BN Ori is a young emission-line star with a peculiar light curve. During the first half of this century the star showed strong irregular brightness- variations, similar to those of Herbig Ae stars. In the current half of the century the light curve resembles that of a FU Ori-object (FUOR, after Ambartsumian 1971). It can be characterised by an initial large-scale rise in brightness followed by a gradual decay over a period of about 15 Sears. From various photometric patrol programmes we concluded that the star remained at the same brightness level for the last 30 years. Between 1991 and 1995 the spectrum of BN Ori was intensively observed and was found to exhibit some unique pecularities. We have analysed the visual spectra obtained with the 6m BTA telescope of the SAG, the 60cm and 48cm telescopes at Mt. Maidanak, the 1.4m CAT and the 1.5 In telescope of ESO and the UV-spectra obtained with the IUE in 1984 and 1986. The spectra show certain similarities with those of classical FUORs, such as a large-scale thermal stratification, with Balmer lines showing A6 - A7 spectral type wings, while other lines are typical for late F-type stars. However, in contrast to classical FUORs, BN Ori is not of high luminosity-class.On the other hand the spectrum also shows similarities with those of Herbig A7e stars, although these stars have much stronger absorption lines in the UV from their outer shell and their circumstellar dust excesses are much larger than for BN Ori. Also, the rotation rate nu sin i of BN Ori is between 180 and 220 km s(-1), which is typical for A-type stars of 2 - 5 M-circle dot. This may indicate that BN Ori is a fast rotating FUOR with an intermediate-mass precursor on which the outburst had a different effect than in the case of the classical FUORs (which are rotating slower and have a low-mass T Tauri star as precursor). In the BN Ori outburst most of the massive gas- and dust shell, characteristic for Herbig Ae stars, seems to have been removed, except for the H alpha- and Mg II-emission region close to the photosphere. The disappearance of the circumstellar dust shell may explain the drastic variability-change in the light curve of BN Ori. The mass-accretion rates of BN Ori and several Herbig A7e stars (derived from the analysis of their UV-spectra in terms of disc-accretion) suggest that the FUOR outburst in BN Ori was due to a thermal runaway (Bell 1994) in its inner accretion-disc which was triggered by a modest increase in the accretion rate.

The role of photoevaporation on gaseous globules was shortly ago presented as a possible new mode of star formation (SF) in NGC 6611. In this paper we study this process by the distribution of the population of young stars and strong near-IR sources in the field of NGC 6611. We show that the locations of these sources do not coincide with those of the EGGs, but are found in regions shielded from the photoevaporation by the surrounding clouds. We therefore conclude that SF in NGC 6611 takes place at the other side of dark clouds to where the EGGs can be found.

The distance and formation history of the very young open cluster NGC 6530 were investigated by study ing a total of 132 probable member stars of this open cluster. It was concluded that the distance to NGC 6530 is 1.8 +/- 0.2 kpc, and its interstellar reddening E(B - V) is 0.(m)30. Furthermore. rye conclude that the extinction law of the intracluster material is normal (i.e. R-v = 3.1), but anomalous extinction laws were found for several more embedded stars in the cluster. Of the 132 stars included in this study, 11 are suspected to be variable, five show Ha in emission and nine show an infrared excess. Among the member stars of NGC 6530, three were found to be part of the Herbig Ae/Be stellar class, whereas two others are possible members of this stellar group as well. Also, one cluster member is probably a new massive post-AGB star, whereas the same could possibly apply to another member star. Finally, from the distribution of post- and pre-main sequence stars in the cluster's HR-diagram, it was concluded that the process of star formation in NGC 6530 must have started a few times 10(7) years ago and, for the less massive stars, is probably still going on today.

