Pre-main Sequence Candidates Research Articles

A photometric and spectroscopic investigation of star formation in the very young open cluster NGC 6383

The very young open cluster NGC 6383 centered on the O-star binary HD 159176 is an interesting place for studying the impact of early-type stars with strong radiation fields and powerful winds on the formation processes of low-mass stars. To investigate this process, it is necessary to determine the characteristics (age, presence, or absence of circumstellar material) of the population of low-mass pre-main-sequence (PMS) stars in the cluster. We obtained deep U B V (R I)_c H-alpha photometric data of the entire cluster as well as medium-resolution optical spectroscopy of a subsample of X-ray selected objects. Our spectroscopic data reveal only very weak H-alpha emission lines in a few X-ray selected PMS candidates. We photometrically identify a number of H-alpha emission candidates but their cluster membership is uncertain. We find that the fainter objects in the field of view have a wide range of extinction (up to A_V = 20), one X-ray selected OB star having A_V ~ 8. Our investigation uncovers a population of PMS stars in NGC 6383 that are probably coeval with HD 159176. In addition, we detect a population of reddened objects that are probably located at different depths within the natal molecular cloud of the cluster. Finally, we identify a rather complex spatial distribution of H-alpha emitters, which is probably indicative of a severe contamination by foreground and background stars.

The Young Population of the Chamaeleon II Dark Cloud

We discuss the results of the optical spectroscopic follow-up of pre-main-sequence (PMS) objects and candidates selected in the Chamaeleon II dark cloud based on data from the Spitzer Legacy survey From Molecular Cores to Planet Forming Disks (c2d) and from previous surveys. Our sample includes both objects with infrared excess selected according to c2d criteria and referred to as young stellar objects and other cloud members and candidates selected from complementary optical and near-infrared data. We characterize the sample of objects by deriving their physical parameters. The vast majority of objects have masses M ≤ 1 M_☉ and ages <6 Myr. Several of the PMS objects and candidates lie very close to or below the hydrogen-burning limit. A first estimate of the slope of the initial mass function in Cha II is consistent with that of other T associations. The star formation efficiency in the cloud (1%-4%) is consistent with our own estimates for Taurus and Lupus, but significantly lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds may reflect a different history of star formation. We also find that the Cha II cloud is turning some 8 M_☉ into stars every megayear, which is less than the star formation rate in the other c2d clouds. However, the star formation rate is not steady and evidence is found that the star formation in Cha II might have occurred very rapidly. The Hα emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated.

TheSpitzerc2d Survey of Large, Nearby, Interstellar Clouds. X. The Chamaeleon II Pre–Main‐Sequence Population as Observed with IRAC and MIPS

We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M☉). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M☉ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8 < θ < 6.0 (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M☉ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.

Pre-main sequence stars in open clusters

Context. We present the basic ideas and first results from the project we are carrying out at present, the search for and characterisation of pre-main sequence (PMS) stars among the members of Galactic young clusters. The observations of 10 southern clusters, nine of them located in the Carina-Sagittarius spiral arm of the Milky Way are presented. Aims. We aim at listing candidate PMS member stars in young clusters. The catalogued stars will serve as a basis for future spectroscopic studies of individual objects to determine the properties of stellar formation in the last phases before the main sequence stage. Properties such as the presence of residual envelopes or disks, age spread among PMS members, and the possible presence of several episodes of star formation in the clusters, are to be addressed. Methods. Multicolour photometry in the $UBVR_{\rm C}I_{\rm C}$ system has been obtained for 10 southern young clusters in the fourth Galactic quadrant, located between Galactic longitudes l = 238° and l = 310°. For six clusters in the sample, the observations presented here provide the first published study based on CCD photometry. A quantitative comparison is performed with post-MS isochrones, and PMS isochrones from three different evolutionary models are used in the photometric membership analysis for possible PMS stars. Results. The observations produce photometric indices in the Johnson-Cousins photometric systems for a total of 26 962 stars. The matching of our pixel coordinates with corresponding fields in the 2MASS data base provides astrometric calibration for all cataloged stars and JHK 2MASS photometric indices for 60% of them. Post-MS cluster ages range from 4 to 60 Myr, whereas the photometric membership analysis assigns PMS membership to a total of 842 stars, covering an age range between 1 and 10 Myr. This information on the PMS candidate members has been collected into a catalogue, named DAY-I, which contains 16 entries for 842 stars in the field of 10 southern clusters.

