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IC 2118 (Witch Head nebula)



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A radiation driven implosion model for the enhanced luminosity of protostars near HII regions
Context: Molecular clouds near H II regions tend to harbor more luminousprotostars. Aims: We investigate whether a radiation-drivenimplosion mechanism enhances the luminosity of protostars near regionsof high ionizing fluxes. Methods: We performed numericalsimulations to model collapse of cores exposed to UV radiation from Ostars. We investigated the dependence of mass loss rates on the initialdensity profiles of cores and variation of UV fluxes. We derived simpleanalytic estimates of accretion rates and final masses of protostars.Results: The radiation-driven implosion mechanism can increaseaccretion rates of protostars by 1-2 orders of magnitude. On the otherhand, mass loss due to photo-evaporation is not high enough to have asignificant impact on the luminosity. The increase in accretion rateresults in luminosity 1-2 orders of magnitude higher than those ofprotostars that form without external triggering. Conclusions:Radiation-driven implosion can help explain the observed higherluminosity of protostars in molecular clouds near H II regions.

Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in theformation of low- and intermediate-mass stars in OB associations (theλ Ori region, Ori OB1, and Lac OB1 associations). We find thatthe classical T Tauri stars and Herbig Ae/Be stars tend to line upbetween luminous O stars and bright-rimmed or comet-shaped clouds; thecloser to a cloud the progressively younger they are. Our positional andchronological study lends support to the validity of theradiation-driven implosion mechanism, where the Lyman continuum photonsfrom a luminous O star create expanding ionization fronts to evaporateand compress nearby clouds into bright-rimmed or comet-shaped clouds.Implosive pressure then causes dense clumps to collapse, prompting theformation of low-mass stars on the cloud surface (i.e., the bright rim)and intermediate-mass stars somewhat deeper in the cloud. These starsare a signpost of current star formation; no young stars are seenleading the ionization fronts further into the cloud. Young stars inbright-rimmed or comet-shaped clouds are likely to have been formed bytriggering, which would result in an age spread of several megayearsbetween the member stars or star groups formed in the sequence.

Multigenerational Star Formation in L1551
The L1551 molecular cloud is undergoing a long and sustained period ofrelatively high efficiency star formation. It contains two smallclusters of Class 0 and I protostars, as well as a halo of more evolvedClass II and III YSOs, indicating a current and at least one past burstof star formation. We present here new, sensitive maps of 850 and 450μm dust emission covering most of the L1551 cloud; new CO J=2-1 dataof the molecular cloud; and a new, deep, optical image of [S II]emission (6730 Å). Compact submillimeter emitters are concentratedin two subclusters: L1551 IRS5 and L1551 NE, and the HL Tauri group.Both stellar groups show significant extended emission and outflow/jetactivity. A jet, terminating at HH 265 and with a very weak associatedmolecular outflow, may originate from LkHα 358 or from a binarycompanion to another member of the HL Tauri group. Several Herbig-Haroobjects associated with L1551 IRS5/L1551 NE were clearly detected in thesubmillimeter, as were faint ridges of emission tracing outflow cavitywalls. We confirm a large-scale molecular outflow originating from L1551NE, parallel to that from L1551 IRS5, and suggest that the ``hollowshell'' morphology is more likely due to two interacting outflows. Weconfirm the presence of a prestellar core (L1551 MC) of mass 2-3Msolar northwest of L1551 IRS5. The next-generation clustermay be forming in this core. The L1551 cloud appears cometary inmorphology and appears to be illuminated and eroded from the directionof Orion, perhaps explaining the multiple episodes of star formation.

Sailing along River Eridani.
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Triggered Star Formation in the Orion Bright-rimmed Clouds
We have developed an empirical and effective set of criteria, based onthe Two Micron All Sky Survey (2MASS) colors, to select candidateclassical T Tauri stars (CTTSs). This provides a useful tool to studythe young stellar population in star-forming regions. Here we presentour analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN1616, LDN 1634, and Ori East to show how massive stars interact withmolecular clouds to trigger star formation. Our results support theradiation-driven implosion model, in which the ionization fronts from OBstars compress a nearby cloud until the local density exceeds thecritical value, thereby inducing the cloud to collapse to form stars. Wefind that only BRCs associated with strong IRAS 100 μm emission (atracer of high density) and Hα emission (a tracer of ionizationfronts) show signs of ongoing star formation. Relevant timescales,including the ages of O stars, expanding H II regions, and the ages ofCTTSs, are consistent with sequential star formation. We also find thatCTTSs are only seen between the OB stars and the BRCs, with those closerto the BRCs being progressively younger. There are no CTTSs leading theionization fronts, i.e., within the molecular clouds. All of thesefindings provide strong evidence of triggered star formation and showthe major roles massive stars play in sustaining the star-formingactivities in the region.

