Hα Emission Strength Research Articles

Disentangling the two sub-populations of early Herbig Be stars using VLT/X-shooter spectra

Context. Early Herbig Be (HBe) stars are massive, young stars accreting through the boundary layer mechanism. However, given the rapid (<2 Myr) evolution of early Herbig stars to the main-sequence phase, studying the evolution of the circumstellar medium around these stars can be a cumbersome exercise. Aims. In this work, we study the sample of early (B0–B5) HBe stars using the correlation between Hα emission strength and near–infrared excess, complemented by the analysis of various emission features in the X-shooter spectra. Methods. We segregate the sample of 37 early HBe stars based on the median values of Hα equivalent width (EW) and near–infrared index (n(J–H)) distributions. The stars with |Hα EW| > 50 Å and n(J–H) > −2 are classified as intense HBe stars and stars with |Hα EW| < 50 Å and n(J–H) < −2 as weak HBe stars. Using the VLT/X–shooter spectra of five intense and eight weak HBe stars, we visually checked for the differences in intensity and profiles of various HI and metallic emission lines commonly observed in Herbig stars. Results. We propose that the intense HBe stars possess an inner disk close to the star (as apparent from the high near-infrared excess) and an active circumstellar environment (as seen from the high Hα EW value and presence of emission lines belonging to FeII, CaII, OI, and [OI]). However, for weak HBe stars, the inner disk has cleared, and the circumstellar environment appears more evolved than for intense HBe stars. Furthermore, we compiled a sample of ~58 000 emission-line stars published in Gaia DR3 to identify more intense HBe candidates. Further spectroscopic studies of these candidates will help us to understand the evolution of the inner (approximately a few au) disk in early HBe stars.

Optical Spectroscopy of Classical Be Stars in Old Open Clusters

We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100 Myr. Ours is the first spectroscopic study of classical Be stars in open clusters older than 100 Myr. We found that the Hα emission strength of most of the stars is less than 40 Å, in agreement with previous studies. Our analysis further suggests that one of the stars, [KW97] 35–12, might be a weak Hα emitter in nature, showing Hα equivalent width of −0.5 Å. Interestingly, we also found that the newly detected classical Be star LS III +47 37b might be a component of the possible visual binary system LS III +47 37, where the other companion is also a classical Be star. Hence, the present study indicates the possible detection of a binary Be system. Moreover, it is observed that all 16 stars exhibit a lesser number of emission lines compared to classical Be stars younger than 100 Myr. Furthermore, the spectral type distribution analysis of B-type and classical Be stars for the selected clusters points out that the existence of CBe stars can depend on the spectral type distribution of B-type stars present in these clusters.

Stellar Wind Variations during the X‐Ray High and Low States of Cygnus X‐11,2

We present results from Hubble Space Telescope ultraviolet spectroscopy of the massive X-ray and black hole binary system, HD 226868 = Cyg X-1. The spectra were obtained at both orbital conjunction phases in 2002 and 2003, when the system was in the X-ray high/soft state. The UV stellar wind lines suffer large reductions in absorption strength when the black hole is in the foreground due to the X-ray ionization of the wind ions. We constructed model UV wind line profiles assuming that X-ray ionization occurs everywhere in the wind except the zone where the supergiant blocks the X-ray flux. The good match between the observed and model profiles indicates that the wind ionization extends to near the hemisphere of the supergiant facing the X-ray source. We also present contemporaneous spectroscopy of the Hα emission that forms in the high-density gas at the base of the supergiant's wind and the He II λ4686 emission that originates in the dense, focused wind gas between the stars. The Hα emission strength is generally lower in the high/soft state than in the low/hard state, but the He II λ4686 emission is relatively constant between X-ray states. The results suggest that mass transfer in Cyg X-1 is dominated by the focused wind flow that peaks along the axis joining the stars, and that the stellar wind contribution from the remainder of the hemisphere facing the X-ray source is shut down by X-ray photoionization effects (in both X-ray states).