New pre-main sequence candidates in the Taurus-Auriga star forming region

Aims. We have studied the X-ray source population of the Taurus Molecular Cloud (TMC) to search for new members of the Taurus-Auriga star forming region. Methods. Candidate members have been selected among the X-ray sources detected in 24 elds of the XMM-Newton Extended Survey of the Taurus Molecular Cloud, having an IR counterpart in the 2MASS catalog, based on color-magnitude and color-color diagrams. Their X-ray spectral properties have been compared with those of known members and other X-ray sources in the same elds but without a NIR counterpart. A search for are-lik e variability in the time series of all new candidates and the analysis of the X-ray spectra of the brightest candidates have been used to identify sources with a high probability of membership. Results. We have found that 347 of 1909 detected X-ray sources have an infrared counterpart in the 2MASS catalogue. Among them, we have selected 57 sources that are consistent with being new pre-main sequence star candidates at the distance of the Taurus-Auriga star forming region; the X-ray spectral properties of this sample are, on the whole, similar to the properties of known TMC members and di erent from those of X-ray sources without an IR counterpart, most of which are likely to be of extragalactic origin. For 12 such candidates, the likelihood of membership is very high, based on the relatively high plasma temperatures derived from their X-ray spectra and/or the observation of powerful ares in their light curves.

A Large-Area Search for Low-Mass Objects in Upper Scorpius. I. The Photometric Campaign and New Brown Dwarfs

We present a wide-field photometric survey covering ~200 deg^2 toward the Upper Scorpius OB association. Data taken in the R and I bands with the Quest-2 camera on the Palomar 48-inch telescope were combined with the 2MASS JHK survey and used to select candidate pre-main sequence stars. Follow-up spectroscopy with the Palomar 200-inch telescope of 62 candidate late-type members identified 43 stars that have surface gravity signatures consistent with association membership. From the optical/near-infrared photometry and derived spectral types we construct an HR diagram for the new members and find 30 likely new brown dwarfs, nearly doubling the known substellar population of the Upper Scorpius OB association. Continuation of our spectroscopic campaign should reveal hundreds on new stellar and substellar members.

The Low-Mass Pre-Main-Sequence Population of the Stellar Association LH 52 in the Large Magellanic Cloud Discovered withHubble Space TelescopeWFPC2 Observations

We report on the serendipitous discovery of ~500 low-mass candidate pre-main-sequence (PMS) stars in the vicinity of the stellar association LH 52 in the Large Magellanic Cloud. We present evidence that the red faint sequence of these stars seen in the CMD of LH 52 from Hubble Space Telescope (HST) WFPC2 observations belongs only to the association and follows almost perfectly isochrone models for PMS stars of masses down to ~0.3 M☉. We find that this feature has a Galactic counterpart and that the mass spectrum of the candidate PMS stars in LH 52 seems to correspond to a Salpeter initial mass function with a slope Γ -1.26 in the mass range 0.8-1.4 M☉.

Pre-main sequence star Proper Motion Catalogue

We measured the proper motions of 1250 pre-main sequence (PMS) stars and of 104 PMS candidates spread over all-sky major star-forming regions. This work is the continuation of a previous effort where we obtained proper motions for 213 PMS stars located in the major southern star-forming regions. These stars are now included in this present work with refined astrometry. The major upgrade presented here is the extension of proper motion measurements to other northern and southern star-forming regions including the well-studied Orion and Taurus-Auriga regions for objects as faint as V ≤ 16.5. We improve the precision of the proper motions which benefited from the inclusion of new observational material. In the PMS proper motion catalogue presented here, we provide for each star the mean position and proper motion as well as important photometric information when available. We provide also the most common identifier. The rms of proper motions vary from 2 to 5 mas/yr depending on the available sources of ancient positions and depending also on the embedding and binarity of the source. With this work, we present the first all-sky catalogue of proper motions of PMS stars.

