The present work aims at performing a comprehensive census and characterisation of the pre-main sequence (PMS) population in the cometary cloud L1615/L1616, in order to assess the significance of the triggered star formation scenario and investigate the impact of massive stars on its star formation history and mass spectrum. Our study is based on UBVRcIc and JHKs photometry, as well as optical multi-object spectroscopy. We performed a physical parametrisation of the young stellar population in L1615/L1616. We identified 25 new T Tauri stars mainly projected on the dense head of the cometary cloud, almost doubling the current number of known members. We studied the spatial distribution of the cloud members as a function of the age and H$α$ emission. The star formation efficiency in the cloud is about 7-8 %, as expected for molecular clouds in the vicinity of OB associations. The slope of the initial mass function (IMF), in the mass range 0.1<M<5.5 $M_{\odot}$, is consistent with that of other T and OB associations, providing further support of an universal IMF down to the hydrogen burning limit, regardless of environmental conditions. The cometary appearance, as well as the high star formation efficiency, can be explained in terms of triggered star formation induced by the strong UV radiation from OB stars or supernovae shockwaves. The age spread as well as both the spatial and age distribution of the PMS objects provide strong evidence of sequential, multiple events and possibly still ongoing star formation activity in the cloud.
We present molecular line observations of the star-forming cloud around RNO6 along with a newly discovered nearby molecular cloud that we name RNO6NW. Both clouds display striking similarities in their cometary structures and overall kinematics. By using 13CO line observations, we estimate that these clouds have similar sizes (~4.5 pc) and masses (~200 solar masses). Both molecular clouds RNO6 and RNO6NW are active in star formation. From new high resolution near-IR narrowband images, we confirm that RNO6 hosts an embedded IR cluster that includes a Herbig Be star. A conspicuous H2 filament is…
With its cometary appearance and a reflection nebula near its edge facing some bright Orion stars, the Lynd's cloud L1616 shows ample evidence for being affected by one or more of these massive stars. To estimate its mass and star formation efficiency as well as to determine if it is gravitationally bound, we mapped this cloud in J=1${\rightarrow}$0 transitions of $^{12}$CO and $^{13}$CO. It is found that the distribution of the emission in the line {\it wings} show clear evidence for substantial mass motions. Also, the ``virial'' mass of the cloud is found to be five times the actual cloud ma…
We propose a model for the origin of the isolated nonthermal filaments observed at the Galactic center based on an analogy to cometary plasma tails. We invoke the interaction between a large scale magnetized galactic wind and embedded molecular clouds. As the advected wind magnetic field encounters a dense molecular cloud, it is impeded and drapes around the cloud, ultimately forming a current sheet in the wake. This draped field is further stretched by the wind flow into a long, thin filament whose aspect ratio is determined by the balance between the dynamical wind and amplified magnetic fie…
Cometary ultracompact HII regions have been modelled as the interaction of the hypersonic wind from a moving star with the molecular cloud which surrounds the star. We here show that a similar morphology can ensue even if the star is stationary with respect to the cloud material. We assume that the HII region is within a stellar wind bubble which is strongly mass loaded: the cometary shape results from a gradient in the distribution of mass loading sources. This model circumvents problems associated with the necessarily high spatial velocities of stars in the moving star models.
Red coloured rain occurred in many places of Kerala in India during July to September 2001 due to the mixing of huge quantity of microscopic red cells in the rainwater. Considering its correlation with a meteor airbust event, this phenomenon raised an extraordinary question whether the cells are extraterrestrial. Here we show how the observed features of the red rain phenomenon can be explained by considering the fragmentation and atmospheric disintegration of a fragile cometary body that presumably contains a dense collection of red cells. Slow settling of cells in the stratosphere explains t…
We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an HII region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slit-less spectroscopy we detected ten H_alpha emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally towards an O9.5 star, but has evolved to align towards a younger O6 star when that formed. Nea…
A thin, bright dust cloud, which is associated with the Rosetta mission target object (67P/Churyumov-Gerasimenko), was observed after the 2002 perihelion passage. The neckline structure or dust trail nature of this cloud is controversial. In this paper, we definitively identify the dust trail and the neckline structure using a wide-field CCD camera attached to the Kiso 1.05-m Schmidt telescope. The dust trail of 67P/Churyumov-Gerasimenko was evident as scattered sunlight in all images taken between September 9, 2002 and February 1, 2003, whereas the neckline structure became obvious only after…
Recent observations with the Hubble Space Telescope have discovered in the nearest planetary nebula, the Helix Nebula, thousands of gigantic comet-like objects with planet like masses in a ring around the central brilliant star at a distance comparable to our own Oort cloud of comets. We propose that such circumstellar rings of planets exist around most stars and that the gas clouds which emit the broad optical lines from quasars are radiation ablated planets which have been stripped off by gravitational collisions from stars that orbit near the central massive black hole. We propose that coll…
The structure of the high galactic latitude Cometary Globule 12 (CG 12) has been investigated by means of radio molecular line observations. Detailed, high signal to noise ratio maps in C18O (1-0), C18O (2-1) and molecules tracing high density gas, CS (3-2), DCO+ (2-1) and H13CO+ (1-0), are presented. The C18O line emission is distributed in a 10' long North-South elongated lane with two strong maxima, CG12 N(orth) and CG12 S(outh). In CG12 S the high density tracers delineate a compact core, DCO+ core, which is offset by 15" from the C18O maximum. The observed strong C18O emission traces the …
