Emission Stars Research Articles

Searches for HH objects and emission stars in star formation regions. V. Two new cometary nebulae in Perseus

A small region surrounding the emission star LkHα 326 in the Perseus dark cloud is studied in the optical range. Two new cometary nebulae are described and their relationship to the Herbig-Haro objects in this region is examined. An HH-jet is discovered near the central star of one of these nebulae. Six emission stars, of which four are new, are detected by slitless spectroscopy.

THE INITIAL MASS FUNCTION AND YOUNG BROWN DWARF CANDIDATES IN NGC 2264. III. PHOTOMETRIC DATA

We have performed deep wide-field CCD photometry of the young open cluster NGC 2264 to study the extent of star-forming regions (SFRs) and the shape of the initial mass function. In this paper, we present VRI and Hα photometry for more than 67,000 stars. From the spatial distribution of the selected Hα emission stars, we identify two active SFRs and a less active halo region surrounding these two SFRs. There are several Hα emission stars in the field region outside the halo region, and these may be newly formed stars in the Mon OB1 association surrounding the cluster. The locus of pre-main-sequence (PMS) stars in the IC versus V − IC diagram is revised from the distribution of Hα and X-ray emission stars in the diagram. The mean reddening of late-type PMS stars is estimated to be E(B − V) 0.2 mag using the distribution of X-ray emission stars in the 2MASS color-color diagram. We can confirm that the Hα emission stars below the PMS locus (so-called BMS stars) are bona-fide members of NGC 2264 from their spatial distribution as well as from their near-IR excess in the 2MASS color-color diagram. In addition, four objects around IRS-2 detected with the Spitzer IRAC are also classified as BMS stars.

Search for HH objects and emission stars in star formation regions. IV. New Herbig-Haro flows and objects associated with cometary nebulae

As part of a project to search for new Herbig-Haro (HH) objects in star formation regions, observations are reported on the neighborhoods of five cometary nebulae: MacC H12, MacC sH15, GM 1-14, RNO 33, and Pars 17. We have been able to identify 9 previously unknown HH objects in those regions. Almost all of these objects belong to directed flows whose sources are, with high probability, the central stars of these nebulae. In the cases of MacC H12 and GM 1-14, the outflows have a distinct bipolar structure. The sources of the outflows are located on a J-H/H-K diagram in order to classify them.

Stellar contents and star formation in the young star cluster Be 59

We present UBV Ic CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between E(B−V) ≃ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as 3.7 ± 0.3. The distance of the cluster is found to be 1.00 ± 0.05 kpc. Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range 2.5 < M/M⊙≤ 28 is found to be −1.01 ± 0.11 which is shallower than the Salpeter value (−1.35), whereas in the mass range 1.5 < M/M⊙≤ 2.5 the slope is almost flat. The slope of the K-band luminosity function is estimated as 0.27 ± 0.02, which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Hα emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.

Evidence for weak magnetic fields in early‐type emission stars

AbstractWe report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi‐mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&amp;K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

SpitzerObservations of NGC 2362: Primordial Disks at 5 Myr

We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars.

HD 65949: the highest known mercury excess of any CP star?

ESO spectra of HD 65949 show it to be unlike any of the well-known types within its temperature range $\approx$ 13600K. It is neither a silicon, nor a mercury-manganese star, though it has a huge Hg II line at $\lambda$3984. We estimate $\log({\rm Hg/H}) + 12.0 \approx 7.4$. This is higher than any published stellar mercury abundance. HD 65949 is a member of a nearby open cluster, NGC 2516, which is only slightly older than the Pleiades, and has been of recent interest because of its numerous X-ray emission stars, including HD 65949 itself, or a close companion. A longitudinal magnetic field of the order of -290 Gauss at the 4.7~$\sigma$ level was very recently diagnosed from accurate circular spectropolarimetric observations with FORS 1 at the VLT. The spectral lines are sharp, allowing a thorough identification study. Second spectra of Ti, Cr, and Fe are rich. Mn II is well identified but not unusually strong. Numerous lines of S II and P II are found, but not Ga II. The resonance lines of Sr II are strong. While many Y II lines are identified, and Nb II is very likely present, {\it no} Zr II lines were found. Xe II is well identified. Strong absorptions from the third spectra of the lanthanides Pr, Nd, and Ho are present, but lines from the second spectra of lanthanides are extremely weak or absent. Among lines from the heavier elements, those of Pt II are clearly present, and the heaviest isotope, $^{198}$Pt, is indicated. The uncommon spectrum of Re II is certain, while Os II and Te II are highly probable. Several of the noted anomalies are unusual for a star as hot as HD 65949.