X-Ray Observations of the Young Cluster NGC 2264

We present results from an X-ray imaging survey of the young cluster NGC 2264, carried out with the European Photon Imaging Cameras (EPIC) on board the XMM-Newton spacecraft. The X-ray data are merged with extant optical and near-infrared photometry, spectral classifications, Hα emission strengths, and rotation periods to examine the interrelationships between coronal and chromospheric activity, rotation, stellar mass, and internal structure for a statistically significant sample of pre-main-sequence stars. A total of 300 distinct X-ray sources can be identified with optical or near-infrared counterparts. The sources are concentrated within three regions of the cluster: in the vicinity of S Mon, within the large emission/reflection nebulosity southwest of S Mon, and along the broad ridge of molecular gas that extends from the Cone Nebula to the NGC 2264 IRS 2 field. From the extinction-corrected color-magnitude diagram of the cluster, ages and masses for the optically identified X-ray sources are derived. A median age of ∼2.5 Myr and an apparent age dispersion of ∼5 Myr are suggested by pre-main-sequence evolutionary models. The X-ray luminosity of the detected sources appears well correlated with bolometric luminosity, although there is considerable scatter in the relationship. Stellar mass contributes significantly to this dispersion, while isochronal age and rotation do not. X-ray luminosity and mass are well correlated such that LX ∝ (M/M⊙)1.5, similar to the relationship found within the younger Orion Nebula Cluster. No strong evidence is found for a correlation between EH-K, the near-infrared color excess, and the fractional X-ray luminosity, which suggests that optically thick dust disks have little direct influence on the observed X-ray activity levels. Among the X-ray-detected weak-line T Tauri stars, the fractional X-ray luminosity, LX/Lbol, is moderately well correlated with the fractional Hα luminosity, LHα/Lbol, but only at the 2 σ level of significance. The cumulative distribution functions for the X-ray luminosities of the X-ray-detected classical and weak-line T Tauri stars within the cluster are comparable, assuming the demarcation between the two classes is at an Hα equivalent width of 10 Å. However, if the nondetections in X-rays for the entire sample of Hα emitters known within the cluster are taken into account, then the cumulative distribution functions of these two groups are clearly different, such that classical T Tauri stars are underdetected by at least a factor of 2 relative to the weak-line T Tauri stars. Examining a small subsample of X-ray-detected stars that are probable accretors based on the presence of strong Hα emission and near-infrared excess, we conclude that definitive nonaccretors are ∼1.6 times more X-ray luminous than their accreting counterparts. In agreement with earlier published findings for the Orion Nebula Cluster, we find a slight positive correlation (valid at the 2 σ confidence level) between LX/Lbol and rotation period in NGC 2264. The lack of a strong anticorrelation between X-ray activity and rotation period in the stellar population of NGC 2264 suggests that either the deeply convective T Tauri stars are rotationally saturated or the physical mechanism responsible for generating magnetic fields in pre-main-sequence stars is distinct from the one that operates in evolved main-sequence stars.

The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars

We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the FeH band near 1 μm, we categorize the integrated surface magnetic flux as low (well under 1 kG), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) for a set of more than 20 stars ranging from M2 down to M9. Along with the field, we also measure the rotational broadening (v sin i) and Hα emission strength. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M dwarfs. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measurable, and the strongest fields are associated with with the most rapid rotators. Interestingly, these very cool rapid rotators appear to have the largest magnetic flux in the whole sample (greater than in the classical dMe stars). Hα emission is found to be a good proxy for magnetic fields, although the relation between the fractional emission and the magnetic flux varies with effective temperature. The drop-off in this fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and activity in cool and fully convective objects.

