IRAS Fluxes Research Articles

Chemistry of nebulae around binary post-AGB stars: A molecular survey of mm-wave lines

Context. There is a class of binary post-asymptotic giant branch (post-AGB) stars that exhibit remarkable near-infrared excess. Such stars are surrounded by Keplerian or quasi-Keplerian disks, as well as extended outflows composed of gas escaping from the disk. This class can be subdivided into disk- and outflow-dominated sources, depending on whether it is the disk or the outflow that represents most of the nebular mass, respectively. The chemistry of this type of source has been practically unknown thus far. Aims. Our objective is to study the molecular content of nebulae around binary post-AGB stars that show disks with Keplerian dynamics, including molecular line intensities, chemistry, and abundances. Methods. We focused our observations on the 1.3, 2, 3 mm bands of the 30mIRAM telescope and on the 7 and 13 mm bands of the 40 m Yebes telescope. Our observations add up ~600 h of telescope time. We investigated the integrated intensities of pairs of molecular transitions for CO, other molecular species, and IRAS fluxes at 12, 25, and 60 μm. Additionally, we studied isotopic ratios, in particular 17O/18O, to analyze the initial stellar mass, as well as 12CO/13CO, to study the line and abundance ratios. Results. We present the first single-dish molecular survey of mm-wave lines in nebulae around binary post-AGB stars. We conclude that the molecular content is relatively low in nebulae around binary post-AGB stars, as their molecular lines and abundances are especially weaker compared with AGB stars. This fact is very significant in those sources where the Keplerian disk is the dominant component of the nebula. The study of their chemistry allows us to classify nebulae around AC Her, the Red Rectangle, AI CMi, R Sct, and IRAS 20056+1834 as O-rich, while that of 89 Her is probably C-rich. The calculated abundances of the detected species other than CO are particularly low compared with AGB stars. The initial stellar mass derived from the 17O/18O ratio for the Red Rectangle and 89 Her is compatible with the central total stellar mass derived from previous mm-wave interferometric maps. The very low 12CO/13CO ratios found in binary post-AGB stars reveal a high 13CO abundance compared to AGB and other post-AGB stars.

Multiphase ISM in Nearby Early Type Galaxy IC 5063

A multiwavelength study of a nearby dust lane early-type galaxy IC 5063 is presented. The objectives are to investigate dust extinction properties and the association of interstellar dust with other phases of ISM. The color-index maps as well as the extinction maps derived from the analysis of deep CCD observations in optical passbands revealed a prominent dust lane along its optical major axis in the inner region. In addition, two more fainter and extended dust patterns are apparent in the color index map as well as extinction maps. These features are also evident in the smooth model subtracted residual maps. The extinction curve derived for this galaxy revealed that dust grains in it are identical to the canonical grains in the Milky Way with the dust grains little larger than the canonical grains. The total extinction measured in the V band extinction map enabled us to quantify the dust content of this galaxy to be equal to , an order of magnitude shorter than that estimated using the IRAS flux densities at 60 and 100 μm. A Multiphase ISM study revealed a surprising similarity in the morphologies of the Hα emitting ionized gas distribution and X-ray emitting gas. Systematic analysis of high resolution X-ray observations using Chandra telescope enabled us to detect 18 discrete X-ray sources within optical D25 region of IC 5063, out of which 17 sources were separated out as the low mass X-ray binaries and one as the high mass X-ray binary source in the X-ray color-color plot.

Herschelimaging of the dust in the Helix nebula (NGC 7293)

In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACS and SPIRE instruments on board the Herschel satellite. The broadband maps show the dust distribution over the main Helix nebula to be clumpy and predominantly present in the barrel wall. We determined the spectral energy distribution of the main nebula in a consistent way using Herschel, IRAS, and Planck flux values. The emissivity index of 0.99 +/- 0.09, in combination with the carbon rich molecular chemistry of the nebula, indicates that the dust consists mainly of amorphous carbon. The dust excess emission from the central star disk is detected at 70 micron and the flux measurement agree with previous measurement. We present the temperature and dust column density maps. The total dust mass across the Helix nebula (without its halo) is determined to be 0.0035 solar mass at a distance of 216 pc. The temperature map shows dust temperatures between 22 and 42 K, which is similar to the kinetic temperature of the molecular gas, strengthening the fact that the dust and gas co-exist in high density clumps. Archived images are used to compare the location of the dust emission in the far infrared (Herschel) with the ionized (GALEX, Hbeta) and molecular hydrogen component. The different emission components are consistent with the Helix consisting of a thick walled barrel-like structure inclined to the line of sight. The radiation field decreases rapidly through the barrel wall.