X-ray emission from T Tauri stars in the Lupus 3 star-forming region

Aims. In this paper, I present analysis results of an $\it{XMM-Newton}$ observation of the Lupus 3 region that contains a high proportion of young low mass ($M Methods. The detection of X-ray sources in 0.5 to 4.5 keV images of the Lupus 3 core was performed using the standard source detection method of the $\it{XMM-Newton}$ Science Analysis Software. The detected sources were correlated with a list of Herbig-Haro objects and H$_{\rm \alpha}$ emission stars that contains mainly classical T Tauri stars, with a catalogue of weak-line T Tauri Stars and with a recent list of new low-mass members of the Lupus 3 dark cloud found in a visible-light spectroscopic survey at the center of the Lupus 3 star-forming core. The light curves and spectra of the brightest X-ray sources with known T Tauri star counterparts were analysed. Results. One hundred and two X-ray sources were detected in the 30´ diameter field-of-view of the EPIC cameras, of which 25 have visible or near-IR counterparts that are known as pre-main sequence stars. Their X-ray luminosity ranges from 3 $\times$ 10 28 to 3 $\times$ 10 30 erg s -1 . Two of these objects with mass estimates lower than 0.075 $M_{\odot}$ have an X-ray luminosity of about 4–7 $\times$ 10 28 erg s -1 , comparable with that of flaring young brown dwarfs. A linear correlation is found between the X-ray luminosity and the mass or volume of the stars that is qualitatively expected from some models of distributed turbulent dynamos. The EPIC spectra of the X-ray brightest sources can be fitted using optically thin plasma emission models with two components at temperatures in the ranges 3–9 $\times$ 10 6 K and 1–50 $\times$ 10 7 K, respectively. The large emission measure of hot plasma may be caused by disruptions of magnetic fields associated with an intense flaring activity, while the X-ray emission from the “cool” plasma components may result from solar-type active regions. The emission measures of the plasma components are of the order of 10 52 cm -3 , typical of the values expected from coronal plasmas in T Tauri stars, post-T Tauri stars, and active late-type dwarfs in close binary systems. One property of the X-ray brightest stars in Lupus 3 that seems common among pre-main sequence stars is the low abundance of Fe.

Spectral and photographic studies of the known T association in the neighborhood of Cyg OB7

Spectral and photographic observations of an 11′×11′ region in the center of the known association Cyg OB7 are presented. The observations were made on the 2.6-m telescope of the Byurakan Astrophysical Observatory in 2004. The spectral camera SCORPIO, a multiregime focal reducer for the primary focus, was used. One new HH node and three new emission stars were discovered in the region under study. One of the previously known emission stars is found to have spectral class M0Ve and to lie at a distance of 200 pc. Some variability is detected in the brightness of a known infrared nebula and the cometary nebula CN2.

NGC 2401: a template of the young population of the Norma–Cygnus arm in the Third Galactic Quadrant

Abstract Based on a deep optical CCD [UBV(RI)C] photometric survey and on the Two-Micron All Sky Survey (2MASS) data, we derived the main parameters of the open cluster NGC 2401. We found that this cluster is placed at 6.3 ± 0.5 kpc (VO−MV= 14.0 ± 0.2) from the Sun and is 25 Myr old, which allows us to identify NGC 2401 as a member of the young population belonging to the innermost side of the extension of the Norma–Cygnus spiral arm in the Third Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies in the cluster area revealed that it is a B0Ve star; however, we could not confirm that it is a cluster member. We also constructed the cluster luminosity function (LF) down to V∼ 22 and the cluster initial mass function (IMF) for all stars with masses above M∼ 1–2 M⊙. It was found that the slope of the cluster IMF is x≈ 1.8 ± 0.2. The presence of a probable pre-main-sequence (PMS) star population associated with the cluster is weakly revealed.

