Rosette Molecular Cloud Research Articles

Ammonia characterisation of dense cores in the Rosette Molecular Cloud

Context. The Rosette molecular cloud complex is a well-known Galactic star-forming region with a morphology pointing towards triggered star formation. The distribution of its young stellar population and the gas properties point to the possibility that star formation is globally triggered in the region. Aims. We focus on the characterisation of the most massive pre- and protostellar cores distributed throughout the molecular cloud in order to understand the star formation processes in the region. Methods. We observed a sample of 33 dense cores, identified in Herschel continuum maps, with the Effelsberg 100-m telescope. Using NH3 (1,1) and (2,2) measurements, we characterise the dense core population, computing rotational and gas kinetic temperatures and NH3 column density with multiple methods. We also estimated the gas pressure ratio and virial parameters to examine the stability of the cores. Using results from Berschel data, we examined possible correlations between gas and dust parameters. Results. Ammonia emission is detected towards 31 out of the 33 selected targets. We estimate kinetic temperatures to be between 12 and 20 K, and column densities within the 1014−2 × 1015 cm−2 range in the selected targets. Our virial analysis suggests that most sources are likely to be gravitationally bound, while the line widths are dominated by non-thermal motions. Our results are compatible with large-scale dust temperature maps suggesting that the temperature decreases and column density increases with distance from NGC 2244 except for the densest protoclusters. We also identify a small spatial shift between the ammonia and dust peaks in the regions most exposed to irradiation from the nearby NGC 2244 stellar cluster. However, we find no trends in terms of core evolution with spatial location, in the prestellar to protostellar core abundance ratio, or the virial parameter. Conclusions. Star formation is more likely based on the primordial structure of the cloud in spite of the impact of irradiation from the nearby cluster, NGC 2244. The physical parameters from the NH3 measurements suggest gas properties in between those of low- and high-mass star-forming regions, suggesting that the Rosette molecular cloud could host ongoing intermediate-mass star formation, and is unlikely to form high-mass stars.

The JCMT BISTRO-2 Survey: The Magnetic Field in the Center of the Rosette Molecular Cloud

Abstract We present the first 850 μm polarization observations in the most active star-forming site of the Rosette Molecular Cloud (d ∼ 1.6 kpc) in the wall of the Rosette Nebula, imaged with the SCUBA-2/POL-2 instruments of the James Clerk Maxwell telescope, as part of the B-Fields In Star-forming Region Observations 2 (BISTRO-2) survey. From the POL-2 data we find that the polarization fraction decreases with the 850 μm continuum intensity with α = 0.49 ± 0.08 in the p ∝ I −α relation, which suggests that some fraction of the dust grains remain aligned at high densities. The north of our 850 μm image reveals a “gemstone ring” morphology, which is a ∼1 pc diameter ring-like structure with extended emission in the “head” to the southwest. We hypothesize that it might have been blown by feedback in its interior, while the B-field is parallel to its circumference in most places. In the south of our SCUBA-2 field the clumps are apparently connected with filaments that follow infrared dark clouds. Here, the POL-2 magnetic field orientations appear bimodal with respect to the large-scale Planck field. The mass of our effective mapped area is ∼174 M ⊙, which we calculate from 850 μm flux densities. We compare our results with masses from large-scale emission-subtracted Herschel 250 μm data and find agreement within 30%. We estimate the plane-of-sky B-field strength in one typical subregion using the Davis–Chandrasekhar–Fermi technique and find 80 ± 30 μG toward a clump and its outskirts. The estimated mass-to-flux ratio of λ = 2.3 ± 1.0 suggests that the B-field is not sufficiently strong to prevent gravitational collapse in this subregion.

