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Polarization Shadows of Extragalactic Sources by the Local Magnetoionic Interstellar Medium
We report depolarization of extragalactic sources in the NRAO VLA SkySurvey (NVSS) by local structures in the interstellar medium. The skydensity of polarized sources drops by a factor of 2-4 in regions withangular scales ~10°, implying up to 40% depolarization on averageper source. Some of these polarization shadows are associated with H IIregions, but three are associated with regions of depolarized diffuseGalactic emission. The absence of a correlation between the depth ofpolarization shadows and Hα intensity suggests that some shadowsare related to structure in the magnetic field. At least somepolarization shadows are caused by partial bandwidth depolarization inthe NVSS. Alternatively, some may be caused by regions with small-scale(<~1") variations in rotation measure.
| A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy
We report on observations of several optical emission lines toward avariety of newly revealed faint, large-scale Hα-emitting regionsin the Galaxy. The lines include [N II] λ6583, [N II]λ5755, [S II] λ6716, [O III] λ5007, and He Iλ5876 obtained with the Wisconsin Hα Mapper (WHAM) towardsight lines that probe superbubbles, high-latitude filamentary features,and the more diffuse warm ionized medium (WIM). Our observations includemaps covering thousands of square degrees toward the well-knownOrion-Eridanus bubble, a recently discovered60deg×20deg bipolar superbubble centered inPerseus, plus several classical H II regions surrounding OB stars andhot evolved stellar cores. We use the emission-line data to explore thetemperature and ionization conditions within the emitting gas and theirvariations between the different emission regions. We find that in thediffuse WIM and in the faint high-latitude filamentary structures theline ratios of [N II]/Hα and [S II]/Hα are generally high,while [O III]/Hα and He I/Hα are generally low compared tothe bright classical H II regions. This suggests that the gas producingthis faint widespread emission is warmer, in a lower ionization state,and ionized by a softer spectrum than gas in classical H II regionssurrounding O stars, the presumed ionization source for the WIM. Inaddition, we find differences in physical conditions between the largebubble structures and the more diffuse WIM, suggesting that theionization of superbubble walls by radiation from interior Oassociations does not account entirely for the range of conditions foundwithin the WIM, particularly the highest values of [N II]/Hα and[S II]/Hα.
| Polarization Observations of the Anomalous Microwave Emission in the Perseus Molecular Complex with the COSMOSOMAS Experiment
The anomalous microwave emission detected in the Perseus molecularcomplex by Watson et al. has been observed at 11 GHz through dualorthogonal polarizations with the COSMOSOMAS experiment. Stokes U and Qmaps were obtained at a resolution of ~0.9d for a 30°×30°region including the Perseus molecular complex. Faint polarized emissionhas been measured; we find Q=-0.2%+/-1.0% andU=-3.4+1.8-1.4%, both at the 95% confidence level,with a systematic uncertainty estimated to be lower than 1% determinedfrom tests of the instrumental performance using unpolarized sources inour map as null hypothesis. The resulting total polarization level isΠ=3.4+1.5-1.9%. These are the first constraintson the polarization properties of an anomalous microwave emissionsource. The low level of polarization seems to indicate that theparticles responsible for this emission in the Perseus molecular complexare not significantly aligned in a common direction over the wholeregion, as a consequence of either a high structural symmetry in theemitting particle or a low-intensity magnetic field. Our weak detectionis fully consistent with predictions from electric dipole emission andresonance relaxation at this frequency.
| An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz. Observations and data reduction
A new polarization survey of the northern sky at 1.41 GHz is presented.The observations were carried out using the 25.6 m telescope at theDominion Radio Astrophysical Observatory in Canada, with an angularresolution of 36 arcmin. The data are corrected for ground radiation toobtain Stokes U and Q maps on a well-established intensity scale tied toabsolute determinations of zero levels, containing emission structuresof large angular extent, with an rms noise of 12 mK. Survey observationswere carried out by drift scanning the sky between -29° and +90°declination. The fully sampled drift scans, observed in steps of0.25° to ˜ 2.5° in declination, result in a northern skycoverage of 41.7% of full Nyquist sampling. The survey surpasses by afactor of 200 the coverage, and by a factor of 5 the sensitivity, of theLeiden/Dwingeloo polarization survey that was until now the mostcomplete large-scale survey. The temperature scale is tied to theEffelsberg scale. Absolute zero-temperature levels are taken from theLeiden/Dwingeloo survey after rescaling those data by the factor of0.94. The paper describes the observations, data processing, andcalibration steps. The data are publicly available athttp://www.mpifr-bonn.mpg.de/div/konti/26msurvey orhttp://www.drao.nrc.ca/26msurvey.