GQ Lup B Visible and Near-Infrared Photometric Analysis

We have reanalyzed archival HST and Subaru data of the recently discovered planetary mass companion (PMC) GQ Lup B. With these we produce the first R- and I-band photometry of the companion and fit a radius and effective temperature using detailed model atmospheres. We find an effective temperature of 2335 ± 100 K, a radius of 0.38 ± 0.05 R☉, and a luminosity of log(L/L☉) = -2.42 ± 0.07 (at 140 pc). Since we fit wavelengths that span most of the emitted radiation from GQ Lup, this luminosity estimate is robust, with uncertainty dominated by the distance uncertainty (±50 pc). The radius obtained for 140 pc (0.38 R☉) is significantly larger than the one originally derived and larger than model predictions. The mass of the object is much more model-dependent than the radiative properties, but for the Gaia dusty models we find a mass between 10MJup and 20MJup, in the range of the brown dwarf and PMC deuterium-burning boundary. Assuming a distance of 140 pc, observations fit to 1 σ the Baraffe evolution model for an ~15MJup brown dwarf. Additionally, the F606W photometric band is significantly overluminous compared to model predictions and other brown dwarfs. Such overluminosity could be explained by a bright Hα emission from chromospheric activity, interaction with another undetected companion, or accretion. Assuming that GQ Lup B has a bright Hα emission line, its Hα emission strength is 10-1.71±0.10Lbol, significantly larger than field late-type dwarfs. GQ Lup B might be strongly accreting and might still be in its formation phase.

Edge-on T Tauri stars

Using the U echelle spectrograph at the ESO VLT we obtained two-dimensional high-resolution (R = 50 000) spectra of the edge-on disk objects HH30 ∗ , HK Tau B, and HV Tau C. For comparison purposes we also observed with the same equipment both the classical T Tauri star HL Tau and the active late-type star LDN 1551-9. The spectra of all three observed edge-on disks consist of a T Tauri emission and absorption line spectrum with superimposed jet emission lines. Analysis of the spectra confirmed that the disks are completely opaque at visible wavelengths and that light from the central objects reaches us only via scattering layers above and below the disk planes. The central objects of our targets were found to be normal T Tauri stars showing moderate but different amounts of veiling of their photospheric spectra, indicating different accretion rates or evolutionary stages. We suggest that all classical T Tauri stars (CTTSs) show this observed morphology when viewed edge-on. Part of the jet emission from edge-on systems is directly visible to us in the forbidden lines as well as in Hα and He , a finding which contradicts the present paradigm of a pure magnetospheric accretion origin for the formation of hydrogen and helium emission lines in moderately active CTTSs. From a comparison with those Taurus-Auriga CTTSs for which the inclination is reliably known, we conclude that the view angle of CTTS systems is one of the key parameters governing apparent Hα emission strength in the T Tauri class. We discuss the various possible formation regions for the Na  D lines and show that profiles similar to observed ones can be formed at the base of the disk wind.

Parallaxes of Brown Dwarfs at USNO

Trigonometric parallaxes have been measured by Dahn et al. (2002) for 28 cool dwarfs and brown dwarfs, including 17 L dwarfs and three T dwarfs. Broadband CCD and near-IR photometry (VRIz*JHK) have been obtained for these objects and for 24 additional late-type dwarfs. These data have been supplemented with astrometry and photometry from the literature, including parallaxes for the brighter companions of ten L and two T dwarfs. The absolute magnitudes and colors are reviewed here. The I - J color and the spectral type are both good predictors of absolute magnitude for late-M and L dwarfs. MJ becomes monotonically fainter with I - J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. In contrast, the J - K color correlates poorly with absolute magnitude for L dwarfs. Using several other parameters from the literature (Li detection, Hα emission strength, projected rotation velocity, and tangential velocity), we fail to uncover any measurable parameter that correlates with the anomalous J - K color.

Activity and Kinematics of Late M, L, and T Dwarfs

The kinematics of late M and certainly L dwarfs indicate that they have a mean age of no more than a few Gyr. This result does not rule out a mix of young brown dwarfs and ZAMS stars. Relative Hα emission strengths begin to decline in the late M dwarfs, even though about 100% of the M7-8 dwarfs show the weaker emission. Both the frequency and likely strength of this emission declines with spectral type among the L's and T's. A larger sample of late M and L dwarfs, which may provide more definitive tests of the kinematic and activity properties, is being assembled from our NStars project.Two unusual objects, one an M9.5e and the other a T6e, show persistent Hα emission at least an order of magnitude stronger than their counterparts of similar spectral type. If these are hypothesized to be very young stellar objects (≲10 Myr) like some observed in Rho Oph, Sigma Ori and Taurus – evolutionary models would require them to have very low masses: In particular, for an entity to fade to a late T dwarf temperature and luminosity within 10 Myr, its mass should be no more than several Jupiters. This seems implausible for an isolated object not near any obvious site of recent star formation. However, our attempt to show that the emission might be due to accretion in a binary system was unsuccessful.