Assessing the performance of molecular gas clump identification algorithms

The detection of clumps (cores) in molecular clouds is an important issue in sub-millimetre astronomy. However, the completeness of the identification and the accuracy of the returned parameters of the automated clump identification algorithms are still not clear. In this work, we test the performance and bias of the GaussClumps, ClumpFind, FellWalker, Reinhold, and Dendrograms algorithms in identifying simulated clumps. By designing the simulated clumps with various sizes, peak brightness, and crowdedness, we investigate the characteristics of the algorithms and their performance. In the aspect of detection completeness, FellWalker, Dendrograms, and GaussClumps are the first, second, and third best algorithms, respectively. The numbers of correct identifications of the six algorithms gradually increase as the size and signal-to-noise ratio (SNRs) of the simulated clumps increase and they decrease as the crowdedness increases. In the aspect of the accuracy of retrieved parameters, FellWalker and Dendrograms exhibit better performance than the other algorithms. The average deviations in clump parameters for all algorithms gradually increase as the size and SNR of clumps increase. Most of the algorithms except FellWalker exhibit significant deviation in extracting the total flux of clumps. Taken together, FellWalker, GaussClumps, and Dendrograms exhibit the best performance in detection completeness and extracting parameters. The deviation in virial parameter for the six algorithms is relatively low. When applying the six algorithms to the clump identification for the Rosette molecular cloud, ClumpFind1994, ClumpFind2006, GaussClumps, FellWalker, and Reinhold exhibit performance that is consistent with the results from the simulated test.

A Large-scale Survey of CO and Its Isotopologues toward the Rosette Molecular Cloud

Using the PMO-13.7m millimeter telescope at Delingha in China, we have conducted a large-scale simultaneous survey of $^{12}$CO, $^{13}$CO, and C$^{18}$O J=1-0 emission toward the Rosette molecular cloud (RMC) region with a sky coverage of 3.5 $\times$ 2.5 deg$^2$. The majority of the emission in the region comes from the RMC complex with velocities lying in the range from -2 km s$^{-1}$ to 20.5 km s$^{-1}$. Beyond this velocity range, 73 molecular clumps are identified with kinematic distances from 2.4 kpc to 11 kpc. Based on the spatial and velocity distribution, nine individual clouds, C1-C9, have been identified for the RMC complex. It appears that the C3 cloud is different from other clouds in the RMC complex in view of its characteristic velocity, excitation temperature, and velocity dispersion. Most of the young stellar clusters in the region are located in positions of both high column density and high excitation temperature. Seven new molecular filaments are discovered in the RMC complex. Evidence for cloud-cloud collision is found in the region of young stellar clusters REFL9 and PouF, showing that these young stellar clusters probably result from a cloud-cloud collision. The abundance ratios of $^{13}$CO to C$^{18}$O in the region have a mean value of 13.7 which is 2.5 times larger than the solar system value, showing that UV photons from the nearby OB clusters have strong influence on the chemistry of clouds in the RMC complex.

Pre- and protostellar cores in the Rosette Nebula

AbstractRecent theories on the formation of the Solar System turned the attention to the study of low mass cloud cores in massive star forming regions. The Rosette Molecular Cloud is a well-known star forming area having highly filamentary structure with dense cores covering a wide range of masses. These pre- and protostellar cores were observed by Herschel and key core properties were derived from its data. With the Effelsberg 100m telescope a sample of these cores with masses ranging between 3-40 M⊙ were observed in ammonia inversion lines. In this work we are examining the correlations between these two datasets with the aim of gaining insight of the processes behind the star formation of the region.

Hub-filament System in IRAS 05480+2545: Young Stellar Cluster and 6.7 GHz Methanol Maser

Abstract To probe the star formation (SF) process, we present a multi-wavelength study of IRAS 05480+2545 (hereafter I05480+2545). Analysis of Herschel data reveals a massive clump (M clump ∼ 1875 peak N(H2) ∼ 4.8 × 1022 cm−2; A V ∼ 51 mag) containing the 6.7 GHz methanol maser and I05480+2545, which is also depicted in a temperature range of 18–26 K. Several noticeable parsec-scale filaments are detected in the Herschel 250 μm image and seem to be radially directed to the massive clump. It resembles more of a “hub-filament” system. Deeply embedded young stellar objects (YSOs) have been identified using the 1–5 μm photometric data, and a significant fraction of YSOs and their clustering are spatially found toward the massive clump, revealing the intense SF activities. An infrared counterpart (IRc) of the maser is investigated in the Spitzer 3.6–4.5 μm images. The IRc does not appear as a point-like source and is most likely associated with the molecular outflow. Based on the 1.4 GHz and Hα continuum images, the ionized emission is absent toward the IRc, indicating that the massive clump harbors an early phase of a massive protostar before the onset of an ultracompact H ii region. Together, the I05480+2545 is embedded in a very similar “hub-filament” system to those seen in the Rosette Molecular Cloud. The outcome of the present work indicates the role of filaments in the formation of the massive star-forming clump and cluster of YSOs, which might help channel material to the central hub configuration and the clump/core.

