Results
Fibers in the massive Orion Integral Filament
From the combination of 2x 150-pointing ALMA mosaics and large-scale IRAM30m observations, we have investigated the internal structure of the Orion Integral Shape Filament (ISF). Our observations prove the presence of dense gas fibers within this massive filament with identical kinematic properties than those found in low-mass clouds. (Hacar et al 2018) (ADS)
From left to right: (a) SCUBA-850mu continuum map (Johnstone & Bally 1999); (b) ALMA + IRAM30m integrated N2H+ (1-0) emission map; (c) N2H+ fibers extracted HiFIVE. The left panel indicates the position of all the protostars (cyan triangles) and continuum sources (cyan crosses) detected in previous studies as well as the most important regions within this cloud. Note that the dense gas detected by ALMA forms a complex fiber network with multiple hub-like structures.
Narrow fibers in Orion
Previous Herschel observations suggested that filaments may present universal widths of ~0.1pc. Contrary to these Herschel studies, we have detected narrow fibers with median FWHM of ~0.035pc. Our results demonstrates that these filamentary structures present a wide range of widths probably depending on their initial gas density. (Hacar et al 2018) (ADS)
(Left): ALMA + IRAM30m integrated N2H+ (1-0) emission map of both OMC-2 South (upper panel) and OMC-1 Ridge (lower panel) regions. (Centre and Right): N2H+ (1-0) emission cut perpendicular to the main gas structure. The red dashed-line indicates the best gaussian fit to these profiles with a FWHM indicated in the top-left corner of each subplot. Note the narrow emission distribution of all the ISF fibers in comparison with the typical 0.1 pc width proposed in previous studies (see scales on the ALMA emission maps). At the distance of Orion, this analysis is only possible thanks to the enhanced ALMA resolution (4.5" or ~0.009pc) compared to other single-dish studies (e.g., Herschel with 36").
Gas at ultra-high densities (>107 cm-3) in the OMC-1 region
We used the new SEPIA660 receiver installed at the APEX12m radiotelescope to investigate the molecular emission in the OMC-1 region at frequencies of ~650 GHz (ALMA Band 9). The first detection of bright and extended N2H+ (7-6) emission demonstrates presence of large mass reservoirs of gas at ultra-high densities, typically above >107 cm-3 , in this high-mass star-forming region. The detection of this emission opens a new window on the study of the origin of massive clusters at high frequencies (Hacar et al 2020b) (ADS).
New SEPIA660 observations along the OMC-1 region. From left to right: (a) VISTA-IR Ks band (Meingast et al. 2016), (b) APEX- SEPIA660 N2H+ (7–6), and (c) APEX-SEPIA660 C18O(6–5) maps (this work). All molecular maps are convolved into a common Nyquist grid with a final resolution of 10". The intensity the H41α emission tracing the extension of the ONC HII nebula (red contours, Hacar et al. 2020a) is indicated in the VISTA image. For reference, the position of the Trapezium (white stars) and the Orion BN source (yellow star), as well as the first contour of the N2H+ (1–0) emission (white contour) (Hacar et al. 2018), are displayed in all panels.
HCN/HNC intensity ratio: a new chemical thermometer for the molecular ISM
Our ORION-4D survey mapped the ISF region in both 1mm and 3mm bands with the IRAM-30m radiotelescope. Our broad band observations obtained simultaneous OTF-maps of both HCN and HNC (1-0) lines. Answering an old question in astrochemistry, our analysis demonstrates that it is possible to use the line ratio of these two isomers (i.e., I(HCN)/I(HNC)) as direct probe of the gas kinetic temperatures in the ISM (Hacar, Bosman, & van Dishoeck 2020a) (ADS)
Comparison between different TK estimates along the ISF: (a) TK(HCN/HNC) (this work), (b) ammonia-derived gas kinetic temperatures TK(NH3) (Friesen et al. 2017), (c) Herschel dust temperatures Tdust (Lombardi et al. 2014), and (d) 12CO (1–0) excitation temperatures Tex(12CO) (Nakamura et al. 2012; Shimajiri et al. 2014), all represented with the same colour scale. We note that both dust temperature and column density are underestimated towards the OMC-1 and BN/KL regions due to saturation effects and the lack of proper spectral energy distribution (SED) fits in the Herschel maps provided by Lombardi et al. (2014). Also we notice the rapid decrease of the Tex(CO) at the northern end our maps, likely due to subthermal excitation of the 12CO (i.e., Tex(12CO) < TK) lines at the low densities expected in this region. To facilitate the comparison of these figure, we indicate the intensity contour with I(N2H+)=1.5 K km s−1 in panels (b-d) (see Hacar et al 2017).