| Detection of Anomalous Microwave Emission in the Perseus Molecular Cloud with the COSMOSOMAS Experiment
We present direct evidence for anomalous microwave emission in thePerseus molecular cloud, which shows a clear rising spectrum from 11 to17 GHz in the data from the COSMOSOMAS experiment. By extending thefrequency coverage using W ilkinson Microwave Anisotropy Probe mapsconvolved with the COSMOSOMAS scanning pattern, we reveal a peak fluxdensity of 42+/-4 Jy at 22 GHz integrated over an extended area of 1.65d×1.0d centered on R.A.=55.4d+/-0.1d and decl.=+31.8d+/-0.1d(J2000). The flux density that we measure at this frequency is nearly anorder of magnitude higher than can be explained in terms of normalGalactic emission processes (synchrotron, free-free, and thermal dust).An extended IRAS dust feature, G159.6-18.5, is found near this position,and no bright unresolved source that could be an ultracompact H IIregion or gigahertz-peaked source could be found. An adequate fit forthe spectral density distribution can be achieved from 10 to 50 GHz byincluding a very significant contribution from electric dipole emissionfrom small spinning dust grains.
| Deep Imaging Surveys of Star-forming Clouds. III. Herbig-Haro Objects in the Perseus Molecular Cloud
We present a catalog of 72 new Herbig-Haro (HH) objects discovered inthe Perseus molecular cloud. There are 69 previously cataloged HHobjects in this region, and the new discoveries bring the total numberof known HH objects in Perseus to 141. Individual outflows often containseveral distinct HH objects. These observations demonstrate that thePerseus molecular cloud is one of the most active star-forming regionsin the solar vicinity. We explore different methods for probing themomentum injection rate of outflows and examine whether outflows candrive turbulence within the molecular cloud. On the scale of the entirePerseus cloud, the shocks produced by outflows from young stars may notinject momentum at a sufficient rate to counter the rate at whichmomentum decays. However, intense outflow activity within individualcloud cores with high star formation rates, such as NGC 1333, may besufficient to locally support or even disrupt the core.
| DEFPOS and Its First Results
A spectrometer was built to examine the interstellar medium (ISM) usingthe hydrogen Balmer α line. It is called Dual Etalon Fabry-PerotOptical Spectrometer (DEFPOS). DEFPOS will be coupled to coudéexit of the 150 cm telescope (RTT150) installed at TÜBITAK NationalObservatory (TUG). DEFPOS was ready for observations about two yearsago, but work was still continuing on the RTT150 coudé exitalignment. So we have started observing HII regions with DEFPOS withoutthe RTT150. We present here some characteristics of the instrument andsome of the results obtained.
| Deep Imaging Surveys of Star-Forming Clouds II. A New Giant Herbig-Haro Flow in L1451
We report the discovery of a new giant Herbig-Haro flow near the L1451region of the Perseus molecular cloud. The east-west oriented flowcontains two known HH objects (HH 280 and HH 317), two new HH objects(HH 492 and HH 493), and is 2.1 pc long. The flow is driven by the ClassI protostar IRAS 03235+3004 embedded in a bright-rimmed, sharp-edgedcometary cloud facing northeast. The flow source is embedded in awest-facing conical reflection nebula. The cometary cloud appears tohave been shaped by O and B stars in the Per OB2 association, includingthe B0.5 star 40 Per, located along the well-defined symmetry axis ofthe cloud and with a projected separation of 26 pc. A soft-UV inducedphotoablation model is proposed to explain the properties of thecometary cloud.