THE PHYSICAL ENVIRONMENT OF THE MASSIVE STAR-FORMING REGION W42

We present an analysis of multi-wavelength observations from various datasets and Galactic plane surveys to study the star formation process in the W42 complex. A bipolar appearance of W42 complex is evident due to the ionizing feedback from the O5-O6 type star in a medium that is highly inhomogeneous. The VLT/NACO adaptive-optics K and L' images (resolutions ~0".2-0".1) resolved this ionizing source into multiple point-like sources below ~5000 AU scale. The position angle ~15 deg of W42 molecular cloud is consistent with the H-band starlight mean polarization angle which in turn is close to the Galactic magnetic field, suggesting the influence of Galactic field on the evolution of the W42 molecular cloud. Herschel sub-millimeter data analysis reveals three clumps located along the waist axis of the bipolar nebula, with the peak column densities of ~3-5 x10^{22} cm^{-2} corresponding to visual extinctions of AV ~32-53.5 mag. The Herschel temperature map traces a temperature gradient in W42, revealing regions of 20 K, 25 K, and 30-36 K. Herschel maps reveal embedded filaments (length ~1-3 pc) which appear to be radially pointed to the denser clump associated with the O5-O6 star, forming a hub-filament system. 512 candidate young stellar objects (YSOs) are identified in the complex, ~40% of which are present in clusters distributed mainly within the molecular cloud including the Herschel filaments. Our datasets suggest that the YSO clusters including the massive stars are located at the junction of the filaments, similar to those seen in Rosette Molecular Cloud.

RETHINKING A MYSTERIOUS MOLECULAR CLOUD

I present high-resolution column density maps of two molecular clouds having strikingly different star formation rates. To better understand the unusual, massive G216-2.5, a molecular cloud with no massive star formation, the distribution of its molecular gas is compared to that of the Rosette Molecular Cloud. Far-infrared data from Herschel are used to derive $N(\mathrm{H}_2)$ maps of each cloud and are combined with $I_{\mathrm{CO}}$ data to determine the CO-to-H$_2$ ratio, $X_{\mathrm{CO}}$. In addition, the probability distribution functions (PDFs) and cumulative mass fractions of the clouds are compared. For G216-2.5, $< N(\mathrm{H}_2) >=7.8\times 10^{20} cm^{-2}$ and $< X_{\mathrm{CO}} > =2.2\times 10^{20} (K km s^{-1})^{-1}$; for the Rosette, $< N(\mathrm{H}_2) > =1.8\times 10^{21} cm^{-2}$ and $ < X_{\mathrm{CO}} > =2.8\times 10^{20} (K km s^{-1})^{-1}$. The PDFs of both clouds are log-normal for extinctions below $\sim 2$ mag and both show departures from log-normality at high extinctions. Although it is the less-massive cloud, the Rosette has a higher fraction of its mass in the form of dense gas and contains $1389 M_\odot$ of gas above the so-called extinction threshold for star formation, $A_V = 7.3$ mag. The G216-2.5 cloud has $874 M_\odot$ of dense gas above this threshold.