We present an interferometric and single dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105+/-5 K and an average production rate ratio Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observatio…
Spitzer photometry and spectroscopy of the star HD69830 reveal an excess of emission relative to the stellar photosphere between 8 and 35 $μ$m dominated by strong features attributable to crystalline silicates with an emitting surface area more than 1,000 times that of our zodiacal cloud. The spectrum closely resembles that of the comet C/1995 O1 (Hale-Bopp). Since no excess is detected at 70 $μ$m, the emitting material must be quite warm, be confined within a few AU of the star, and originate in grains with low long-wavelength emissivity, i.e. grains much smaller than $70 μ$m$/2π\sim 10 $ $μ$…
The feasibility of observing the C18O (3-2) spectral line in cold clouds with the APEX telescope has been tested. As the line at 329.330 GHz lies in the wing of a strong atmospheric H2O absorption it can be observed only at high altitude observatories. Using the three lowest rotational levels instead of only two helps to narrow down the physical properties of dark clouds and globules. The centres of two C18O maxima in the high latitude low mass star forming region CG 12 were mapped in C18O (3-2) and the data were analyzed together with spectral line data from the SEST. The T_MB(3-2)/T_MB(2-1) …
A novel explanation for the origin of the cometary globules within NGC 7293 (the "Helix" planetary nebula) is examined; that these globules originate as massive cometary bodies at large astrocentric radii. The mass of such hypothetical cometary bodies would have to be several orders of magnitude larger than any such bodies observed in our solar system in order to supply the observed mass of neutral gas. It is however shown that comets at "outer Oort cloud" like distances are likely to survive past the red giant and asymptotic giant branch evolutionary phases of the central star, allowing them …
We present simultaneous optical and near-infrared high angular resolution observations of the close Herbig Ae/Be binary star NX Pup which is associated with cometary globule~1. The reconstructed images have a diffraction--limited resolution of 62 mas in V, 75 mas in R (speckle masking reconstruction), and 115 mas in H, 156 mas in K (adaptive optics + post--processing). Compared to previous results we were able to derive better estimates on spectral type and luminosity and hence put better constraints on the evolutionary status (mass & age) of NX Pup A and B: with NX Pup A of spectral type F0-F…
In this chapter we review the properties of the Orion outlying clouds at b < -21 degrees. These clouds are located far off the Orion giant molecular cloud complex and are in most cases small cometary-shaped clouds, with their head pointing back towards the main Orion clouds. A wealth of data indicate that star formation is ongoing in many of these clouds. The star formation in these regions might have been triggered due to the strong impact of the massive stars in the Orion OB association. Some of the clouds discussed here may be part of the Orion-Eridanus bubble. An overview on each individua…
We have obtained maps of the 1.25mm thermal dust emission and the molecular gas emission over a region of 20' by 10' arcmin around the Trifid Nebula (M20), with the IRAM 30m and the CSO telescopes as well as in the mid-infrared wavelength with ISO and SPITZER. Our survey is sensitive to features down to N(H2) \sim 10^{22} cm-2 in column density. The cloud material is distributed in fragmented dense gas filaments (n(H2) \sim 1000 cm-3) with sizes ranging from 1 to 10 pc. A massive filament, WF, with properties typical of Infra Red Dark Clouds, connects M20 to the W28 supernova remnant. These fi…
We present photometric and spectroscopic results for the low mass pre-main sequence (PMS) stars with spectral types K - M in the cometary globule (CG) 30/31/38 complex. We obtained multi-object high resolution spectra for the targets selected as possible PMS stars from multi-wavelength photometry. We identified 11 PMS stars brighter than V = 16.5 with ages < 5 Myr at a distance of approximately 200 pc. The spatial distribution of the PMS stars, CG clouds, and ionizing sources (O stars and supernova remnants) suggests a possible triggered origin of the star formation in this region. We confirm …
We present results of optical spectroscopic and photometric observation of the pre-main sequence stars associated with the cometary shaped dark cloud Lynds 1622, and 12CO and 13CO observations of the cloud. We determined the effective temperatures and luminosities of 14 pre-main sequence stars associated with the cloud from their positions in the Hertzsprung--Russell diagram, as well as constructed their spectral energy distributions using optical, 2MASS and Spitzer IRAC and MIPS data. We derived physical parameters of L1622 from the molecular observations. Our results are not compatible with …
We review recent studies of organic molecule formation in dense molecular clouds and in comets. We summarise the known organic inventories of molecular clouds and recent comets, particularly Hale-Bopp. The principal chemical formation pathways involving gas phase reactions, as well as formation by catalytic reactions on grain surfaces or through dust fragmentation, are identified for both dense clouds and cometary comae. The processes leading to organic molecules with known biological function, carbon chains, deuterium fractionation, HNC and S-bearing compounds are described. Observational sea…
We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I~26.0 mag. It reveals not only a young population of blue main-sequence stars (age <30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr<age<100 Myr), and an older evolved populations of asymptotic giant branch (AGB) stars (age >100 Myr) and red giant branch (RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag o…
The IC 1396N cometary globule within the large nearby HII region IC 1396 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler~37 dispersed throughout the HII region, and 25 are associated with young stars formed within the globule. Spitzer/2MASS photometry shows the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the…
We calculate the formation of dust clouds in atmospheres of giant gas-planets. The chemical structure and the evolution of the grain size distribution in the dust cloud layer is discussed based on a consistent treatment of seed formation, growth/evaporation and gravitational settling. Future developments are shortly addressed.