| First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Preliminary Maps and Basic Results
We present full-sky microwave maps in five frequency bands (23-94 GHz)from the Wilkinson Microwave Anisotropy Probe (WMAP) first-year skysurvey. Calibration errors are less than 0.5%, and the low systematicerror level is well specified. The cosmic microwave background (CMB) isseparated from the foregrounds using multifrequency data. The sky mapsare consistent with the 7° FWHM Cosmic Background Explorer (COBE)maps. We report more precise, but consistent, dipole and quadrupolevalues. The CMB anisotropy obeys Gaussian statistics with-58
| Interstellar extinction in the direction of the Aquila Rift
The distance dependence of interstellar extinction in the direction ofthe Aquila Rift is investigated using 473 starsobserved in the Vilnius photometric system. The front edge of the darkclouds in the area is found to be at 225+/-55 pc and the thickness ofthe cloud system is about 80 pc. The maximum extinction AV inthe clouds is close to 3.0 mag. Two stars with larger extinction arefound and discussed. Since the new distance of the clouds is larger thanthe previously accepted distance, the cloud system mass should beincreased to 2.7*E5 Msun which is close to thevirial mass estimated from the CO velocity dispersion. Additionalarguments are given in favor of the genetic relation between the Serpensand the Scorpio-Ophiuchus dark clouds.
| Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features
We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6μm for a sample of 45 disk galaxies from the US Infrared SpaceObservatory Key Project on Normal Galaxies. The galaxies were selectedto span the range in global properties of normal, star-forming diskgalaxies in the local universe. The spectra can be decomposed into threespectral components: (1) continuum emission from stellar photospheres,which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region;(2) a weak NIR excess continuum, which has a color temperature of~103 K, carries a luminosity of a few percent of the totalfar-infrared (FIR) dust luminosity LFIR and most likelyarises from the interstellar medium (ISM); and (3) the well-known broademission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generallyattributed to aromatic carbon particles. These aromatic features inemission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part ofthe spectrum and resemble the so-called type A spectra observed in manynonstellar sources and the diffuse ISM in our own Galaxy. The fewnotable exceptions include NGC 4418, where a dust continuum replaces theAFEs in MIR, and NGC 1569, where the AFEs are weak and the strongestemission feature is [S IV] 10.51 μm. The relative strengths of theAFEs vary by 15%-25% among the galaxies. However, little correlation isseen between these variations and either IRAS 60 μm/100 μm fluxdensity ratio R(60/100) or the FIR/blue luminosity ratioLFIR/LB, two widely used indicators of the currentstar formation activity, suggesting that the observed variations are nota consequence of the radiation field differences among the galaxies. Wedemonstrate that the NIR excess continuum and AFE emission arecorrelated, suggesting that they are produced by similar mechanisms andsimilar (or the same) material. On the other hand, as the current starformation activity increases, the overall strengths of the AFEs and theNIR excess continuum drop significantly with respect to that of the FIRemission from large dust grains. In particular, the summed luminosity ofthe AFEs falls from ~0.2 LFIR for the most ``IR-quiescent''galaxies to ~0.1 LFIR for the most ``IR-active'' galaxies.This is likely a consequence of the preferential destruction in intenseradiation fields of the small carriers responsible for the NIR/AFEemission.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries, France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| Interstellar Extinction in the Direction of the Barnard 1 Dark Cloud in Perseus
Spectral and luminosity classes, absolute magnitudes, color excesses,interstellar extinctions and distances are determined for 98 stars downto 13 mag in the Barnard 1 dark cloud belonging to the Per OB2association. The classification of stars is based on theirphotoelectric photometry in the Vilnius seven-color photometric system.The extinction vs. distance diagram exhibits the presence of two dustlayers at 150 and 230 pc distances. The distance of the first cloud,which gives an extinction A_V of 0.3 mag, coincides with the distance ofthe Taurus dark cloud complex. The second cloud with much largerextinction is about at the same distance as the clouds in the directionof the nearby objects: reflection nebula NGC 1333 and open cluster IC348.