Pillars and globules at the edges of H ii regions

Pillars and globules are present in many high-mass star-forming regions, such as the Eagle nebula (M16) and the Rosette molecular cloud, and understanding their origin will help characterize triggered star formation. The formation mechanisms of these structures are still being debated. Recent numerical simulations have shown how pillars can arise from the collapse of the shell in on itself and how globules can be formed from the interplay of the turbulent molecular cloud and the ionization from massive stars. The goal here is to test this scenario through recent observations of two massive star-forming regions, M16 and Rosette. The column density structure of the interface between molecular clouds and H ii regions was characterized using column density maps obtained from far-infrared imaging of the Herschel HOBYS key programme. Then, the DisPerSe algorithm was used on these maps to detect the compressed layers around the ionized gas and pillars in different evolutionary states. Finally, their velocity structure was investigated using CO data, and all observational signatures were tested against some distinct diagnostics established from simulations. The column density profiles have revealed the importance of compression at the edge of the ionized gas. The velocity properties of the structures, i.e. pillars and globules, are very close to what we predict from the numerical simulations. We have identified a good candidate of a nascent pillar in the Rosette molecular cloud that presents the velocity pattern of the shell collapsing on itself, induced by a high local curvature. Globules have a bulk velocity dispersion that indicates the importance of the initial turbulence in their formation, as proposed from numerical simulations. Altogether, this study re-enforces the picture of pillar formation by shell collapse and globule formation by the ionization of highly turbulent clouds.

THE PROGRESSION OF STAR FORMATION IN THE ROSETTE MOLECULAR CLOUD

Using Spitzer Space Telescope and Chandra X-ray Observatory data, we identify YSOs in the Rosette Molecular Cloud (RMC). By being able to select cluster members and classify them into YSO types, we are able to track the progression of star formation locally within the cluster environments and globally within the cloud. We employ nearest neighbor method (NNM) analysis to explore the density structure of the clusters and YSO ratio mapping to study age progressions in the cloud. We find a relationship between the YSO ratios and extinction which suggests star formation occurs preferentially in the densest parts of the cloud and that the column density of gas rapidly decreases as the region evolves. This suggests rapid removal of gas may account for the low star formation efficiencies observed in molecular clouds. We find that the overall age spread across the RMC is small. Our analysis suggests that star formation started throughout the complex around the same time. Age gradients in the cloud appear to be localized and any effect the HII region has on the star formation history is secondary to that of the primordial collapse of the cloud.

Cluster-formation in the Rosette molecular cloud at the junctions of filaments (Corrigendum)

Cluster-formation in the Rosette molecular cloud at the junctions of filaments (<i>Corrigendum</i>)

Cluster-formation in the Rosette molecular cloud at the junctions of filaments

For many years feedback processes generated by OB-stars in molecular clouds, including expanding ionization fronts, stellar winds, or UV-radiation, have been proposed to trigger subsequent star formation. However, hydrodynamic models including radiation and gravity show that UV-illumination has little or no impact on the global dynamical evolution of the cloud. The Rosette molecular cloud, irradiated by the NGC2244 cluster, is a template region for triggered star-formation, and we investigated its spatial and density structure by applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and probability density functions (PDFs) on Herschel column density maps, obtained within the HOBYS key program. The analysis reveals not only the filamentary structure of the cloud but also that all known infrared clusters except one lie at junctions of filaments, as predicted by turbulence simulations. The PDFs of sub-regions in the cloud show systematic differences. The two UV-exposed regions have a double-peaked PDF we interprete as caused by shock compression. The deviations of the PDF from the log-normal shape typically associated with low- and high-mass star-forming regions at Av~3-4m and 8-10m, respectively, are found here within the very same cloud. This shows that there is no fundamental difference in the density structure of low- and high-mass star-forming regions. We conclude that star-formation in Rosette - and probably in high-mass star-forming clouds in general - is not globally triggered by the impact of UV-radiation. Moreover, star formation takes place in filaments that arose from the primordial turbulent structure built up during the formation of the cloud. Clusters form at filament mergers, but star formation can be locally induced in the direct interaction zone between an expanding HII--region and the molecular cloud.