The Pre-Main-Sequence Population of L988

L988 is a large (~05 ? 07) dark cloud complex at about 600 pc that contains several bright pre-main-sequence objects (such as V1331 Cyg and LkH? 321), but this paper deals in detail only with a small region on its eastern edge, near the HAeBe star LkH? 324. That star and its distant companion LkH? 324SE lie at the apex of a V-shaped area apparently excavated from the edge of L988, and are the brightest members of a small cluster containing about 50 H?-emission stars. A median age of about 0.6 Myr (with large dispersion) is inferred from its color-magnitude diagram, constructed from VRI photometry to V = 22. Keck HIRES spectra show that LkH? 324SE is probably also an HAeBe. Its image is nonstellar, and within 3'' to the northwest are three condensations having complex [S II] and [O I] profiles and radial velocities up to -200 km s-1. They probably originate in an outflow from LkH? 324SE. A bright Ap star with strong Si II lines is embedded in the heavy obscuration 8' to the west. It illuminates a small reflection nebulosity, has several faint H? emitters nearby, and shares the radial velocity of L988, so clearly it was formed in that cloud. It demonstrates again that such chemical peculiarities can be established very early in young stars of moderate mass.

The Young Cluster NGC 2362

An Hα emission survey of the young cluster NGC 2362 resulted in the detection of 130 Hα emission stars in an 11' × 11' field approximately centered on the fourth magnitude O9 Ib multiple star τ CMa. The survey was carried out using the wide-field grism spectrograph on the University of Hawaii 2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. Deep optical VRCIC (to V ~ 23.0) and near-infrared (NIR) photometry (JHK) to K ~ 16 were obtained for several fields within the cluster. Spectra covering the 6000–8000 A region at a resolution of R ~ 3000 (adequate for the determination of Li I λ6708 line strengths) were also acquired for ~200 pre-main-sequence (PMS) candidates with GMOS. Ages and masses for the Hα emitters in NGC 2362 were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli, as well as those of Baraffe et al. An estimated cluster age of ~1.8 Myr follows from the models of D'Antona & Mazzitelli and 3.5–5.0 Myr from those of Baraffe et al. The fraction of the T Tauri star (TTS) population that is composed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 for the TTS population of NGC 2264. On the basis of W(Hα) alone, the fraction of TTSs still undergoing accretion is 5%–9%, comparable to the inner disk fraction determined from JHKL-band excesses by Haisch and coworkers (12%). Approximately 15% of the PMS sample in this study exhibits possible NIR excess, having EH-K > 0.1 mag. Given the lack of NIR excess and strong Hα emission from the majority of cluster members, it is inferred that the inner disk regions of the TTS population have dissipated significantly. The mean level of chromospheric activity among the WTTS population of NGC 2362 is log(LHα/Lbol) = -3.65, significantly greater than that of the low-mass population of the 600 Myr old Hyades cluster, log(LHα/Lbol) = -3.90. The total mass of the Hα emitters and the OB stellar population of NGC 2362 defines a lower limit for the cluster mass of ~300 M⊙. Allowance for A- and F-type stars still on the radiative track, multiplicity, outlying members, and the low-mass population lying below the completeness limit of the Hα emission survey increases this lower limit to well over 500 M⊙. The derived relaxation, disruption, and evaporation timescales for the cluster imply that NGC 2362 will likely survive beyond the age of the Pleiades, but statistics of galactic cluster lifetimes favor its disruption well before the age of the Hyades.