C II] 158 μm LUMINOSITIES AND STAR FORMATION RATE IN DUSTY STARBURSTS AND ACTIVE GALACTIC NUCLEI

Results are presented for [CII] 158 micron line fluxes observed with the Herschel PACS instrument in 112 sources with both starburst and AGN classifications, of which 102 sources have confident detections. Results are compared with mid-infrared spectra from the Spitzer Infrared Spectrometer and with L(IR) from IRAS fluxes; AGN/starburst classifications are determined from equivalent width of the 6.2 micron PAH feature. It is found that the [CII] line flux correlates closely with the flux of the 11.3 micron PAH feature independent of AGN/starburst classification, log [f([CII] 158 micron)/f(11.3 micron PAH)] = -0.22 +- 0.25. It is concluded that [CII] line flux measures the photodissociation region associated with starbursts in the same fashion as the PAH feature. A calibration of star formation rate for the starburst component in any source having [CII] is derived comparing [CII] luminosity L([CII]) to L(IR) with the result that log SFR = log L([CII)]) - 7.08 +- 0.3, for SFR in solar masses per year and L([CII]) in solar luminosities. The decreasing ratio of L([CII]) to L(IR) in more luminous sources (the "[CII] deficit") is shown to be a consequence of the dominant contribution to L(IR) arising from a luminous AGN component because the sources with largest L(IR) and smallest L([CII])/L(IR) are AGN.

The discovery based on GLIMPSE data of a protostar driving a bipolar outflow

We report the discovery based on GLIMPSE data of a proto-stellar system driving a bipolar outflow . The bipolar outflow closely resembles the shape of an hourglass in the infrared. The total luminosity of L_total=5507 L_sun, derived from IRAS fluxes, indicates the ongoing formation of a massive star in this region. The spectral energy distribution (SED) of the driving source is fitted with an online SED fitting tool, which results in a spectral index of about 1.2. This, along with the presence of a bipolar outflow, suggests the detection of a Class I protostar. The driving source indicates prominent infrared excesses in color-color diagrams based on archived 2MASS and GLIMPSE data, which is in line with an early evolutionary stage of the system.

DUST AROUND R CORONAE BOREALIS STARS. I.SPITZER/INFRARED SPECTROGRAPH OBSERVATIONS

Spitzer/IRS spectra from 5 to 37 um for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the IRAS fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 um and 25 um are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 um luminosity and [8.0]-[24.0] color index.

Arp 220: EXTINCTION AND MERGER-INDUCED STAR FORMATION

We analyse new spatially resolved integral field spectroscopic H- and K-band data at a resolution of 0.3arcsec (100pc) and re-analyse interferometric CO(2-1) line observations of the prototypical merging system Arp220. We find that the majority of the K-band luminosity is due to a 10Myr old starburst, with a significant contribution from an underlying ca 1Gyr old stellar population, and a small contribution from stars less than 8Myr old. The Calzetti (2000) reddening law provides the best fit to photometric datapoints spanning 0.45um -- 2.12um. Furthermore, estimates of the bolometric luminosity from IRAS fluxes in conjunction with our stellar population analysis indicate that we observe less than 10% of the emitted K-band light. The stellar and CO(2-1) kinematic centre of the western nucleus coincides with the compact hot dust emission, indicating that the latter marks the centre of the gravitational potential. In the eastern nucleus, the CO(2-1) data are well matched by a model in which the gas orbits around the peak of the dust emission. This, and the similarity of the K-band tracer kinematics, shows that despite the irregular morphology, the eastern nucleus is also a kinematically coherent structure. Comparison of the extinction map and EW_CO and EW_Br-gamma maps indicates that the lower half of the eastern nucleus is significantly more extincted than the upper half, suggesting that the lower half is buried in the larger-scale gas disk.

