Parsecs In Diameter Research Articles

A Gaia view of the two OB associations Cygnus OB2 and Carina OB1: the signature of their formation process

ABSTRACT OB associations are the prime star-forming sites in galaxies. However, the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help in tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5–8 parsecs in diameter and display small dispersions in proper motion, while the haloes spread over tens of parsecs with two to three times larger dispersions in proper motion. This is reminiscent of the so-called line width–size relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters.

Disruption of the Orion molecular core 1 by wind from the massive star θ1 Orionis C.

Massive stars inject mechanical and radiative energy into the surrounding environment, which stirs it up, heats the gas, produces cloud and intercloud phases in the interstellar medium, and disrupts molecular clouds (the birth sites of new stars1,2). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds3-7. Theoretical studies predict that momentum injection by radiation should dominate that by stellar winds8, but this has been difficult to assess observationally. Velocity-resolved large-scale images in the fine-structure line of ionized carbon ([CII]) provide an observational diagnostic for the radiative energy input and the dynamics of the interstellar medium around massive stars. Here we report observations of a one-square-degree region (about 7 parsecs in diameter) of Orion molecular core 1-the region nearest to Earth that exhibits massive-star formation-at a resolution of 16 arcseconds (0.03 parsecs) in the [CII] line at 1.9 terahertz (158 micrometres). The results reveal that the stellar wind originating from the massive star θ1 Orionis C has swept up the surrounding material to create a 'bubble' roughly four parsecs in diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per second. This finding demonstrates that the mechanical energy from the stellar wind is converted very efficiently into kinetic energy of the shell and causes more disruption of the Orion molecular core 1 than do photo-ionization and evaporation or future supernova explosions.

The Stellar Content of the Infalling Molecular Clump G286.21+0.17

Abstract The early evolution during massive star cluster formation is still uncertain. Observing embedded clusters at their earliest stages of formation can provide insight into the spatial and temporal distribution of the stars and thus probe different star cluster formation models. We present near-infrared imaging of an 8′ × 13′ (5.4 pc × 8.7 pc) region around the massive infalling clump G286.21+0.17 (also known as BYF73). The stellar content across the field is determined and photometry is derived in order to obtain stellar parameters for the cluster members. We find evidence for some sub-structure (on scales less than a parsec in diameter) within the region with apparently at least three different sub-clusters associated with the molecular clump based on differences in extinction and disk fractions. At the center of the clump we identify a deeply embedded sub-cluster. Near-infrared excess is detected for 39%–44% in the two sub-clusters associated with molecular material and 27% for the exposed cluster. Using the disk excess as a proxy for age, this suggests the clusters are very young. The current total stellar mass is estimated to be at least 200 M ⊙. The molecular core hosts a rich population of pre-main-sequence stars. There is evidence for multiple events of star formation both in terms of the spatial distribution within the star-forming region and possibly from the disk frequency.

A possible formation scenario for the ultramassive cluster W3 in NGC 7252

The intermediate age star cluster W3 (age � 300‐500 Myr) in NGC 7252 is the most luminous star cluster known to date with a dynamical mass estimate of 8±2·10 7 M⊙. With an effective radius of about 17.5 pc and a velocity dispersion of 45 kms −1 this object may be viewed as one of the recently discovered ultra-compact dwarf galaxies (UCDs). Its intermediate age, however, precludes an origin as a stripped nucleated dwarf elliptical galaxy. The galaxy NGC 7252 is a merger remnant from two gas-rich disc galaxies. Interactions between two gas-rich galaxies lead to bursts of intense star formation. The age of the interaction and the age of W3 are in good agreement, suggesting that was W3 probably formed in the starburst. We propose a formation scenario for W3. Observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star cluster in confined regions of up to several hundred parsec in diameter. The total mass of new stars in these regions can reach 10 7 or even 10 8 M⊙. By means of numerical simulations we have shown that the star clusters in these regions merge on short time-s cales of a few Myr up to a few hundred Myr. We apply this scenario to W3 and predict properties which could be observable and the future evolution of this object. This work lends credence to the notion that at least some of the UCDs may be evolved star cluster complexes formed during early hierarchical mergers.

The central dusty torus in the active nucleus of NGC 1068.

Active galactic nuclei (AGNs) display many energetic phenomena--broad emission lines, X-rays, relativistic jets, radio lobes--originating from matter falling onto a supermassive black hole. It is widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.