| Study of the Per OB2 star forming complex. II. Structure and kinematics
The study of a sky area with a 20 degree diameter centered on the nearbyassociation Per OB2 has been carried out on the basis of the ``CompiledCatalogue of Astronomical Data in the Per OB2 Field'' presented in aprevious paper. The catalogue contains about 30 000 stars with accurateproper motions in the Hipparcos system as well as magnitudessupplemented with spectral classes and other relevant data. We appliedthe reduced proper motion technique to estimate individual distances foralmost all catalogue stars. The distributions of reddening andinterstellar extinction were determined on the basis of the spectralclassification of stars in the area. Proper motions were corrected fordifferential rotation of the Galactic disk and parallactic motioneffects and used for the evaluation of association membership. Theanalysis of a uniform subset of main sequence (MS) stars earlier than A7- which is complete within 500 pc - has clearly revealed the associationas an area of enhanced density with an angular size of about7o. In comparison to the previous results on Per OB2, we showthat the presently determined area extends the association from itsclassical position towards the California nebula and the Auriga darkcloud. There are 1025 MS proper motion members residing in this region.The distance to the association centre derived from a kinematiccalibration is found to be about 300 pc i.e., in perfect agreement withthe Hipparcos data. The shape of the association is almost spherical,and its diameter found from stellar counts is about 40 pc. Anexamination of internal systematic motions from proper motion data hasshown that there is no evidence of an expansion or a rotation of theassociation around the line of sight. The average tangential and spatialvelocity vectors of Per OB2 with respect to the LSR are determined as(Vl, Vb)=(+7.2+/-0.4,+2.0+/-0.3) km s-1and (U, V, W)=(+12.7+/- 1.6,-3.0+/- 0.6,-0.9+/- 0.8) km s-1.
| The Unusual Herbig Ae/Be Star XY Per A
The results of an investigation of XY Per based on photoelectricobservations in the Strömgren system are presented. It is shownthat the brighter component in the binary system XY Per AB, classifiedearlier as A2 II, is a variable of the Herbig Ae/Be type. At the sametime, our estimates gave M V (XY Per A) = +1.25. An analysis of thevariation of the indices and c showed that XY Per A is a typical shellstar: along with intense hydrogen absorption lines it displays anappreciable emission deficit in the Balmer continuum. The mostnoteworthy are the results of observations indicating that as thebrightness decreases, the opacity of the shell increases both in linesand in the Balmer continuum. This observational fact can scarcely beexplained in terms of the model of brightness variation due to dustclouds revolving around the star.
| Interferometric Observations of the H II Region Around xi Per
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| Interstellar extinction in the California Nebula region.
Vilnius seven-color photometry has been obtained for 238 stars down to ~13 mag in the area of the California Nebula inPerseus. For nearly all of the stars, photometric spectral classes,luminosity classes, absolute magnitudes, interstellar reddenings,extinctions and distances are determined. The ``extinction versusdistance'' diagrams give evidence for the presence of one dust layer at~ 160 pc distance in the direction of the CaliforniaNebula and its nearest surroundings and of two dust layers atdistances of 160 pc and 300 pc north-west of the nebula, in thedirection of the dark clouds L 1449 and L1456. The front layer contributes extinction between 0.3 and1.3 mag, and the second layer gives about 1 mag of additionalextinction. It appears likely that the front dust layer is the extensionof the Taurus dark clouds. The second dust layer probably belongs to thecomplex of dark clouds found in other areas of the southern part ofPerseus (in the directions of the open cluster IC348, the reflection nebula NGC 1333, etc.).Both cloud complexes run more or less parallel to the Galactic plane.
| A Submillimeter View of Star Formation near the H II Region KR 140
We present the results of 450 and 850 μm continuum mapping of the HII region KR 140 using the Submillimeter Common-User Bolometer Array(SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). KR 140is a small (5.7 pc diameter) H II region at a distance of 2.3+/-0.3 kpc.Five of the six IRAS point sources near KR 140 were mapped in thisstudy. Our analysis shows that two of these IRAS sources are embeddedlate B-type stars lying well outside the H II region, two are a part ofthe dust shell surrounding the H II region, and one is the combinedemission from an ensemble of smaller sources unresolved by IRAS. We havediscovered a number of relatively cold submillimeter sources not visiblein the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from0.5 to 130 Msolar. The distribution of masses for all sourcesis well characterized by a power law N(>M)~M-α withα=0.5+/-0.04, in agreement with the typical mass function forclumped structures of this scale in molecular clouds. Several of thesubmillimeter sources are found at the H II molecular gas interface andhave probably been formed as the result of the expansion of the H IIregion. Many of the submillimeter sources we detect are gravitationallybound and most of these follow a mass-size relationship expected forobjects in virial equilibrium with nonthermal pressure support. Upon theloss of nonthermal support, they could be sites of star formation. Alongwith the two B stars that we have identified as possible cluster membersalong with VES 735, we argue that five nearby highly reddened stars arein a pre-main-sequence stage of evolution.