This paper reports on computational evidence for the formation of cloud-like dust structures around C-rich AGB stars. This spatio-temporal structure formation process is caused by a radiative/thermal instability of dust forming gases as identified by Woitke et al.(2000). Our 2D (axisymmetric) models combine a time-dependent description of the dust formation process according to Gail & Sedlmayr (1988) with detailed, frequency-dependent continuum radiative transfer by means of a Monte Carlo method (Niccolini et al.2003) in an otherwise static medium (v=0). These models show that the formation of…
We have performed optical imaging observations of the dark cloud L1251 at multiple wavelengths, B, V, R, and I, using the 105 cm Schmidt telescope at the Kiso Observatory, Japan. The cloud has a cometary shape with a dense "head" showing star formation activity and a relatively diffuse "tail" without any signs of star formation. We derived extinction maps of A_B and A_V with a star count method, and also revealed the color excess (E_{B-V}, E_{V-R}, and E_{V-I}) distributions. On the basis of the color excess measurements we derived the distribution of the ratio of total to selective extinction…
We revisit the problem of the star formation timescale and the ages of molecular clouds. The apparent overabundance of star-forming molecular clouds over clouds without active star formation has been thought to indicate that molecular clouds are "short-lived" and that star formation is "rapid". We show that this statistical argument lacks self-consistency and, even within the rapid star-formation scenario, implies cloud lifetimes of approximately 10 Myr. We discuss additional observational evidence from external galaxies that indicate lifetimes of molecular clouds and a timescale of star forma…
We demonstrate that single and binary star clusters can be formed during cloud-cloud collisions triggered by the tidal interaction between the Large and Small Magellanic clouds. We run two different sets of self-consistent numerical simulations which show that compact, bound star clusters can be formed within the centers of two colliding clouds due to strong gaseous shocks, compression, and dissipation, providing the clouds have moderately large relative velocities (10-60 km s$^{-1}$). The impact parameter determines whether the two colliding clouds become a single or a binary cluster. The sta…
We consider evolution of interstellar clouds in Local Group dwarf spheroidal galaxies (dSphs) in the context of their observed star formation histories. The Local Group dSphs generally experienced initial bursts of star formations in their formation epochs ($\sim 15$ Gyr ago), when hot gas originating from the supernovae can make the cold interstellar clouds evaporate. We find that the maximum size of evaporating cloud is 10 pc. Thus, clouds larger than 10 pc can survive during the initial star formation. These surviving clouds can contribute to the second star formation to produce ``intermedi…
We present results of hydrodynamic simulations of star formation triggered by cloud-cloud collisions. During the early stages of star formation, low-mass objects form by gravitational instabilities in protostellar discs. A number of these low-mass objects are in the sub-stellar mass range, including a few objects of planetary mass. The disc instabilities that lead to the formation of low-mass objects in our simulations are the product of disc-disc interactions and/or interactions between the discs and their surrounding gas.
Observations of both star-forming regions and young, gas-free stellar associations indicate that most nearby molecular clouds form stars only over a short time span before dispersal; large-scale flows in the diffuse interstellar medium have the potential for forming clouds sufficiently rapidly, and for producing stellar populations with ages much less than the lateral crossing times of their host molecular clouds. We identify four important factors for understanding rapid star formation and short cloud lifetimes. First, much of the accumulation and dispersal of clouds near the solar circle mig…
We have posted the source code for our cloud model for public use as a tool for the intercomparison of planetary radiation transport models attempting to incorporate the physics of cloud condensation.
We demonstrate that the observationally inferred rapid onset of star formation after parental molecular clouds have assembled can be achieved by flow-driven cloud formation of atomic gas, using our previous three-dimensional numerical simulations. We post-process these simulations to approximate CO formation, which allows us to investigate the times at which CO becomes abundant relative to the onset of cloud collapse. We find that global gravity in a finite cloud has two crucial effects on cloud evolution. (a) Lateral collapse (perpendicular to the flows sweeping up the cloud) leads to rapidly…