On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud

We investigate a sample of 18 Large Magellanic Cloud Herbig Ae/Be candidate stars looking at their (1) spectral types (2) brightness variability mechanism and (3) near infra-red JHK emission. We find that the majority of the target stars have Hα emission, are of spectral type early- to mid-B and lack strong JHK excess emission. Their Balmer decrements are found to be similar to that of Galactic Be stars in general. Their erratic brightness variability is evaluated by using the observed optical color excess and the color gradient from the light curves and is subsequently interpreted as being due to variable dust obscuration or variable bf-ff emission from circumstellar ionized gas. For approximately half of the target stars in our sample the type of variability seems to be dissimilar to the mechanism involving bound-free and free-free emission, but could be interpreted as caused by variable dust obscuration, as we have proposed in earlier studies. It is therefore suggested that they are pre-main sequence objects, despite the fact that they nearly all lack thermal dust emission in the near infra-red; mid/far infra-red observations for these objects are warranted. One star is observed to have JHK excess emission and an inspection of its 7.5 year MACHO light curve confirms its erratic photometric behavior. The object displays deep photometric minima with a quasi-period of 191.3 days, as generally seen in the Galactic pre-main sequence subgroup of the UX Orionis stars.

Photometric and Spectroscopic Study of Stars in the Field of the Young Open Cluster Roslund 4

We present photometric and spectroscopic observations performed in the field of the Galactic open cluster Roslund 4, which contains the two cataloged nebulae IC 4954 and IC 4955. UBVRI photometry was carried out in a field of 11' × 6' around the cluster center. Medium-resolution optical spectroscopy has been obtained for 41 stars, including previously selected main-sequence and pre–main-sequence candidate cluster members. Narrowband Hα, [S II], and continuum images have been secured. The observations allow us the measurement of different physical parameters for the cluster. Assuming an absorption coefficient AV/E(B - V) = 3.1 and a reddening slope E(U - B)/E(B - V) = 0.72, we obtain a color excess E(B - V) = 1.1 ± 0.2 and a distance modulus DM0 = 11.7 ± 0.5. Fitting of isochrones to the color-magnitude diagrams gives log age(yr) = 7.2 ± 0.2, and the spectroscopic measurements provide the value VR = -15.7 ± 5.2 km s-1 for the heliocentric radial velocity. Among the 41 stars with spectroscopic data, 11 stars are probable cluster members on the basis of their radial velocity, and another 10 are considered as possible members. Two probable member stars of spectral types A5 and G1 show hints of absorption in the Li I 6708 A line, with respective equivalent widths of 0.10 and 0.28 A, and 22 stars of spectral types from B2 to G0 show different degrees of emission in Hα, [N II], and [S II] lines. With the exception of three stars of spectral type earlier than A0, the emission of which is mainly photospheric, all other emissions seen in Hα and forbidden lines have a nebular origin. They arise in an ionized cloud that surrounds the cluster and is causing diffuse emission and, possibly, local variations in the extinction law from star to star. In addition to the stars, spectra of several nebular condensations with relatively higher excitation have been analyzed. Two of them have been suggested to be Herbig-Haro (HH) objects. Our spectra indicate that only one of these condensations could be considered as an HH object.

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-latitude molecular clouds associated with the reflection nebula IC 2118, around I ∼ 208° and b ∼ -27°. The stars were selected as T Tauri candidates in objective prism plates obtained with the Schmidt telescope of Konkoly Observatory. Results of spectroscopic follow-up observations, carried out with the FLAIR spectrograph installed on the UK Schmidt and with ALFOSC on Nordic Optical Telescope, are presented in this paper. Based on spectral types, presence of emission lines and lithium absorption line, we identified five classical T Tauri stars and a candidate weak-line T Tauri star projected on the molecular clouds, as well as two candidate pre-main sequence stars outside the nebulous region. Using the near infrared magnitudes obtained from the 2MASS All Sky Catalog (IPAC 2003) we determined the masses and ages of these stars. We found that the five classical T Tauri stars projected on the clouds are physically related to them, whereas the other stars are probably background objects. Adopting a distance of 210 pc for IC 2118 (Kun et al. 2001) and using Palla & Stahler's (1999) evolutionary tracks we derived an average age of 2.5 x 10 6 yrs and a mass interval of 0.4-1.0 M ○. for the members of the IC 2118 association.