The photometric behaviour of the peculiar B[e] star HD 45677 over the last 25 years is investigated. We conclude that the photometric variations (V = 7.(m)22-8.(m)85) on such a time scale can be well explained by obscurations, possibly due to large (> 1 mu m) circumstellar (CS) dust grains which were created after an explosive event around 1950. Intermediate time scale variations are also identified and can be well explained by infall of CS material.Evidence is also found for smaller, pulse-like, amplitude variations. The time scale of this ''flickering'' ranges from days down to hours and is explained by instabilities in accretion flows towards HD 45677. The accretion mechanisms can be the origin of the hypothesized existence of a bipolar flow.HD 45677, seen edge-on, shows evidence for the presence of a circumstellar disk. The significant accretion flows in this disk probably increased some time after the 1950 event, either due to a fall-back of part of the ejected material by a blow-out around 1950 or either due to the explosive dissociation of a large cometary-like body. Dynamic effects of such infalling and outflowing material close to the stellar surface are also detected by high resolution spectroscopy in the Ha! and He I profiles. These short time scale variations are also seen in the cool material as detected by the variations in the violet part of the Na I D profiles, which are probably due to collisions of the outflowing material with the outer disk.The presence of a disk is often indicated and here by the emission of the sodium lines with a strong absorption component at the systemic velocity (about 20 km s(-1)). Most detected lines, except Her, are well centered at this velocity. Apart from the red [SII] lines, all other nebular lines as well as many other emission lines, including the Fe II emission spectrum, were seen in spectra taken far before and after the 1950 event as well as in the latest spectra. So, the situation of the gaseous stellar environment seems to be stable in spite of the large photometric variations. The Fe II emission lines are double peaked with a velocity separation of about 32 km s(-1). This could mean that the inner disk material, in which these lines are thought to be formed, rotates with about 16 km s(-1). In our 1992 data this rotation velocity seems to range up to 30 km s(-1), which could be due to material accelerated by the 1950 event.We discuss in this paper the evolutionary status of HD 45677. Because of its rather isolated position in the sky and because of the very fast evolution of a B2 type star, we think that HD 45677 could be young, but not in the sense of being a pre-main sequence object. Options like an evolved object such as a LBV or PN are not suitable because of the probable luminosity class III, IV or V and its rather cool central source, respectively. HD 45677 shows no evidence of any companion. The slight possibility of it being symbiotic can be added as well as instabilities in its unknown post-main sequence phase and the option that HD 45677 could be a hot post-AGE star.

Fundamental astrophysical parameters (distance, temperature, luminosity, mass, age) of a sample of 10 Herbig Ae/Be candidates and 3 non-emission line A and B stars in star forming regions were computed combining Hipparcos parallaxes with data from literature. All genuine Herbig stars in our sample are located between the birthline and the zero-age main sequence (ZAMS) in the Hertzsprung-Russell diagram (HRD), in accordance with what is expected for pre-main sequence stars. The region in the HRD close to the birthline is relatively devoid of stars when compared to the region closer to the ZAMS, in agreement with the expected evolutionary time scales. The Herbig Ae/Be stars not associated with star forming regions were found to be located close to the ZAMS. Additionally we discuss a possible evolutionary scenario for the circumstellar environment of Herbig stars.

Studies of the evolutionary precursors of beta Pic have demonstrated that high velocity, accreting gas features are routinely detectable when the viewing geometry is similar to the beta Pic system. We present a first exploration of the composition of the accreting gas toward 4 nearby, UV-luminous Herbig Ae/Be stars viewed through their circumstellar dust disks. We find that in addition to refractory materials, plausibly interpreted as sublimates of silicate grains, the accreting gas features in these stars contain carbon, mildly refractory species such as Zn II, and volatiles. Overall, the composition of the accreting gas is consistent with detection of the gaseous comae of star-grazing planetesimals of either cometary or carbonaceous chondritic composition.