| Detection of [N II] λ5755 Emission from Low-Density Ionized Interstellar Gas
The extremely faint, temperature-sensitive ``auroral'' emission line [NII] λ5755 has been detected from the low-density ionized gasalong the sight line toward l=130.0d, b=-7.5d using the WisconsinHα Mapper. The intensity of this emission line, relative to thered nebular line [N II] λ6584, is found to be twice that observedin bright, classical H II regions surrounding O stars. This implies thatthe electron temperature of the ionized gas along this sight line isabout 2000 K higher than the H II regions and that the enhanced [N II]λ6584/Hα and [S II] λ6716/Hα intensity ratiosin this low-density gas are due at least in part to an elevatedtemperature.
| The Milky Way in Molecular Clouds: A New Complete CO Survey
New large-scale CO surveys of the first and second Galactic quadrantsand the nearby molecular cloud complexes in Orion and Taurus, obtainedwith the CfA 1.2 m telescope, have been combined with 31 other surveysobtained over the past two decades with that instrument and a similartelescope on Cerro Tololo in Chile, to produce a new composite CO surveyof the entire Milky Way. The survey consists of 488,000 spectra thatNyquist or beamwidth (1/8deg) sample the entire Galactic plane over astrip 4deg-10deg wide in latitude, and beamwidthor 1/4deg sample nearly all large local clouds at higher latitudes.Compared with the previous composite CO survey of Dame et al. (1987),the new survey has 16 times more spectra, up to 3.4 times higher angularresolution, and up to 10 times higher sensitivity per unit solid angle.Each of the component surveys was integrated individually using clippingor moment masking to produce composite spatial and longitude-velocitymaps of the Galaxy that display nearly all of the statisticallysignificant emission in each survey but little noise. The composite mapsprovide detailed information on individual molecular clouds, suggestrelationships between clouds and regions widely separated on the sky,and clearly display the main structural features of the molecularGalaxy. In addition, since the gas, dust, and Population I objectsassociated with molecular clouds contribute to the Galactic emission inevery major wavelength band, the precise kinematic information providedby the present survey will form the foundation for many large-scaleGalactic studies. A map of molecular column density predicted fromcomplete and unbiased far-infrared and 21 cm surveys of the Galaxy wasused both to determine the completeness of the present survey and toextrapolate it to the entire sky at |b|<32deg. The closeagreement of the observed and predicted maps implies that only ~2% ofthe total CO emission at |b|<32deg lies outside ourcurrent sampling, mainly in the regions of Chamaeleon and the GumNebula. Taking into account this small amount of unobserved emission,the mean molecular column density decreases from~3×1020 cm-2 at |b|=5deg to~0.1×1020 cm-2 at |b|=30deg thisdrop is ~6 times steeper than would be expected from a plane-parallellayer, but is consistent with recent measurements of the mean molecularcolumn density at higher latitudes. The ratio of the predicted molecularcolumn density map to the observed CO intensity map provides acalibration of the CO-to-H2 mass conversion factorX≡NH2/WCO. Out of the Galacticplane (|b|>5deg), X shows little systematic variation withlatitude from a mean value of (1.8+/-0.3)×1020cm-2 K-1 km-1 s. Given the large skyarea and large quantity of CO data analyzed, we conclude that this isthe most reliable measurement to date of the mean X value in the solarneighborhood.