H [CSC]ii[/CSC] Emission from a Complete Spectroscopic Survey of B[CLC]e[/CLC] Stars in [CLC]h[/CLC] and χ Persei

We describe data for emission-line B stars observed in a spectroscopic survey of h and χ Persei. The survey is complete to V = 12.5 and covers an area of ~1100 arcmin2 roughly centered on the two clusters. We detect 32 Be stars; some have low Hα emission strength. Seven of these are new identifications; seven others are confirmations of Be stars previously identified using photometry. Five of the observed Be stars show significant Hα profile variations from epoch to epoch. We show that spectral indices yield physical characteristics of the H II emission region. This automatic method is robust and easily applied to large spectroscopic samples. We infer Hα : Hβ flux ratios of 2–5 and observe a linear relationship between Hα emission and J-K color for these stars. We include a Hertzsprung-Russell diagram for the B-type stars in the clusters.

ITAL]UBVI[/ITAL] and Hα Photometry of the [CLC][ITAL]h[/ITAL][/CLC] and χ Persei Cluster

UBVI and Hα photometry is presented for 17319 stars in vicinity of the young double cluster h and χ Persei. Our photometry extends over a 37' × 1° field centered on the association. We construct reddening contours within the imaged field. We find that the two clusters share a common distance modulus of 11.75 ± 0.05 and ages of log age(yr) = 7.1 ± 0.1. From the V-Hα color, a measure of the Hα emission strength, we conduct a survey for emission line objects within the association. We detect a sample of 33 Be stars, eight of which are new detections. We present a scenario of evolutionary enhancement of the Be phenomenon to account for the peak in Be fraction toward the top of the main sequence in the population of h and χ Persei and similar young clusters.

The Development and Behavior of an Active Region on/near the Photosphere of the B2[CLC]e[/CLC] Star μ Centauri

An unusual mass-loss event observed in the B2e star μ Centauri during the course of 5 days in 1994 April is described and discussed within the framework of contemporary ideas on the Be phenomenon. The onset of the activity occurred in less than 1 day and was characterized by variable emission in He I λ6678 that displayed a distinctive character. Unlike the transient microemission in He I that frequently occurs in μ Cen and other Be stars, the emission-line variations seen in this event took place more slowly in three discrete velocity intervals. On two occasions, violet (v)- and red (r)-shifted emission components declined on a timescale of 2 hr, while the emission at/near the line center increased. The short timescale and observed velocity behavior suggest that the site of the activity was near the photosphere. The possible importance of nonradial pulsations and magnetic fields in precipitating the event is discussed. A scenario is suggested to explain the observations in which material originating from an active site on the photosphere is injected into a slab. Layers in the active region become visible in He I λ6678 as the prevailing density builds to values favorable for the production of this emission line. It is estimated that the slab covered ~30% of the star. A 22% increase in the Hα emission strength by the final day of the observations indicates that the activity did indeed add material to the circumstellar disk.

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).

Mass motions in the atmospheres of red giants in the globular clusters M55 and M13

We present Na D and Hα stellar line profiles for a sample of 19 stars in M55 and M13, in a search for spectral features indicative of mass motions in the atmospheres of red giants. We combine the observations of the stars in these two clusters with those made previously for stars in ω Cen, M22 and M4 to give a total sample of 63 globular cluster red giants. From this total sample we deduce that only stars with a luminosity above log(L/L⊙)≈2.8 show Hα emission. The luminosity limit for the onset of significant Na D coreshift is log(L/L⊙)≈2.9, while that for significant Hα coreshifts is ≈0.4 dex lower at log(L/L⊙)≈2.5, and this therefore represents the threshold for the first detectable mass-motion phenomena as the star evolves up the giant branch. By comparing our results with those in the literature, time variation of the strength of Hα emission is observed for two stars, while five stars display variations in Na D coreshifts over periods greater than six months.