For the search of Herbig Ae/Be objects in the extremely young open cluster NGC 6611 we have selected a sample of 52 pre-main sequence candidates, discovered by Walker (1961), Sagar & Joshi (1979), Chini & Wargau (1990) and The et al. (1990). We continue the approach of the last paper by studying each star individually with new and unpublished Walraven WULBV, Johnson/Cousins UBV(RI)(C) and Johnson JHKLM photometric data as well as low resolution spectroscopy.Each object is shown to have its own extinction law, which is investigated using their spectral energy distribution (SED). There does not seem to be a clear relationship between the location of a star and the extinction law. This means that the extinction is generated locally and its correction must be taken individually. For each object accurate astrophysical parameters are then derived. Plotting the objects in an HR-diagram, together with the values for the E(B - V), the probability of membership value P and the extinction characteristics, helps to discriminate between cluster members and non-cluster members.Most foreground stars are of late spectral type and are labeled as Group III objects. Group I, to which most members of this cluster belong, contains objects of early spectral type. Part of them seem to be in their post ZAMS phase and the other part in their pre-ZAMS stage. By comparing the evolutionary tracks of Palla & Stahler (1993) for pre-MS objects and of Maeder & Meyenet (1988) for post-MS stars we have concluded that the cluster contains objects of a few 0.1 Myr as well as objects of about 6 Myr. As most of the Group I objects do not show well-known Herbig Ae/Be characteristics, the time scale of clearing the disk material must be typically less than about 0.1 Myr for the more massive objects. Objects that show an IR-excess are found among the less luminous ones. They could still be in their pre-ZAMS phase, having an age of about 1 Myr. Such an age is appropriate for the Group II objects, which are of intermediate spectral type. As they are located close to the stellar birthline they should have been formed recently. It seems that an efficient clearing mechanism must have taken place, because not many of these objects are embedded and show only some IR-anomalies. This could be the reason that we have found no more than four typical Herbig Ae/Be candidates.We support the hypothesis of Hillenbrand et al. (1993) that there is an age spread in NGC 6611. The stars with the highest Av values are located in the centre of the cluster, somewhat to the northwest. This coincides with the location of many embedded sources more towards the northwest, a region in which star-formation is probably still taking place. The age of the most evolved objects is about 6 Myr.It would be interesting to study these regions and the Group II objects since investigating the youngest objects which are still partly embedded may help us to understand the first phases of star formation and the rapid cleaning of the circumstellar material. Young ''naked'' stars can also be formed by evaporating gaseous globules (EGGs) as was recently discovered in dark regions of this cluster (Hester & Scowen 1995). The lack of angular momentum of such objects could explain the paucity of HAeBe candidates.

This short paper presents previously unpublished, as well as new, photometric observations of the peculiar B[e] star HD 45677. The photometric measurements were made in the Walraven WULBV, Stromgren uvby, Johnson/Cousins UBV(RI)(C) and the ESO JHKLM photometric systems between 1977 and 1994. Together with all the previously published photometry, these data will be analyzed in a separate paper, which will also investigate the spectroscopic behaviour of this fascinating object.

The photometric behaviour of the irregular variable Herbig Ae star V 351 Ori was investigated combining data from the literature with new photometric data in the Stromgren system. It is shown that this enigmatic object changed its photometric behaviour from that of a Herbig Ae star with strong photometric variations, due to extinction by circumstellar dust clouds, to that of an almost non-variable star. Such a behaviour is not unique; it has been found also in the star BN Ori (Shevchenko et al. 1996). This suggests that such transitions must occur quite often during the evolution of intermediate mass HAeBe stars towards the main-sequence (MS). However, it is equally probable that before such a star arrives at the MS, it will become strongly variable again after collecting enough dust from its environment.During the process of V 35 1 Ori's transition from a Herbig Ae star with strong photometric variations to a non-variable one, the maximum brightness in the Stromgren v, b and y magnitudes decreased, whereas that in u increased, causing a strong blueing effect in the colour-magnitude diagrams. It should be noted that this effect happened at a decrease in visual brightness by about 0.(m)2 magnitude only. It is therefore caused by a different mechanism than that in V 351 Ori before the transition and in UX Ori type stars, where the blueing effect occurs at a decrease in V of about 2.(m)5.The transition, from a strongly variable star to the state in which V 351 Ori is nearly constant at a level equal to that of its maximum brightness before the transition, happened very fast (within 50 days), and went along the interstellar reddening line in the colour-magnitude diagrams. A provisional model to explain V 351 Ori's behaviour, in which it is assumed that a temporarily strong accretion of matter onto the star took place, is proposed.