MAPPING THE DIFFUSE ULTRAVIOLET SKY WITH THEGALAXY EVOLUTION EXPLORER

We present a map of the diffuse ultraviolet cosmic background in two wavelength bands (FUV: 1530 {\AA}; NUV: 2310 {\AA}) over almost 75% of the sky using archival data from the GALEX mission. Most of the diffuse flux is due to dust-scattered starlight and follows a cosecant law with slopes of 545 photons cm-2 s-1 sr-1 {\AA}-1 and 433 photons cm-2 s-1 sr-1 {\AA}-1 in the FUV and NUV bands, respectively. There is a strong correlation with the 100 {\mu}m IRAS flux with an average UV/IR ratio of 300 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the FUV band and 220 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the NUV but with significant variations over the sky. In addition to the large scale distribution of the diffuse light, we note a number of individual features including bright spots around the hot stars Spica and Achernar.

The pre-ZAMS nature of Mol160/IRAS 23385+6053 confirmed by Spitzer

Context. The formation of massive stars goes through phases that remain heavily obscured until the object is well along on the main sequence. The identification of massive YSOs in different evolutionary phases is therefore particularly difficult, and requires a statistical approach with large samples of candidate objects to determine the observational signatures of these different phases. Aims. A mandatory activity in this context is the identification and characterisation of all the phases that a massive forming YSO undergoes. It is of particular interest to verify the observability of the phase in which the object is rapidly accreting while not yet igniting the fusion of hydrogen that marks the arrival on the ZAMS. Methods. One of the candidate prototypical objects for this phase is Mol160/ΙRAS 23385+6053, which has been the subject of detailed studies that confirmed and strengthened the possibility that this massive YSO may be in a pre-hot core stage. We further investigate this issue by means of Spitzer imaging and spectroscopy in the 5-70 μm range. Results. The dense core of Mol160/ΙRAS 23385+6053, which up to now had only been detected at submillimeter and millimeter wavelengths, with only upper limits below 20 pm, has been revealed for the first time at 24 and 70 μm by Spitzer. These observations confirm the earlier assumptions that this object is dominant at far-IR wavelengths. The complete 24 μm-3.4 mm continuum cannot be fitted with a standard model of a zero-age main-sequence (ZAMS) star embedded in an envelope. A simple greybody fit yields a mass of 220 M ⊙ . The luminosity is slightly in excess of 3000 L ⊙ , which is a factor of 5 less than previous estimates when only IRAS fluxes were available between 20 and 100 μm. The source is under-luminous by the same factor with respect to UCHII regions or hot-cores of similar circumstellar mass, and simple models show that this is compatible with an earlier evolutionary stage. Spectroscopy between 5-40 μm shows that the physical conditions are typical of a photo-dissociated or photo-ionized region. The required UV illumination can be provided by some of the other sources revealed at A ≤ 24 μm in the same star-forming region, that can be plausibly modeled as moderately embedded intermediate-mass ZAMS stars. Conclusions. Our results strengthen the suggestion that the central core in Mol160/IRAS 23385+6053 is a massive YSO actively accreting from its circumstellar envelope and that it has not yet begun hydrogen fusion.

Properties of dust in early-type galaxies

We report optical extinction properties of dust for a sample of 26 early-type galaxies based on the analysis of their multicolour CCD observations. The wavelength dependence of dust extinction for these galaxies is determined and the extinction curves are found to run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic environment are quite similar to those of the Milky Way. For the sample galaxies, value of the parameter RV, the ratio of total extinction in V band to selective extinction in B and V bands, lies in the range 2.03-3.46 with an average of 3.02, compared to its canonical value of 3.1 for the Milky Way. A dependence of RV on dust morphology of the host galaxy is also noticed in the sense that galaxies with a well defined dust lane show tendency to have smaller RV values compared to the galaxies with disturbed dust morphology. The dust content of these galaxies estimated using total optical extinction is found to lie in the range 104 to , an order of magnitude smaller than those derived from IRAS flux densities, indicating that a significant fraction of dust intermixed with stars remains undetected by the optical method. We examine the relationship between dust mass derived from IRAS flux and the X-ray luminosity of the host galaxies.The issue of the origin of dust in early-type galaxies is also discussed.