The discovery of low-mass pre-main-sequence stars in Cepheus OB3b

We report the discovery of a low-mass pre-main-sequence (PMS) stellar population in the younger subgroup of the Cepheus OB3 association, Cep OB3b, using UBVI CCD photometry and follow-up spectroscopy. The optical survey covers approximately 1300 arcmin2 on the sky and gives a global photometric and astrometric catalogue for more than 7000 objects. The location of a PMS population is well defined in a V versus (V−I) colour–magnitude diagram. Multifibre spectroscopic results for optically selected PMS candidates confirm the T Tauri nature for 10 objects, with equal numbers of classical TTS (CTTS) and weak-line TTS (WTTS). There are six other objects that we classify as possible PMS stars. The newly discovered TTS stars have masses in the range ∼0.9–3.0 M⊙ and ages from Ã1 to nearly 10 Myr, based on the Siess, Dufour & Forestini isochrones. Their location close to the O and B stars of the association (especially the O7n star) demonstrates that low-mass star formation is indeed possible in such an apparently hostile environment dominated by early-type stars and that the latter must have been less effective in eroding the circumstellar discs of their lower-mass siblings compared with other OB associations (e.g. λ-Ori). We attribute this to the nature of the local environment, speculating that the bulk of molecular material, which shielded low-mass stars from the ionizing radiation of their early-type siblings, has only recently been removed.

Candidate Pre–Main-Sequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and [CLC][ITAL]uvby[/ITAL][/CLC] Data

We propose a method that uses near-infrared plus uvby photometry to identify potentially extensive circumstellar dusty environments around F and A stars. The method has been applied to a sample of ~900 metal-rich reddened F stars with 2 Micron All Sky Survey and uvby data, suggesting the presence of circumstellar dust emitting in the near-infrared for ~70 stars. The log Te-MV diagram suggests that most, if not all, are likely pre-main-sequence (PMS) stars. They seem to be a continuation of the class of Herbig Ae/Be PMS stars in the spectral type F. Their number drops sharply downward of log Te ~ 3.84 (spectral types later than ~F5), which may provide new clues to the PMS evolution of 1-2 M☉ stars. We present a list of 21 of the most conspicuous candidate stars with circumstellar dust. About half of them are associated with the extended star-forming region around ρ Ophiuchi. The brightest of these 21 stars, with V ≲ 7.5, turn out to be IRAS sources, suggesting the presence of heated dust emitting in the far-infrared. Also in this list, HD 81270 is reported as a very unusual star moving away from the Galactic plane at a projected speed of 70 km s-1.

The Galactic Open Cluster NGC 6531 (M21)

UBV RI and H photometry has been performed for the open cluster NGC 6531. A total of 56 bright main sequence (MS) members were selected from their positions in photometric diagrams. We also classified 7 pre-main sequence (PMS) stars and 6 PMS candidates with Ha: emission from H photometry. We determined a reddening of = 0.29 0.03 and a distance modulus of Vo - Mv = 10.5 for the cluster. From the comparison of our photometric results to theoretical evolution models, we derived a MS turnoff age of 7.5 Myr and a PMS age spread of 4 Myr. The IMF slope , calculated in the mass range of 0.45 log m 1.35 is a steep value of = -1.8 0.6.

ITAL]UBVRI[/ITAL] and Hα Photometry of the Young Open Cluster NGC 6530

New UBVRI and Hα photometry has been obtained for the young open cluster NGC 6530 located 10' east of the Lagoon Nebula (M8). We found 37 pre–main-sequence (PMS) stars with Hα emission and nine PMS candidates using the R-Hα color, a measure of Hα emission. From this new photometry we derived the cluster parameters (V0 - MV = 11.25 ± 0.1 mag and E(B-V) = 0.35 mag), typical age (1.5 Myr), and an age spread of about 5 Myr. We confirmed the presence of a small amount of differential reddening across the cluster. We also verified the abnormal reddening law for the cluster, especially for a heavily embedded O7 V star, Herschel 36. Comparisons were made for several PMS evolution models with regard to the resulting age distribution and the initial mass function.