From Stromgren uvby photometry, obtained during the last 14 years, we demonstrate that HR 6000 shows long-term photometric variations with a small (about 0(m).03 in u, 0(m).02 in v and 0(m).01 in b) amplitude. Whereas previous authors classified HR 6000 spectroscopically as A0/3 IIIp and photometrically as B7, we conclude that the observed properties of this system may be explained by inferring that HR 6000 is in fact a binary system, consisting of a B6 V star and a T Tauri companion. The combined spectrum of these two components may easily have been mistaken for a chemically peculiar early A-type star. This scenario also explains the observed small amount of infrared excess and the observed ROSAT X-ray flux of this system as arising from a T Tauri companion. Other traces of this companion have not been detected in our studies. Detection of the T Tauri companion with near-IR spectroscopy may be possible due to the significant contribution of the companion to the total flux at these wavelengths.

We present a survey of high dispersion UV and optical spectra of Herbig Ae/Be (HAeBe) and related stars. We find accreting, circumstellar gas over the velocity range +100 to +400 km/s, and absorption profiles similar to those seen toward Beta Pic, in 36% of the 33 HAeBe stars with IUE data as well as in 3 non-emission B stars. We also find evidence of accretion in 7 HAeBe stars with optical data only. Line profile variability appears ubiquitous. As a group, the stars with accreting gas signatures have higher v sin i than the stars with outflowing material, and tend to exhibit large amplitude (greater than or equal to 1(sup m)) optical light variations. All of the program stars with polarimetric variations that are anti-correlated with the optical light, previously interpreted as the signature of a dust disk viewed close to equator-on, also show spectral signatures of accreting gas. These data imply that accretion activity in HAeBe stars is preferentially observed when the line of sight transits the circumstellar dust disk. Our data imply that the spectroscopic signatures of accreting circumstellar material seen in Beta Pic are not unique to that object, but instead are consistent with interpretation of Beta Pic as a comparatively young A star with its associated circumstellar disk.

It has recently become possible to reduce the raw IRAS Low Resolution Spectrograph (LRS) data in an interactive way. In this paper several problems concerning the LRS data of extended objects are addressed. We investigate the presence of the 21 mum emission feature in HII regions as reported by Cox (1990). These sources appear to be extended in the IRAS data. A careful re-analysis of the IRAS data shows that observing an extended source with a slit-less spectrograph like the LRS results in spurious features at the edges of the LRS band. The extent of an object results in a degradation of the wavelength resolution and a decrese of the LRS spectrum at the wavelength edges. Furthermore, the point source flux density listed in the IRAS Point Source Catalog (PSC) is under-estimated for extended objects. It is concluded that these effects can well explain the presence of the 21 mum emission feature in the HII regions discussed by Cox (1990).

IUE spectra of the Herbig Ae star UX Ori reveal the presence of large amplitude light and color changes, which are an extension to shorter wavelengths of the ''blueing'' effect seen in the optical. At optical maximum the UV spectrum of UX Ori is dominated by heavy line blanketing from accreting circumstellar gas with velocities as high as +200 km s(-1). At minimum light, prominent mid-UV emission from Fe II and Mg II is present, and the overall spectrum closely resembles spectra of more heavily embedded Herbig Ae stars. Comparison of the UV and optical data for UX Ori suggest that single parameter ISM-like extinction curves do not fit the observed data near optical maximum light, when contamination by scattered circumstellar dust is minimized. The best fit to the UV color-magnitude diagram is for circumstellar extinction dominated by silicate grains with a(min) greater than or equal to 0.15 mu m, and with a power law distribution n(alpha) = alpha(-2), suggesting that both the particle size distribution and grain chemistry in the inner disk have evolved considerably from those characteristic of molecular clouds in the 2-3 Myr since the formation of UX Ori.