Near infra-red and optical colour gradients in E-type galaxies

Colour gradients are considered for a sample of circa 50 E-type galaxies in the Local Supercluster. The new data includes isophotal colour profiles in J - H, J - K, V - J and V - K, measured using 2MASS frames mostly from the Large Galaxies Atlas, V frames from previous work and V profiles from the literature. This is supplemented by U - B, B - V, B - R, V - I colour gradients obtained anew from published photometric data. Colour gradients in E galaxies show remarkably large variations from object to object and do not correlate with other properties. Metallicity gradients are the primary cause as shown before. Age gradients with opposite effects are possibly needed to explain objects with small colour gradients. Some empirical evidence of such age effects has been found for a subset of objects with morphological peculiarities and younger stars mixed. Dust has only modest effects on colour gradients, as shown by the fact that objects with zero IRAS 100 μ flux have the same average values of the gradients, except in V - J and V - K, as those with non zero flux (cf. Table 7). This last subsample however exhibits poor but definite correlations between IRAS flux and gradients, which might be caused by the presence of a few relatively dusty galaxies in the sample. Given the absence of a correlation between any gradients and galaxy velocity dispersion (and hence mass), the observations do not agree with the predictions of the monolithic scenario for the formation of E galaxies. Simulated datasets of dummy objects mimicking the hierarchical scenario have been obtained, and used to test a technique for estimating the dust content of E-galaxies from the comparison of the V - K (or V - J) colour gradients with the U - B (or B - V) ones: the contents of diffuse dust, gauged in terms of published models, are obtained for a dozen objects.

ISO observations of the Wolf-Rayet galaxies NGC 5430, NGC 6764, Mrk 309 and VII Zw 19

Observations of four WR galaxies (NGC 5430, NGC 6764, Mrk 309 and VII Zw 19) using the Infrared Space Observatory are presented here. ISOCAM maps of NGC 5430, Mrk 309 and NGC 6764 revealed the location of star formation regions in each of these galaxies. ISOPHOT spectral observations from 4 to 12 microns detected the ubiquitous PAH bands in the nuclei of the targets and several of the disk star forming regions, while LWS spectroscopy detected [OI] and [CII] emission lines from two galaxies, NGC 5430 and NGC 6764. Using a combination of ISO and IRAS flux densities, a dust model based on the sum of modified blackbody components was successfully fitted to the available data. These models were then used to calculate new values for the total IR luminosities for each galaxy, the size of the various dust populations, and the global SFR. The derived flux ratios, the SFRs, the high L(PAH)/L(40-120 microns) and F(PAH 7.7 microns)/F(7.7 microns continuum) values suggest that most of these galaxies are home to only a compact burst of star formation. The exception is NGC 6764, whose F(PAH 7.7 microns)/F(7.7 microns continuum) value of 1.22 is consistent with the presence of an AGN, yet the L(PAH)/L(40-120 microns) is more in line with a starburst, a finding in line with a compact low-luminosity AGN dominated by the starburst.

The connection between shear and star formation in spiral galaxies

Abstract We present a sample of 33 galaxies for which we have calculated (i) the average rate of shear from published rotation curves, (ii) the far-infrared luminosity from IRAS fluxes, and (iii) the Ks-band luminosity from the Two Micron All Sky Survey (2MASS). We show that a correlation exists between the shear rate and the ratio of the far-infrared to Ks-band luminosity. This ratio is essentially a measure of the star formation rate per unit mass, or the specific star formation rate. From this correlation we show that a critical shear rate exists, above which star formation would turn off in the discs of spiral galaxies. Using the correlation between shear rate and spiral arm pitch angle, this shear rate corresponds to the lowest pitch angles typically measured in near-infrared images of spiral galaxies.