| On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups
We use milli-arcsecond accuracy astrometry (proper motions andparallaxes) from Hipparcos and from radio observations to retrace theorbits of 56 runaway stars and nine compact objects with distances lessthan 700 pc, to identify the parent stellar group. It is possible todeduce the specific formation scenario with near certainty for twocases. (i) We find that the runaway star zeta Ophiuchi and the pulsarPSR J1932+1059 originated about 1 Myr ago in a supernova explosion in abinary in the Upper Scorpius subgroup of the Sco OB2 association. Thepulsar received a kick velocity of ~ 350 km s-1 in thisevent, which dissociated the binary, and gave zeta Oph its large spacevelocity. (ii) Blaauw & Morgan and Gies & Bolton alreadypostulated a common origin for the runaway-pair AE Aur and mu Col,possibly involving the massive highly-eccentric binary iota Ori, basedon their equal and opposite velocities. We demonstrate that these threeobjects indeed occupied a very small volume ~ 2.5 Myr ago, and show thatthey were ejected from the nascent Trapezium cluster. We identify theparent group for two more pulsars: both likely originate in the ~ 50 Myrold association Per OB3, which contains the open cluster alpha Persei.At least 21 of the 56 runaway stars in our sample can be linked to thenearby associations and young open clusters. These include the classicalrunaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei(Cep OB3), and fifteen new identifications, amongst which a pair ofstars running away in opposite directions from the region containing thelambda Ori cluster. Other currently nearby runaways and pulsarsoriginated beyond 700 pc, where our knowledge of the parent groups isvery incomplete.
| Photometric Investigation of the California Nebula Region: Catalogs
Vilnius seven-color photometry has been obtained for 238 stars down to13th mag in the area of the California Nebula in Perseus. For nearly allof the stars photometric spectral classes, luminosity classes, absolutemagnitudes, interstellar reddenings, extinctions and distances aredetermined. The present paper contains the results of photometry and theparameters of the stars.
| Dense Molecular Clouds in the Galactic Center Region. II. Statistical Properties of the Galactic Center Molecular Clouds
We present statistical properties of molecular clouds based on theGalactic center region survey in CS J=1-0 with the 45 m radio telescopeof the Nobeyama Radio Observatory. We identified 159 molecular cloudclumps from the survey data. We found that the velocity widths of theGalactic center molecular clumps are about 5 times larger than those ofthe Galactic disk molecular clouds with the same radius. We estimatedthe virial theorem masses and the LTE masses of the identified clumps.The virial-theorem masses are 1 order of magnitude larger than the LTEmasses. We obtain the mass and size spectra for the Galactic centermolecular clumps. The mass spectrum is dN/dM~M-1.6+/-0.1(M>~1.0×104 Msolar). The spectral indexwas approximately equal to the values obtained in the Galactic diskregions, although the statistical relations, such as the line width-sizerelation, for the Galactic center clouds are much different from thosefor the disk clouds. The size spectrum is dN/dR~R-4.0+/-0.4(R>~3.3 pc). The spectral index is consistent with those for the diskclouds.
| WHAM Observations of Hα, [S II], and [N II] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium
A large portion of the Galaxy (l=123^deg-164 deg, b=-6^deg to -35 deg),which samples regions of the Local (Orion) spiral arm and the moredistant Perseus arm, has been mapped with the Wisconsin Hα Mapper(WHAM) in the [S II] lambda6716 and [N II] lambda6583 lines. Bycomparing these data with the maps from the WHAM Hα Sky Survey, webegin an investigation of the global physical properties of the warmionized medium (WIM) in the Galaxy. Several trends noticed inemission-line investigations of diffuse gas in other galaxies areconfirmed in the Milky Way and extended to much fainter emission. Wefind that the [S II]/Hα and [N II]/Hα ratios increase asabsolute Hα intensities decrease. For the more distant Perseus armemission, the increase in these ratios is a strong function of Galacticlatitude, b, and thus of height, z, above the Galactic plane, while the[S II]/[N II] ratio is relatively independent of Hα intensity.Scatter in this ratio appears to be physically significant, and maps of[S II]/[N II] suggest that regions with similar ratios are spatiallycorrelated. The Perseus arm [S II]/[N II] ratio is systematically lowerthan local emission by 10%-20%. With [S II]/[N II] fairly constant overa large range of Hα intensities, the increase of [S II]/Hαand [N II]/Hα with |z| seems to reflect an increase intemperature. Such an interpretation allows us to estimate thetemperature and ionization conditions in our large sample ofobservations. We find that WIM temperatures range from 6000 to 10,000 K,with temperature increasing from bright to faint Hα emission (lowto high [S II]/Hα and [N II]/Hα), respectively. Changes in[S II]/[N II] appear to reflect changes in the local ionizationconditions (i.e., the S^+/S^++ ratio). We also measure the electronscale height in the Perseus arm to be 1.0+/-0.1 kpc, confirming earlier,less accurate determinations.