The IC 2118 association: New T Tauri stars in high-latitude molecular clouds
We identified new pre-main sequence stars in the region of high-latitudemolecular clouds associated with the reflection nebula IC 2118, around l˜ 208° and b ˜ -27°. The stars were selected as TTauri candidates in objective prism plates obtained with the Schmidttelescope of Konkoly Observatory. Results of spectroscopic follow-upobservations, carried out with the FLAIR spectrograph installed on theUK Schmidt and with ALFOSC on Nordic Optical Telescope, are presented inthis paper. Based on spectral types, presence of emission lines andlithium absorption line, we identified five classical T Tauri stars anda candidate weak-line T Tauri star projected on the molecular clouds, aswell as two candidate pre-main sequence stars outside the nebulousregion. Using the near infrared magnitudes obtained from the 2MASS AllSky Catalog (IPAC \cite{2MASS}) we determined the masses and ages ofthese stars. We found that the five classical T Tauri stars projected onthe clouds are physically related to them, whereas the other stars areprobably background objects. Adopting a distance of 210 pc for IC 2118(Kun et al. \cite{KAY}) and using Palla & Stahler's (\cite{PS})evolutionary tracks we derived an average age of2.5×106 yrs and a mass interval of 0.4-1.0Mȯ for the members of the IC 2118 association.

Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141

Low-Mass Star Formation Induced by the Orion-Eridanus Bubble
During a spectroscopic survey performed with the Nordic OpticalTelescope in La Palma we found five classical T Tauri stars in thesmall, high latitude molecular clouds associated with the reflectionnebula IC2118. The star-forming clouds are located at the outer boundaryof the Orion star forming region, at a mean galactic latitude of-28°. Their positions in space and cometary shapes suggest theirinteraction with the Orion SFR. Using spectroscopic and near-infraredphotometric data we determined the positions of the newly discoveredpre-main sequence stars in the HRD. Comparison of the results withevolutionary models has shown that the masses of these stars are between0.3-0.8Msun and their ages are close to 106 yrs.They are roughly coeval and significantly younger than the known WTTS ofthe same region. We conclude that the birth of the molecular clouds andthe stars was probably induced by the interaction of the Orion -Eridanus Bubble with some small, diffuse high-latitude HI clouds.

Study of Molecular Clouds and Star Formation in the Region of IC 2118
We carried out a12CO survey of molecular clouds in the regionof the reflection nebula IC 2118. Our observations covered the wholearea of IC 2118 (~ 6 deg2) with a 2'.6 beam at a 4' gridspacing. We identified six molecular clouds. Based on a literaturesearch we found a distance value of 210 pc for the reflection nebula andits associated molecular clouds, indicating that the clouds are probablyinteracting with the Orion--Eridanus Bubble. Using this distance value,we determined the physical properties of the clouds. We also made anobjective prism search for Hα emission stars in the region inorder to find possible solar-type stars born in the clouds, andidentified 46 candidate pre-main-sequence stars. Their spectroscopicfollow-up is in progress. Seven of them have so far proved to be T Tauristars.