ANGULAR MOMENTUM IN GIANT MOLECULAR CLOUDS. I. THE MILKY WAY

We present a detailed analysis comparing the velocity fields in molecular clouds and the atomic gas that surrounds them in order to address the origin of the gradients. To that end, we present first-moment intensity-weighted velocity maps of the molecular clouds and surrounding atomic gas. The maps are made from high-resolution 13CO observations and 21-cm observations from the Leiden/Argentine/Bonn Galactic HI Survey. We find that (i) the atomic gas associated with each molecular cloud has a substantial velocity gradient---ranging within 0.02 to 0.07 km/s/pc---whether or not the molecular cloud itself has a substantial linear gradient (ii) If the gradients in the molecular and atomic gas were due to rotation, this would imply that the molecular clouds have less specific angular momentum than the surrounding HI by a factor of 1-6. (iii) Most importantly, the velocity gradient position angles in the molecular and atomic gas are generally widely separated---by as much as 130 degrees in the case of the Rosette Molecular Cloud. This result argues against the hypothesis that molecular clouds formed by simple top-down collapse from atomic gas.

Star formation in the Rosette molecular cloud under the influence of NGC 2244

The Rosette molecular cloud has a characteristic morphology, where the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding HII-region now interacts with the cloud. We use continuum data obtained with the PACS (70 and 160 μ m) and SPIRE instruments (250, 350, 500 μ m) of the Herschel telescope to investigate the spatial distribution of cold and warm gas and to locate all star-forming sites in Rosette. We detected a clear negative temperature gradient and a positive density gradient (running from the HII-region/molecular cloud interface into the cloud), and an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. The existence of such temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster.

Small-scale structure in the Rosette molecular cloud revealed by Herschel

We present a preliminary analysis of the small-scale structure found in new 70-520 <i>μ<i/>m continuum maps of the Rosette molecular cloud (RMC), obtained with the SPIRE and PACS instruments of the <i>Herschel<i/> Space Observatory. We find 473 clumps within the RMC using a new structure identification algorithm, with sizes up to ~1.0 pc in diameter. A comparison with recent <i>Spitzer<i/> maps reveals that 371 clumps are “starless” (without an associated young stellar object), while 102 are “protostellar.” Using the respective values of dust temperature, we determine the clumps have masses (<i>M<i/><sub>C<sub/>) over the range -0.75 <i>≤<i/> log (<i>M<i/><sub>C<sub/>/) <i>≤<i/> 2.50. Linear fits to the high-mass tails of the resulting clump mass spectra (CMS) have slopes that are consistent with those found for high-mass clumps identified in CO emission by other groups.

TheHerschelview of star formation in the Rosette molecular cloud under the influence of NGC 2244

The Rosette molecular cloud is promoted as the archetype of a triggered star-formation site. This is mainly due to its morphology, because the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding HII-region now interacts with the cloud. Studying the spatial distribution of the different evolutionary states of all star-forming sites in Rosette and investigating possible gradients of the dust temperature will help to test the 'triggered star-formation' scenario in Rosette. We use continuum data obtained with the PACS (70 and 160 micron) and SPIRE instruments (250, 350, 500 micron) of the Herschel telescope during the Science Demonstration Phase of HOBYS. Three-color images of Rosette impressively show how the molecular gas is heated by the radiative impact of the NGC 2244 cluster. A clear negative temperature gradient and a positive density gradient (running from the HII-region/molecular cloud interface into the cloud) are detected. Studying the spatial distribution of the most massive dense cores (size scale 0.05 to 0.3 pc), we find an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. No clear gradient is found for the clump (size-scale up to 1 pc) distribution. The existence of temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. A more complete overview of the prestellar and protostellar population in Rosette is required to obtain a firmer result.

Herschelobservations of embedded protostellar clusters in the Rosette molecular cloud