| Distribution of gas, dust and the lambda 6613 Å DIB carrier in the Perseus OB2 association
We present a study of the spatial distribution of the lambda 6613 ÄDIB carrier in the Perseus OB2 association based on high resolutionobservations toward lines of sight representing different interstellarenvironments. We determined that in the studied region, the lambda 6613Ä DIB carrier is concentrated in two distinct clouds withvelocities of 1.4 (+/- 0.4) and 12.0 (+/- 0.9) km s(-1) . We comparedthe lambda 6613 Ä DIB carrier's velocity with the Na I velocitydistribution derived from our survey measurements, as well as with CO,OH, H I and Ca Ii measurements from the literature. We conclude that thebehaviour of the carrier of the lambda 6613 Ä DIB follows theoverall expansion motion of the gas in the association. The DIB velocityis directly linked to that of Ca Ii and H I. The DIB total columndensity is proportional to the total column density of Ca Ii and H Imaking those atoms good tracers of the lambda 6613 Ä DIB carrier.Those new results support the assumption that the lambda 6613 Ä DIBwould arise from a gas phase molecule, possibly single-ionized(Sonnentrucker et al. 1997). We also conclude that the DIB carrier isdistributed in shell structures over the whole association. We finallyshow from the DIB velocity structure that the DIB carrier, gas and dustare well mixed toward the association but that the DIB shells have anangular extent twice larger than that of the dust. Based on observationswith OHP 1.52m Telescope and Aurelie spectrograph.
| The radio emission from the Galaxy at 22 MHz
We present maps of the 22 MHz radio emission between declinations -28degr and +80 degr, covering ~ 73% of the sky, derived from observationswith the 22 MHz radiotelescope at the Dominion Radio AstrophysicalObservatory (DRAO). The resolution of the telescope (EW x NS) is 1.1degr x 1.7 degr secant(zenith angle). The maps show the large scalefeatures of the emission from the Galaxy including the thick non-thermaldisk, the North Polar Spur (NPS) and absorption due to discrete H Iiregions and to an extended band of thermal electrons within 40 degr ofthe Galactic centre. We give the flux densities of nine extendedsupernova remnants shown on the maps. A comparison of the maps with the408-MHz survey of \cite[Haslam et al. (1982]{haslam82}) shows aremarkable uniformity of spectral index (T~ nu (-beta ) ) of most of theGalactic emission, with beta in the range 2.40 to 2.55. Emission fromthe outer rim of the NPS shows a slightly greater spectral index thanthe distributed emission on either side of the feature. The mean localsynchrotron emissivity at 22 MHz deduced from the emission toward nearbyextended opaque H Ii regions is ~ 1.5 10(-40) Wm(-3) Hz(-1) sr(-1) ,somewhat greater than previous estimates.
| Mid-infrared spectroscopy of obscured IRAS galaxies
Spectra from 6 to 12 mu m are presented for the infrared galaxies NGC1266, Arp 148, and IRAS 1713+53. Though they are not markedly luminous(<= 10(12) L_ȯ), their IRAS colors are close to the colors ofultraluminous infrared galaxies, and thus indicative of heavyobscuration. The 6-12 mu m spectra are dominated by emission features ofpolycyclic aromatic hydrocarbon molecules at 6.2, 7.7, and 8.6 mu m.This demonstrates the existence of dust-enshrouded starbursts in thosegalaxies. Based on observations with the Infrared Space Observatory, anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, the Netherlands, and the United Kingdom)with the participation of ISAS and NASA.