Search for Molecular Clouds toward Intermediate-to-High Latitude IRAS Sources in the Southern Sky
We have conducted a search for molecular clouds toward southern IRASpoint sources at intermediate-to-high galactic latitude (|b| >=10deg) in 12CO (J = 1-0) emission with the NANTENtelescope. The main purpose of the survey was to find a new sample ofstar-forming clouds unknown to date. Of the 29 targets, we detected COemission toward 5 IRAS sources, i.e., 04591-0856, 05044-0325,05050-0614, 06345-3023, and 13543-3941. Among 5 detections, themolecular cloud associated with 06345-3023 (G 239.2-16.3) was detectedfor the first time. If we assume the distance as being 1.5 kpc, this\source is ~ 430 pc away from the galactic plane. Three source is ~ 430pc away from the galactic plane. Three sources 04591-0856 (G208.3-28.4), 05044-0325 (G 203.5-24.7), and 05050-0614 (G 206.4-25.9)are located west to the Orion molecular clouds, and 13543-3941 (G316.4+21.2) is located in the cometary globule CG 12. Four of the 5clouds show a cometary shape with a ``head-tail'' distribution, andthree of them have the IRAS source located at the edge opposite to thetail. We find that all of the CO clouds are associated with the opticalnebulosities, which are likely to be reflection nebulae. A comparison ofthe present CO clouds with those in Ophiuchus, Taurus, Chamaeleon, and L1333 indicates that star-forming clouds tend to have a high columndensity, as well as a smaller ratio of the virial mass to the LTE mass.

Remnant molecular clouds in the ORI OB 1 association
We suggest and give some evidence that as an HII region expands and theO star(s) evolve(s) into B giant(s) the remains of the molecular cloudsfirst appear as bright-rimmed clouds, then as cometary globules andfinally as small clouds which are visible by the reflected light fromthe B giants. We propose to call the last one `reflection clouds' andall three categories collectively `remnant clouds'. A list is presentedof about 80 objects of these remnant clouds in the Ori OB 1 association.In the Belt region there is a beautiful spatial sequence frombright-rimmed clouds through cometary globules to reflection clouds. Wesuspect that retarded star formation in remnant clouds can explain thepresence of so-called dispersed T Tau stars in the peripheries of OBassociations.

Briefly noted: Irish astronomy (poem)
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The Orion OB1 association. II. The Orion-Eridanus Bubble.
Observations of the interstellar medium in the vicinity of the Orion OB1association show a cavity filled with hot ionized gas, surrounded by anexpanding shell of neutral hydrogen (the Orion-Eridanus Bubble). In thispaper we examine this cavity and the surrounding bubble with the aid ofdata from the Leiden/Dwingeloo HI survey. We present neutral-hydrogenmaps for the Orion-Eridanus region which allow identification of the HIfilaments and arcs delineating the Bubble and derivation of itsexpansion velocity. The X-ray emission from the Orion-Eridanus region isenhanced with respect to the Galactic background emission. Comparisonwith the HI data shows a detailed anti-correlation of the X-rayenhancement with kinematically-narrow features. This confirms theassociation of the X-ray enhancement with the cavity in HI. Comparisonof the derived column densities in HI with the IRAS 100μm intensitiesshows that the HI shell emission is optically thin. This justifies aderivation of the mass of the HI shell by direct conversion of theobserved HI emission to column densities. Models of wind-blown bubblesare considered, to investigate if the cavity was formed by the stellarwinds and supernovae from Orion OB1. Using a model that takes thedensity stratification of the Galactic HI layer into account, we showthat the stellar winds and supernovae from stars in Orion OB1 canaccount for the size as well as for the expansion velocity of the HIshell. However, density inhomogeneities in the ambient interstellarmedium cause significant discrepancies between our model and theobserved shell.

Red and nebulous objects in dark clouds - A survey
A search on the NGS-PO Sky Survey photographs has revealed 150interesting nebulous and/or red objects, mostly lying in dark clouds andnot previously catalogued. Spectral classifications are presented for 55objects. These indicate a small number of new members of the class ofHerbig-Haro objects, a significant number of new T Tauri stars, and afew emission-line hot stars. It is argued that hot, high-mass stars formpreferentially in the dense cores of dark clouds. The possible symbiosisof high and low mass stars is considered. A new morphology class isdefined for cometary nebulae, in which a star lies on the periphery of anebulous ring.

Untersuchungen über Reflexionsnebel am Palomar Sky Survey I. Verzeichnis von Reflexionsnebeln
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A general study of diffuse galactic nebulae.
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Observation and Astrometry data

Right ascension:05h02m00.00s
Apparent magnitude:99.9

Catalogs and designations:
Proper NamesWitch Head nebula
Witch Head   (Edit)
ICIC 2118

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