Spectroscopy of Pre–Main-Sequence Candidates of Spectral Type AF in the Young Galactic Cluster IC 4996

We present the results of a spectroscopic analysis of the pre–main-sequence (PMS) candidates in IC 4996, proposed by Delgado et al. Spectral types and heliocentric radial velocities are calculated for 16 stars in the field observed by these authors, 13 of them located in the region of the color-magnitude diagram where their proposed PMS stars are located. The estimated heliocentric radial velocity of the cluster is centered around -12 ± 5 km s-1. From the radial velocity distribution, six stars are rejected as cluster members, one of them showing spectral features characteristic of an Am star. The remaining 10 stars are confirmed as cluster members: three B-type stars and seven PMS stars of spectral types A4–F0 (six stars) and early G (one star). One of the proposed PMS members clearly shows radial velocity and spectral type variations, as well as relatively broad Hα absorption. The G-type cluster member is a weak-lined T Tauri star with strong Li I λ6708 absorption [Wλ(Li I) 0.26 A]. These results strongly support the presence in the cluster of a populated sequence of PMS stars of AF spectral type.

Pre–Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study

The results of a UBV CCD photometric study of the cluster IC 4996 are presented. We obtain new values for the cluster parameters: E(B-V) = 0.71 ± 0.08, V0 - MV = 11.9 ± 0.1, and age = (7.5 ± 3) × 106 yr. The combined evidence from the photometric diagrams suggests the presence of a number of pre–main-sequence (PMS) cluster members with spectral types ~A0 to F5. The interpretation of these objects as PMS candidates is further supported by independent results from CCD uvby Hβ observations. If confirmed, these stars would bridge the existing gap in the sample of PMS stars, between the coolest Herbig AeBe stars (HAeBe), and the hottest T Tauri stars. These PMS candidates are located some 0.5 and 1 mag above the main sequence in the V-(B-V) diagram, around the location of spectral types AF. This feature, together with their locations in the (U-B)-(B-V) diagram and the interpretation that they are PMS members, suggest an additional reddening law with a slope α = E(U-B)/E(B-V) 0.55, probably caused by circumstellar material.

ITAL]ROSAT[/ITAL] and [ITAL]Hipparcos[/ITAL] Observations of Isolated Pre–Main-Sequence Stars near HD 98800

We present new observations of the isolated young stars HD 98800 and CD-33_7795. Pointed ROSAT observations show that their X-ray properties, including X-ray luminosity and variability, are consistent with those of pre-main-sequence (PMS) stars. These observations do not reveal any additional PMS candidates in 40' fields centered on HD 98800 and CD-33_7795. Hipparcos observations of TW Hya (Wichmann et al. 1998) and HD 98800 (Soderblom et al. 1998) show that both stars are roughly 50 pc away and are PMS with ages of ~ 10^7 yr. We searched the Hipparcos catalog (complete down to ~ 2-3 L_sun at this distance) for other PMS stars in the same area. In a 10-pc radius volume of space centered on the previously known PMS stars, we find one additional candidate PMS star (CD-36_7429) with a low space velocity, X-ray emission comparable to that of HD 98800, and Li absorption. There are eight other stars in this area that have dwarf spectral types and lie above the main sequence, but based on their weak X-ray emission, high space velocities, and lack of Li in low-resolution spectra (i.e. EW(Li) < 0.1 Angstroms), these are probably mis-classified subgiants or giants. The current positions and proper motions of TW Hya, HD 98800, and CD-36_7429 are inconsistent with them having formed as a group.

ITAL]UBVRI[/ITAL] and Hα Photometry of the Young Open Cluster NGC 6231

New UBVI and Hα photometry has been performed for the young open cluster NGC 2244. We classified 30 OB stars as being members of the cluster using proper-motion data and spectral types from previous investigators, along with photometric diagrams obtained in this study. We measured Hα emission strength of the stars by Hα photometry and set up a selection criterion to select pre–main-sequence (PMS) stars with Hα emission. Fourteen PMS stars and seven PMS candidates were found using the criterion. In addition, six stars found near the positions of ROSAT HRI X-ray sources were assumed to be PMS stars and the optical counterparts of these X-ray sources. We determined a reddening of E(B-V) = 0.47 ± 0.04 for the cluster and a total-to-selective extinction ratio of RV = 3.1 ± 0.2. The distance modulus derived was V0 - MV = 11.1. By comparing our photometric results with theoretical evolution models, we derived a main-sequence turnoff age of 1.9 Myr and a PMS age spread of about 6 Myr. The slope of the initial mass function, Γ, calculated in the mass range 0.5 ≤ log m ≤ 2.0 could be flat (Γ = -0.7 ± 0.1).

Pre-main sequence candidates in the very young open cluster NGC 6611

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 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 , 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 A V 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.