On the incidence of chemically peculiar stars in the Large Magellanic Cloud

With the aim to corroborate the result of a search for chemically peculiar stars in the Large Magellanic Cloud (LMC) we present measurements obtained from CCD-imaging of two fields, one containing a young open cluster (NGC 1711). While for the latter field, including its surrounding we obtain a contribution of three percent of chemically peculiar stars detectable by Delta a photometry (i.e. the magnetic objects of this group), the second field yields about half of this value in good accordance with the finding for NGC 1866 (Maitzen et al., 2001, A&A, 371, L5) the surrounding field of which has been found to exhibit a very low value of such stars - 0.3%. Thus we are faced with the fact, that our incipient impression about a substantially lower appearance of magnetic chemically peculiar stars in the LMC as compared to the Galaxy continues to be valid. Most of the photometrically identified peculiar stars (from their historical origin denominated Ap-stars) are located in the domain of the B-type stars. But this is a selection effect due to the limiting magnitude of our observing conditions impeding the observation of fainter main sequence stars. In addition to objects showing up as positive deviators in Delta a photometry we also discuss nine stars which appear opposite the main line of normal stars, hence are negative deviators. For most of them the interpretation as emission stars of B-type seems to be appropriate. The statistically relevant number of observations obtained so far in the LMC supports the view that the formation of magnetic peculiar stars has occurred there at a significantly lower rate.

The T Tauri Star Population of the Young Cluster NGC 2264

An Hα emission survey of the young cluster NGC 2264 in the Mon OB1 association resulted in the detection of 490 Hα emission stars in a 25' × 40' field approximately centered between the O7 V multiple star S Mon and the Cone Nebula. The survey was carried out with the wide-field grism spectrograph (WFGS) on the University of Hawaii 2.2 m telescope on Mauna Kea. X-ray observations made with the European Photon Imaging Camera on board the European Space Agency's XMM-Newton satellite observatory will be discussed in a subsequent paper. Optical (BVRCIC) photometry was obtained for selected fields to supplement similar data from the literature. Spectra covering the 6000–8000 A region at a resolution of R ~ 3000 (adequate for the determination of Li I λ6708 line strengths) were obtained for 150 Hα and X-ray emission sources with the Gemini Multi-Object Spectrograph. Near-infrared spectra (1–2.5 μm) of a number of T Tauri stars (TTSs), X-ray sources, and LHα25 (W90) were also obtained using SpeX on the Infrared Telescope Facility. Ages and masses for the Hα emitters were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli. The median age for the TTS population is about 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, exists for individual objects. Several fields in the cluster were observed with the WFGS on more than one occasion, permitting an examination of Hα variability over long baselines in time. About 90% of the classical T Tauri stars (CTTSs) showed changes in W(Hα) of at least 10%, while 57% varied at levels of 50% or more. No evidence was found for a significant pool of dormant Hα emitters. Summing the masses of the TTSs and the OB stellar population of NGC 2264, a lower limit for the total stellar mass content of the cluster is about 430 M⊙. This is less than 1% of the total mass of the atomic and molecular gas believed to be associated with NGC 2264. Evidence for hierarchical structure within the cluster is suggested by the spatial distribution of TTSs. Four concentrations of Hα emitters are evident: two near S Mon and two near the Cone Nebula. The median age of the TTSs in the immediate vicinity of S Mon was found to be greater than that of the TTSs near Allen's infrared source (IRS-1), but a significant dispersion is present. From the rotational data of Lamm et al. and Makidon et al., 241 of the TTSs are periodic variables, 150 weak-line T Tauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregular variables (30 WTTSs and 93 CTTSs). A weak-to-moderate positive correlation is found between H - K color and Prot for the CTTSs, in the sense that stars having longer periods tend to have larger H - K colors. A similar positive correlation is found between LHα and Prot among the CTTSs. No statistically significant correlation is found between Prot and theoretical age or between Prot and LX. Other topics discussed include the fraction of Hα emitters that are WTTSs, f(WTTS) = N(WTTS)/N(TTS), for clusters of different ages; the relative detectability of Hα emission using WFGS and narrowband filter imaging techniques; and the correlation of W(Li I) with Te, age, H - K color, and W(Hα).