The Nature of Nearby Counterparts to Intermediate‐Redshift Luminous Compact Blue Galaxies. II. CO Observations

We present the results of a single-dish beam-matched survey of the three lowest rotational transitions of CO in a sample of 20 local (D ≲ 70 Mpc) luminous compact blue galaxies (LCBGs). These ~L*, blue, high surface brightness, starbursting galaxies were selected on the same criteria used to define LCBGs at higher redshifts. Our detection rate was 70%, with those galaxies having LB < 7 × 109 L☉ not detected. We find that the H2 masses of local LCBGs range from 6.6 × 106 to 2.7 × 109 M☉, assuming a Galactic CO-to-H2 conversion factor. Combining these results with our earlier H I survey of the same sample, we find that the ratio of molecular to atomic gas mass is low, typically 5%-10%. Using a large velocity gradient model, we find that the average gas conditions of the entire interstellar medium in local LCBGs are similar to those found in the centers of star-forming regions in our Galaxy and in the nuclear regions of other galaxies. Star formation rates, determined from IRAS fluxes, are a few M☉ yr-1, much higher per unit dynamical mass than normal spiral galaxies. If this rate remains constant, the molecular hydrogen depletion timescales are short, ~10-200 Myr.

Search for massive protostellar candidates in the southern hemisphere

We have observed two rotational transitions of both CS and C 17 O, and the 1.2 mm continuum emission towards a sample of 130 high-mass protostellar candidates with δ δ ≥ $-30^{\circ}$, we have selected from the IRAS Point Source Catalogue 429 sources which potentially are compact molecular clouds on the basis of their IR colours. The sample has then been divided into two groups according to the colour indices [25–12] and [60–12]: the 298 sources with [25–12] ≥ 0.57 and [60–12] ≥ 1.30 have been called High sources, the remaining 131 have been called Low sources. In this paper, we check the association with dense gas and dust in 130 Low sources. We have obtained a detection rate of ~$85\%$ in CS, demonstrating a tight association of the sources with dense molecular clumps. Among the sources detected in CS, ~$76\%$ have also been detected in C 17 O and ~$93\%$ in the 1.2 mm continuum. Millimeter-continuum maps show the presence of clumps with diameters in the range $0.2{-}2$ pc and masses from a few $M_{\odot}$ to $10^{5}~M_{\odot}$; H 2 volume densities computed from CS line ratios lie between ~10 4.5 and 10 5.5 cm -3 . The bolometric luminosities of the sources, derived from IRAS data, are in the range $10^{3}{-}10^{6}~L_{\odot}$, consistent with embedded high-mass objects. Based on our results and those found in the literature for other samples of high-mass young stellar objects, we conclude that our sources are massive objects in a very early evolutionary stage, probably prior to the formation of an Hii region. We propose a scenario in which High and Low sources are both made of a massive clump hosting a high-mass protostellar candidate and a nearby stellar cluster. The difference might be due to the fact that the 12 μ m IRAS flux, the best discriminant between the two groups, is dominated by the emission from the cluster in Lows and from the massive protostellar object in Highs .

Peculiarities and populations in elliptical galaxies

Morphological peculiarities, as defined from isophote asymmetries and number of detected shells, jets or similar features, have been estimated in a sample of 117 E classified galaxies, and qualified by an ad hoc index. The overall frequency of “peculiar” objects (Pec subsample) is 32.5%. It decreases with the cosmic density of the environment, being minimal for the Virgo cluster, the densest environment in the sampled volume. This environmental effect is stronger for galaxies with relatively large . The Pec subsample objects are compared with “normal” objects (Nop subsample) as regards their basic properties. Firstly, they systematically deviate from the Fundamental Plane and the Faber-Jackson relation derived for the Nop subsample, being too bright for their mass. Secondly, the dust content of galaxies, as estimated from IRAS fluxes, are similar in both subsamples. Third, the same is true of the frequency of Kinematically Distinct cores (KDC), suggesting that KDC and morphological peculiarities do not result from the same events in the history of E-galaxies. Using the Nop sample alone, we obtain very tight reference relations between stellar population indicators (, , , , Mg2, Hβ, , Mgb) and the central velocity dispersion . The discussion of the residuals of these relations allows us to classify the Pec galaxies in two families i.e. the YP or NGC 2865 family, and the NP or NGC 3923 one. Galaxies in the first group show consistent evidence for a younger stellar population mixed with the old one, in agreement with classical results (Schweizer et al. [CITE]; Schweizer & Seitzer [CITE]). The second group, however, has “normal“, or reddish, populations. It is remarkable that a fraction (circa 40%) of morphologically perturbed objects do not display any signature of a young population, either because the event responsible for the pecularity is too ancient, or because it did not produce significant star formation (or eventually that the young sub-population has high metallicity). A preliminary attempt is made to interpret the populations of Pec objects by combining a young Single Stellar Population with a Nop galaxy, with only limited success, perhaps largely due to uncertainties in the SSP indices used.

Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars

We present 1300 μ m continuum emission measurements and observations of the $\element[][12]{CO}$ (1–0) and (2–1) transition towards the centers of 64 Magellanic type galaxies (Sdm/Sm) and irregulars (Im/I0/Irr). The sources are selected to have IRAS flux densities $S_{100~\mu{\rm m}} \ge 1000$ mJy and optical diameters mainly below 180´´. We were able to detect $\element[][12]{CO}$ towards 41 and the continuum emission towards 28 galaxies. In addition, we obtained the corresponding data for a set of 6 complementary galaxies of different morphological type.

TheIRASRevised Bright Galaxy Sample

IRAS flux densities, redshifts, and infrared luminosities are reported for all sources identified in the IRAS Revised Bright Galaxy Sample (RBGS), a complete flux-limited survey of all extragalactic objects with total 60 micron flux density greater than 5.24 Jy, covering the entire sky surveyed by IRAS at Galactic latitude |b| > 5 degrees. The RBGS includes 629 objects, with a median (mean) sample redshift of 0.0082 (0.0126) and a maximum redshift of 0.0876. The RBGS supersedes the previous two-part IRAS Bright Galaxy Samples, which were compiled before the final ("Pass 3") calibration of the IRAS Level 1 Archive in May 1990. The RBGS also makes use of more accurate and consistent automated methods to measure the flux of objects with extended emission. Basic properties of the RBGS sources are summarized, including estimated total infrared luminosities, as well as updates to cross-identifications with sources from optical galaxy catalogs established using the NASA/IPAC Extragalactic Database (NED). In addition, an atlas of images from the Digitized Sky Survey with overlays of the IRAS position uncertainty ellipse and annotated scale bars is provided for ease in visualizing the optical morphology in context with the angular and metric size of each object. The revised bolometric infrared luminosity function, phi(L_ir), for infrared bright galaxies in the local Universe remains best fit by a double power law, phi(L_ir) ~ L_ir^alpha, with alpha = -0.6 (+/- 0.1), and alpha = -2.2 (+/- 0.1) below and above the "characteristic" infrared luminosity L_ir ~ 10^{10.5} L_solar, respectively. (Abridged)

Photometry and low resolution spectroscopy of hot post-AGB candidates

We have obtained Johnson U, B, V and Cousins R, I photometry and low resolution spectra of a small sample of hot post-AGB candidates. Using the present data in combination with JHK data from 2MASS, infrared data from the MSX catalog and the IRAS fluxes, we have studied the spectral energy distribution (SED) of these stars. Using the DUSTY code we have estimated the dust temperatures, the distances to the stars, the mass-loss rates, angular radii of the inner boundary of the dust envelopes and dynamical ages from the tip of the AGB. These candidates have also been imaged through a narrow band Hα filter, to search for nebulosity around the central stars. Our Hα images revealed the bipolar morphology of the low excitation PN IRAS 17395–0841 with an angular extent of 2.8´´. The bipolar lobes of IRAS 17423–1755 in Hα were found to have an angular extent of 3.5´´ (south-east lobe) and 2.2´´ (north-west lobe). The dust envelope characteristics, low resolution spectrum and IRAS colors suggest that IRAS 18313–1738 is similar to the proto-planetary nebula (PPN) HD 51585. The SED of IRAS 17423–1755, IRAS 18313–1738 and IRAS 19127+1717 show a warm dust component (in addition to the cold dust) which may be due to recent and ongoing mass-loss.

Non-emission-line young stars of intermediate mass

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be ≲1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities (Lir/L★) and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low Lir/L★, the scarcity of circumstellar dust, the low polarization, the presence of Hα emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars.