that may be detectable with Atacama Large Millimeter Array accompanies the dissociating molecules. Results from this study will help guide the interpretation of near-IR images of externally irradiated jets and outflows such as those obtained with the JWST in high-mass star-forming regions where these conditions may be common.
Near-infrared imaging of the emission from molecular hydrogen is a powerful method for discovering outflows in star-forming regions. We present new near-infrared images, and long-slit and ...integral-field spectroscopy of the ultracompact H ii region G25.65+1.05. These new observations reveal shocked H2 emission associated with a bipolar outflow from a young high-mass star at the centre of the source. The physical parameters of the outflow are discussed and compared with those of outflows from lower mass stars.
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