New variable stars among or near to H‐alpha emission stars

AbstractThis paper is dealing with the discovery of 11 new or new suspected variables, found as a by‐product during the preparation of our Catalogue of Stars in the Northern Milky Way Having H‐alpha in Emission. We give: (1) The coordinates of the variables and of the respective comparison stars; (2) the maximum and the minimum brightnesses of the observed m(pg), which were determined by visual estimation using the brightnesses in Catalogue USNO A2.0; (3) finding charts of the variables together with their comparison stars used (in electronic form only). (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Deep Objective Prism Survey for Classical T Tauri Stars in the σ Orionis Region

ABSTRACTA very deep 1976 CTIO Curtis‐Schmidt objective prism plate of the σ Orionis association taken under excellent conditions has permitted us to search for fainter classical T Tauri stars associated with this cluster. We have discovered 63 Hα emission stars in the area to a limiting magnitude of about 18. Most of these stars are new additions to the growing list of pre–main‐sequence objects identified in this very dense, very young cluster. Since T Tauri stars vary in light and emission‐line strength on all timescales, these data also provide an historical record of the association. By comparing these with later observations, we are able to estimate the average space density of the Hα emission‐line stars in the surveyed area as between 0.52 and 0.86 pc−3 down to a spectral class of dM0. This is about a third of the number of dwarf or subgiant stars in the region that could have been detected as emission‐line stars in these surveys.

Search for HH Objects and Emission Stars in Star Formation Regions. III. PMS Stars in NGC 7129

Slitless spectroscopy is used to discover 22 emission stars in the central and northwest regions of the cluster located in the NGC 7129 nebula. 16 of them are found for the first time. This sample is essentially complete up to V ≤20.0. The emission stars are distributed nonuniformly over the field of the cluster and are concentrated toward its center. Photometry in the V, R, and I bands is conducted on more than a hundred stars in the cluster. This yields an average absorption coefficient AV=1.7±0.27 for this region. Based on their positions in color diagrams for the optical and near IR ranges, most of the emission stars can be regarded as T Tau objects.

Searches for HH-objects and Emission Stars in Star Formation Regions. II. The Region of GM1-61 and V453 Ori

Observations of objects lying in the dark cloud L1582A, which contains the cometary nebulae GM1-61 and V453 Ori, are reported. Five previously unknown HH-objects are discovered in this region; they appear to form several flows. The morphology of the nebulae is examined and possible sources of the HH-flows are discussed. Another reflecting nebula is found that is visible mainly in the infrared.

Deep Fading of the New Herbig Be Star MisV1147

Abstract We report on the discovery of a new variable star, MisV1147, by the MISAO Project and its unusual photometric variations. The light curve of the object is characterized by two distinct states, that is, a relatively calm, bright state and an active state during which repetitive fadings with large amplitudes of $\sim 2 \,\mathrm{mag}$ were observed. The fading events were associated with optical reddening. The object is identified with a catalogued $\mathrm{H}\alpha$ emission star, which has been confirmed by our spectroscopic observations. The $\mathrm{H}\alpha$ emission line is narrow and symmetric both in the bright and faint phases. The object has infrared colors analogous to T Tau stars or Herbig Ae/Be stars. In conjunction with the detection of a NaD absorption line and its position close to Cep OB1, we conclude that MisV1147 is a Herbig Be star with its spectral type of B2. Its optical-infrared spectral energy distribution is typical for Herbig Ae/Be stars, which supports this classification. The overall properties of the light curve are reminiscent of a UX Ori sub-group, showing Algol-type minima. The most noteworthy feature of MisV1147 is that it has a spectral type B2, whereas known UX Ori stars have spectral types later than B8. The object, furthermore, has one of the largest amplitudes of variations among known Herbig Ae/Be stars. Hence the object breaks the very strong correlation between the variation amplitude and the spectral type. This implies that dust clumps in circumstellar matter can significantly obscure a central object even in the early-type system.

Two new bright Ae stars

Two newly identified Ae stars, nu Cyg and kappa UMa, were discovered in the course of the Magnetic Survey of Bright MS stars (Monin et al. 2002). We pre sent their Halpha profiles along with measurements of their equivalent width and parameters of emission features. Emission in the Halpha line of nu Cyg is variable on a time scale of 3 years. kappa UMa exhibits weak emission which is rather stable. The emission is thought to arise from a circumstellar disk, and we have estimated the size of that disk.Both new emission stars are IRAS sources. Their IR color excesses are consistent with those of classical Ae stars. Thus, nu Cyg and kappa UMa appear not to belong to the class of Herbig Ae/Be stars. We argue that the frequency of Ae stars may be underestimated due to the difficulty of detection of weak emission in some A stars.