The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette molecular cloud, providing an unprecedented view of its star formation activity. These new far-infrared data reveal a population of compact young stellar objects whose physical properties we aim to characterise. We compiled a sample of protostars and their spectral energy distributions that covers the near-infrared to submillimetre wavelength range. These were used to constrain key properties in the protostellar evolution, bolometric luminosity, and envelope mass and to build an evolutionary diagram. Several clusters are distinguished including the cloud centre, the embedded clusters in the vicinity of luminous infrared sources, and the interaction region. The analysed protostellar population in Rosette ranges from 0.1 to about 15 Msun with luminosities between 1 and 150 Lsun, which extends the evolutionary diagram from low-mass protostars into the high-mass regime. Some sources lack counterparts at near- to mid-infrared wavelengths, indicating extreme youth. The central cluster and the Phelps & Lada 7 cluster appear less evolved than the remainder of the analysed protostellar population. For the central cluster, we find indications that about 25% of the protostars classified as Class I from near- to mid-infrared data are actually candidate Class 0 objects. As a showcase for protostellar evolution, we analysed four protostars of low- to intermediate-mass in a single dense core, and they represent different evolutionary stages from Class 0 to Class I. Their mid- to far-infrared spectral slopes flatten towards the Class I stage, and the 160 to 70um flux ratio is greatest for the presumed Class 0 source. This shows that the Herschel observations characterise the earliest stages of protostellar evolution in detail.

SPITZERAND NEAR-INFRARED OBSERVATIONS OF A NEW BIPOLAR PROTOSTELLAR OUTFLOW IN THE ROSETTE MOLECULAR CLOUD

We present and discuss \emph{Spitzer} and near-infrared H$_{2}$ observations of a new bi-polar protostellar outflow in the Rosette Molecular Cloud. The outflow is seen in all four IRAC bands and partially as diffuse emission in the MIPS 24 $\mu$m band. An embedded MIPS 24 $\mu$m source bisects the outflow and appears to be the driving source. This source is coincident with a dark patch seen in absorption in the 8 $\mu$m IRAC image. \emph{Spitzer} IRAC color analysis of the shocked emission was performed from which thermal and column density maps of the outflow were constructed. Narrow-band near-infrared (NIR) images of the flow reveal H$_2$ emission features coincident with the high temperature regions of the outflow. This outflow has now been given the designation MHO 1321 due to the detection of NIR H$_2$ features. We use these data and maps to probe the physical conditions and structure of the flow.

DIVERSE PROTOSTELLAR EVOLUTIONARY STATES IN THE YOUNG CLUSTER AFGL961

We present arcsecond resolution mid-infrared and millimeter observations of the center of the young stellar cluster AFGL961 in the Rosette molecular cloud. Within 0.2 pc of each other, we find an early B star embedded in a dense core, a neighboring star of similar luminosity with no millimeter counterpart, a protostar that has cleared out a cavity in the circumcluster envelope, and two massive, dense cores with no infrared counterparts. An outflow emanates from one of these cores, indicating a deeply embedded protostar, but the other is starless, bound, and appears to be collapsing. The diversity of states implies either that protostellar evolution is faster in clusters than in isolation or that clusters form via quasi-static rather than dynamic collapse. The existence of a pre-stellar core at the cluster center shows that that some star formation continues after and in close proximity to massive, ionizing stars.

ACHANDRASTUDY OF THE ROSETTE STAR-FORMING COMPLEX. II. CLUSTERS IN THE ROSETTE MOLECULAR CLOUD

We explore here the young stellar populations in the Rosette Molecular Cloud (RMC) region with high spatial resolution X-ray images from the Chandra X-ray Observatory, which are effective in locating weak-lined T Tauri stars as well as disk-bearing young stars. A total of 395 X-ray point sources are detected, 299 of which (76%) have an optical or near-infrared (NIR) counterpart identified from deep FLAMINGOS images. From X-ray and mass sensitivity limits, we infer a total population of about 1700 young stars in the survey region. Based on smoothed stellar surface density maps, we investigate the spatial distribution of the X-ray sources and define three distinctive structures and substructures within them. Structures B and C are associated with previously known embedded IR clusters, while structure A is a new X-ray-identified unobscured cluster. A high mass protostar RMCX #89 = IRAS 06306+0437 and its associated sparse cluster is studied. The different subregions are not coeval but do not show a simple spatial-age pattern. Disk fractions vary between subregions and are generally 20% of the total stellar population inferred from the X-ray survey. The data are consistent with speculations that triggered star formation around the HII region is present in the RMC, but do not support a simple sequential triggering process through the cloud interior. While a significant fraction of young stars are located in a distributed population throughout the RMC region, it is not clear they originated in clustered environments.