Disk evolution in OMC-2
In addition to our line observations, our 3mm continuum ALMA maps reveal a plethora of protoplanetary disks along the Integral Shape Filament. A statistical analysis of their mass distribution demonstrates that the time evolution of the disk masses in high-mass star-forming regions such as OMC-2 is similar to those in low-mass clouds like Lupus or Taurus (van Terwisga, Hacar, & van Dishoeck 2019) (ADS)
(Left) VISTA Ks observations of along the Integral Shape Filament, including the OMC-1 and ONC regions (South) plus OMC-2 (North) (Meingast et al. 2016). (Right) Spatially filtered ALMA observations of the same region at 3 mm . Point sources detected in the ALMA data are marked: red circles and blue squares indicate disks and protostars from the Megeath et al. (2012)-catalog respectively; sources without detected counterparts in this catalog are marked with black triangles. A total of 26 Protostars, 37 Disks, and 34 Unknown sources are detected in our survey only in the OMC-2 region.
Gravitational collapse of the OMC-1 region
Although originally attributed to the gas explosion of the Orion BN/KL nebula, our work demonstrates that the origin of the so-called molecular fingers might be related to the global gravitational collapse of the OMC-1 cloud. Our detailed analysis of the dense gas kinematics within this region shows a characteristic radial velocity dependence consistent with the expected gravitational accelerations generated by the ONC potential well. These results bring new evidence of the role played by gravity on the assembly of massive clusters. (Hacar et al 2017 (ADS)
From left to right: a) VISTA-NIR image of the ISF; b) IRAM 30 m integrated N2H+ (1-0) emission; c) N2H+ line velocity centroid (VLSR) and d) N2H+ Full-Width-Half-Maximum (FWHM) as a function of declination. Map offsets are referred, in radio projection, to the position of the Orion BN source (white cross). N2H+ contours are equally spaced every 2 K km s-1 . For reference, the first N2H+ (1-0) contour is superposed to the IR image. The magnitude of representative gradients with 1 (green) plus 5, and 7 (blue) km s−1 pc−1 are indicated in the velocity plot.
APOGEE strings: A fossil record of the gas kinematic substructure
For the first time, the accuracy of the stellar radial velocity (RV) measurements provided by APOGEE IN-SYNC survey (~ 0.2 km s−1) allows a direct comparison between the star and gas velocity fields. Their joint study in the Orion A cloud demonstrates that the newly born stars keep memory of the clustering and small velocity dispersion of their parental gas. In particular, chains or strings of stars are originated in the velocity-coherent fibers present in the gas. (Hacar et al 2016) (ADS) (movie)
Position-Position-Velocity diagram (PPV) of the stellar and gas motions along the Orion A cloud. (black triangles) APOGEE stellar RVs; (Blue and red surfaces) overdensities of APOGEE sources with 5 and 10 stars pc−2 (km s−1)−1; (Yellow and Green) 0.5 and 2.0 K km s−1 iso- contours of the 12CO (1-0) emission (Dame et al. 2001) within the area surveyed by APOGEE. The PPV cube is spatially oriented respect to the Galactic Longitude (b; x-axis) and Galactic Latitude (l; y-axis) coordinates, both in degrees units. Both gas and stellar velocities (VLS R ; z-axis) are referred to the LSR in km s−1 within the velocity range between VLSR = [−2.0,20.0] km s−1 . The APOGEE sources are projected in the PP (xy) and PV (xz) planes. The spatial densities of [2,10,20,40) stars pc−2 in the APOGEE sources is also color coded (from red to yellow) in the PP (xy) plane. We note how, with the exception of stars in dynamically hot clusters like the ONC or NGC 1977, the stellar overdensities (blue iso-surfaces) are typically clustered in the PPV space and embedded within the velocity structure of the gas (orange and yellow iso-surfaces). The positions of the most important clusters within Orion A are indicated in the plot.