Anatomy of the Gemini OB1 molecular cloud complex

view Abstract Citations (86) References (71) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Anatomy of the Gemini OB1 Molecular Cloud Complex Carpenter, John M. ; Snell, Ronald L. ; Schloerb, F. Peter Abstract We have investigated the large-scale morphology and properties of the molecular gas in the Gem OB1 cloud complex by mapping over 32 sq deg (177 pc x 221 pc) of the complex in (12)CO(J = 1-0) and (13)CO(J = 1-0) at 50 arcsec sampling with QUARRY on the FCRAO 14 m telescope. The most striking characteristic of the molecular line images are the series of arc- and ring-shaped structures found on spatial scales from a few parsecs in diameter up to at least 35 pc. The morphology and in some instances the kinematics suggest that these features represent swept-up molecular material, most likely from expanding H II regions and wind blown bubbles. The kinetic temperatures and column densities of the molecular gas were derived from the (12)CO and (13)CO data using the Local Thermodynamic Equilibrium (LTE) analysis. Most of the molecular gas was found to have kinetic temperatures of less than or approximately equal to 10 K, and 50% of the mass of gas is contained in lines of sight with H2 column densities less than or approximately equal to 2 x 1021/sq cm. It was found that only 10% of the molecular mass is contained in lines of sight with column densities in excess of 1022/sq cm, and that these regions are found almost exclusively near the massive star forming regions within the arcs and rings of molecular gas. The average H2 densities in areas with (13)CO emission are between 65-120/cu cm, consistent with previous studies of cloud complexes, and is independent of whether the regions contains massive star formation or not. For the Gem OB1 complex as a whole, the average H 2 density is 1.2/cu cm, which is only a few time the average atomic hydrogen density in the interstellar medium. We suggest an overall picture for the Gem OB1 complex in which most of the molecular gas is contained in relatively cold, low column density molecular material. The high column density regions in the Gem OB1 complex form through the external compression of the molecular gas by the winds and H II regions from newly formed massive stars. Thus once massive starr formation is initiated, the structure and further evolution of the cloud complex is largely a result of the interactions of expanding H II regions and stellar winds with the ambient molecular material. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175692 Bibcode: 1995ApJ...445..246C Keywords: Abundance; H Ii Regions; Molecular Clouds; Molecular Spectra; Morphology; Radio Observation; Temperature Distribution; Velocity Distribution; Carbon Monoxide; Density (Mass/Volume); Image Analysis; Luminosity; Radio Astronomy; Spectrum Analysis; Astronomy; ISM: CLOUDS; ISM: INDIVIDUAL NAME: GEMINI OB1; ISM: H II REGIONS; ISM: MOLECULES; ISM: STRUCTURE full text sources ADS | data products SIMBAD (21)

Infrared images of M17

A 256 x 256 near-IR camera at NOAO to image a large portion of the M17 star formation complex. A mosaic image of total size 9 x 9 arcmin is constructed in each of three infrared colors-J, H, and K. The observations determine the size and extent of the obscured cluster of stars which excites the powerful H II region and heats the luminous dust in the adjacent molecular cloud. More than 100 OB stars are found in a clustering several parsecs in diameter within an obscured portion of the H II region. The K luminosity function of this cluster differs significantly from that of the field stars, and has the form expected from the Salpeter initial mass function. The present level of star formation activity in the adjacent molecular clouds M17 SW is found to be considerably lower than that which produced the OB cluster in the H II region. Most of the cluster members are found to have infrared colors which are not consistent with those of purely reddened stellar photospheres. These stars that display IR excesses may be surrounded by circumstellar disks. 26 refs.

Radio Frequency Observations of the Nuclei of Galaxies

For spiral galaxies the nuclear radio emission is usually dominated by a complex distribution of emission with median diameter of 200 pc and median power of 1019 W Hz−1 sr−1 at 1400 MHz. There is a large range in both power and diameter. The power is independent of morphological type for the normal spirals but is correlated with the absolute optical magnitude and with the infrared emission. For Seyfert galaxies the emission is generally stronger, in some cases by several orders of magnitude (e.g. NGC 1275, 3C 120).Elliptical galaxies have been found with very compact radio sources, some less than a parsec in diameter. These are as powerful as the strongest spirals (~ 1021 W Hz−1 sr−1). Even stronger compact nuclear sources are now being found in the nuclei of those elliptical galaxies which also have extended radio sources (the radio galaxies). The presence of nuclear sources of this strength is so highly correlated with the presence of extended sources that this suggests a continuing involvement of the nucleus.

Spectrum of the Intensity Variations in 3C 273B

The intensity variations in radio source 3C 273B have been measured at wavelengths of 31.3, 21.2, and 10.6 centimeters. At 31.3 centimeters the variation is quite small, indicating that the variable component of the source is optically thick at this wavelength. Study of several dijfferent model sources shows that the observed dependence of the intensity variations on frequency can best be explained by an increase of the electron density in a source region about 2 parsecs in diameter. This interpretation is consistent with the distance to 3C 273 determined from Hubble's law and the observed red shift.

PAST EXPLOSIONS IN THE NEARER GALAXIES

In the course of examining the galaxies illustrated in the Hubble Atlas1 the writer has noted a number of circular rings. Such forms can be construed as resulting from explosions but the sizes of the rings are so large that the original explosions would be of almost incredible size. The rings range from 100 to 5000 parsecs in diameter. Following the recent publication by Lynds and Sandage2 of evidence for a tremendous disturbance in M 82 these evidences of past explosions no longer seem irrational, and this note is to call attention to them. Plate la is a Mount Wilson photograph of NGC 5194/95 (M 51) and Figure 1 is a corresponding diagram showing the location of four of the explosion rings. Ring A is peculiar in that it seems to have a spiral structure. If the sense of rotation of NGC 5194 is counterclockwise then the material of ring A should be expected to rotate in the same direction, but the spiral appears to be opposite hand. Ring C is dark and ring D is very faint along its S edge.* Plate Ib and Figure 2 illustrate NGC 628 (M74) in a similar manner. The spiral arm that would be the mate to the major SW arm would seem to have passed through ring A and to have been offset by it so that ring B was formed on it. A vestige of that arm could be interpreted as the bright line extending NE from B. Plate Ha and Figure 3 show NGC 5457 (M 101). Ring A appears to be 2500 to 3000 parsecs in diameter and seems to be nearly centered on a major spiral arm. Very close to the field star which appears to be at its center there are small