| The Young Cluster IC 348
CCD photometry in BVRI was obtained for about 260 stars in and around IC348, and multiobject spectroscopy for 80 of these. A somewhat largerregion was surveyed for stars having H alpha in emission; over 110emission-line stars brighter than about R = 19 were discovered. BecauseH alpha emission could be detected to a limit near W = 3 A, divisioninto weak-line (WTTSs) and classical T Tauri stars (CTTSs) was possibleon purely spectroscopic grounds. There is a steep rise in the number ofemission-line stars below W(H alpha ) = 10 A; the proportion of WTTSs toCTTSs in the area surveyed is 58:51. ROSAT detected only about 58% ofthe spectroscopic WTTSs and about 65% of the CTTSs, although thesenumbers are sensitive to the survey thresholds. The bulk of the ages ofabout 100 stars, read off the theoretical tracks of D'Antona &Mazzitelli, range between about 0.7 and 12 Myr, but the emission-linestars, which are most likely to be members of IC 348, have a mean age of1.3 Myr. Allowance for unresolved binaries would increase this somewhat,but there is a firm upper limit at 2.95 Myr. There is no indication thatthe ages of the emission-line stars depend upon W(H alpha ): the IC 348WTTSs as a population are not systematically older than the CTTSs, butthere is a tendency for the WTTSs to be concentrated toward the centerof IC 348, while the CTTSs are more widely distributed. There is ascattering of emission-H alpha stars over the entire area surveyed.There are too many to be explained as low-mass members of an earliergeneration of star formation in Per OB2 or as foreground dMe stars. Themass frequency function, based on some 125 stars fitted to theoreticaltracks, rises from 1.5 Mȯ to about 0.2 Mȯ, with a slope verymuch like that of the Scalo initial mass function. The optical clusterIC 348 radius is about 4.'0, or 0.37 pc. The total mass of opticallydetectable stars in this volume is 57 Mȯ, while the mean spacedensity is about 520 stars pc-3. The amount of interstellar materialremaining within the cluster is small in comparison. Star formation inthe Per OB2/IC 348 region cannot be characterized by one unique age; itappears that stars have been forming in the region now occupied by theassociation for 10-20 Myr.
| Clump mass spectra of molecular clouds
We present clump mass spectra of the seven molecular clouds L1457, {MCLD126.6+24.5}, NGC 1499SW, Orion B South, S140, M17SW, and NGC 7538, whichwere derived by a Gaussian clump decomposition algorithm from largescale isotopomeric CO maps. We discuss in detail the reliability of themass spectra derived by studying their dependence on the controlparameters of the decomposition algorithm. All clump mass spectra foundare consistent with a power law, dN/dM~ M(-alpha ) , with alpha between1.6 and 1.8. Due to the different resolution of the observations and thedifferent distances of the clouds, the clump masses range from several10(4) ms(-1) ol\ in NGC 7538 down to 10(-4) ms(-1) ol, less than themass of Jupiter, in the nearby cloud L1457. The large dynamic rangecovered by the observations is reflected by the high number of clumpsfound in each cloud, which lie between 100 and 1300. The spectral indexof the clump mass distributions thus is independent of the wide range ofphysical properties of the clouds studied. In particular, there are noindications of a turnover of the clump mass power law index at acharacteristic clump mass such as the Jeans mass, below which the clumpsbecome gravitationally unbound. This is particularly emphasized by theclump properties in L1457 most of which are clearly substellar and whichare far from being gravitationally bound objects.
| Mid-Infrared Continuum of Starburst Nuclei: Contribution from Hot Large Grains within H II Regions?
The IRAS 12 and 25 mu m fluxes are compared with the Br gamma flux in asample of starburst nuclei. Good correlations are found between them.The subsequent analysis indicates the presence of two components in themid-infrared continuum: the nonthermal emission from "small grains"(<=100 A) which are heated transiently by nonionizing photons outsidethe H II regions and the thermal emission from "large grains" which areheated to ~140 K by ionizing, nonionizing, and Ly alpha photons insidethe H II regions. The small grains emitting at 12 mu m are depleted by~20% with respect to cirrus clouds in our Galaxy. The ratio of amountsof large grains to gas in the H II regions is comparable to the standardinterstellar value. The emission from hot large grains appears to bemore enhanced over the emission from small grains in starburst nucleiwith higher excitations.
| Comet Hale-Bopp: Your Viewing Guide
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