Using adaptive optics at the ESO 3.6m telescope, we obtained diffraction limited JHK-images of the region around the Herbig AeBe star NX Pup. We clearly resolved the close companion (sep. 0 '' 128) to NX Pup - originally discovered by HST - and measured its JHK magnitudes. A third object at a separation of 7 ''.0 from NX Pup was identified as a classical T Tauri star so that NX Pup may in fact form a hierarchical triple system. We discuss the evolutionary status of these stars and derive estimates for their spectral types, luminosities, masses and ages.

The newly recognized Herbig Be star, HD 45677 (B2, 7.5 ≤ V ≤ 8.7), traditionally classified as a B[e] star, has recently been found to have the signatures of a young energetic premain sequence (PMS) star, via UV spectropolarimetry, high-dispersion observations, and through the re-analysis of its infrared excess. Furthermore, low-dispersion IUE data have been used to detect significant UV excesses, which are model independent for fluxes between 2000–3000 Å. The UV data also indicate a complex spectral type; the depth of the absorption lines suggest B1-B3 with a luminosity III-IV, whereas the unreddened continuum fluxes are better modeled by an O9-B0 star, making this object one of the earliest PMS star known. This multi-temperature spectral feature is a hallmark of objects seen edge-on, through an optically thick disk, undergoing episodes of mass accretion in their inner regions, such as the well-studied FU Ononis objects. In recent photometric data we have found short-term variability in all Strömgren indices; especially remarkable is ΔVy ~ 0.1 mag in a timescale of hours.

In this paper we present the first results of the study of very young stars having non-periodic Algol type brightness minima. It is based on cooperative observations between the Crimean Astrophysical Observatory (CAO), the European Southern Observatory (ESO) and the National Solar Observatory (NSO). In August-September, 1992, a very deep (Delta V = 2.(m)5), long lasting minimum of the light of the isolated Herbig Ae star UX Ori occurred. At this event the star was observed photometrically (UBVRI) and polarimetrically at the CAO, and spectroscopically (high resolution: H alpha and NaI D) at the ESO. The spectroscopic observations were continued at the NSO with the McMath solar/stellar telescope in October-December, 1992, when the star returned to maximum brightness and again at the ESO in July and October, 1993, when the star was bright. The main results of our observations can be briefly summarized as follows: 1) The photometric and polarimetric results are in agreement with the model according to which UX Ori is surrounded by an edge-on circumstellar disk-like envelope, and its variability is caused by variable obscuration of the star by opaque circumstellar dust clouds. 2) The double-peaked Ha profile observed at maximum light changed to single-peaked at deep minimum; obscuration of a part of the circumstellar gas by an optically thick dust cloud is causing this variation, 3) The inverse P Cygni profiles and variable redshifted absorption components have been observed in the NaI D lines indicating the infall of cool gas onto the star.We believe that, such as in the case of the star beta Pictoris, violent comet-like activity takes place in the young protoplanetary disk of UX Ori which causes the observed variability.

A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and related objects is certainly needed, for both well-seasoned researchers and, in particular, for new investigators starting to study the many interesting astrophysical properties of these very young objects. We present a brief discussion of the current observational characteristics that distinguish this class from their main sequence counterparts. The HAEBE and related stars are listed in five tables, containing 287 objects. Table 1 contains all Ae and Be stars which historically are recognized as true HAEBE stars or potential candidate members. Table 2 gives the stars of spectral type Fe, and emission line stars with very uncertain or unknown spectral type. In Table 3 are given all known Extreme Emission Line Objects (EELOs), of which most have not been identified to belong to any specific group. Table 4a and b list other Bep or B[e] stars with strong IR-excess and unknown spectral type. Table 5 contains the non-emission line possible young objects. Furthermore, Table 6 contains 35 stars rejected from former published lists of HAEBE stars. In these tables we are including coordinates, spectral types, visual magnitudes, ranges in photometric variability and references of several key publications related to each object. Relevant remarks, such as the presence of a nebula in the vicinity of an object, are also given.