Membership of low-mass stars in the open cluster Alpha Persei

The results of a combined astrometric, photometric, and spectroscopic search for low-mass members in the intermediate-age open cluster α Persei are presented. Over 130 low-mass new members have been identified to M V ⇒ 12.5, almost doubling the previous number of known members. The new membership information suggests a slight upward revision of α Per's age to ∼8 × 10 7 yr. α Per is noticebly spatially elongated in a direction parallel to the galactic plane, most likely due to tidal deformation. Analysis of the distribution of relative Hα emission strengths among α Per members confirms the view that the mean Hα strength decreases in increasingly older systems. Allowing for survey incompleteness, one cannot reject the idea that the luminosity function is consistent with the field luminosity function to M v ⇒ 9 or 10

The search for faint members of the Pleiades. II - Colors, spectral types, and H-alpha emission line strengths for M dwarfs in the Pleiades, Hyades, and Gliese field

view Abstract Citations (58) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Search for Faint Members of the Pleiades. II. Colors, Spectral Types, and H-alpha Emission Line Strengths for M Dwarfs in the Pleiades, Hyades, and Gliese Field Prosser, Charles F. ; Stauffer, John ; Kraft, Robert P. Abstract Photoelectric and CCD-based BVRI photometry has been obtained for nearly 100 proper-motion select- ed Pleiades candidates members (Paper I) extending to V ≅ 18. More than three quarters of these stars prove o have positions in a V vs V - I diagram compatible with cluster membership. Low-resolution (∼5 Å) spectra centered on Hα have been obtained for 77 of these proper motion candidates and known (H ii) members of the Pleiades. Additional spectra of K and M dwarf comparison stars, drawn from the Hyades, the Gliese catalog, and MK standard system, were also observed. We derive a relationship between (R - 1)K (Kron system) and spectral type based on the MK system, from which we obtain a mean Pleiades color excess <E(R-I)K> = +0.040±0.008, based on 29 M and late-K dwarfs. This is in excellent agreement with previous determinations based on the colors of early type stars. A comparison of the Hα emission strength for the Pleiades M dwarfs with Hyades and field M dwarfs shows that the mean Hα emission strength decreases by nearly a factor of 2 with increasing stellar age. At a given temperature, there appears to be a 10%-20% difference in maximum Hα emission between the Hyades and Pleiades M dwarfs. With a factor of 10 difference in age between the two clusters, the small difference in peak emission suggests that some physical mechanism acts to limit the maximum Hα activity. Within each group, however, there is a wide range of emission line strengths, and even in the Pleiades a few early M dwarfs have quite weak Hα emission. In a CCD-based spectroscopic Hα survey of the Pleiades field at a spectral resolution of ∼5 Å, we estimate that all but 10%-20% of the proper-motion selected sample of dwarfs having spectral types between MO and MS would be recovered. Publication: The Astronomical Journal Pub Date: April 1991 DOI: 10.1086/115772 Bibcode: 1991AJ....101.1361P Keywords: Dwarf Stars; Emission Spectra; H Alpha Line; M Stars; Open Clusters; Stellar Color; Stellar Spectra; Charge Coupled Devices; Flare Stars; Interstellar Extinction; K Stars; Stellar Spectrophotometry; Astrophysics; CLUSTERS: OPEN; STARS: RED DWARFS; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (134) GCPD (1) Related Materials (1) Part 1: 1991AJ....101..980S

Solar-type stars in Cepheus OB 2: H? emission stars and IRAS point sources

A survey for emission line stars was carried out in 1980 with the Schmidt telescope of Konkoly Observatory in the region of IC 1396 (Kun, 1986a, hereafter referred to as Paper I). This work was aimed at a preliminary mapping of the probable medium mass members of the association Cepheus OB 2. The stars of the region have been followed since then with the same telescope in order to obtain a more complete view on the stellar content of this giant star-forming region. The common variability of the Hα emission strength makes the sense of repeating such surveys. The new survey resulted in the discovery of 65 further emission stars. Equatorial coordinates and finding charts are given for them.

Rapid Variation of Shell Line Parameters in ζ Tau

ζTau is a Be star which probably showed already in 1973 rapid variations in Hα emission strength with time scales of a few minutes (Bahng, 1976). It represents, moreover, the primary of a well-known binary system with a period of 132.91 days (e.g., Hynek and Struve, 1942), and its shell displays long term instabilities with time scales of some years (Delplace and Chambon, 1976). The basis of the present work is a compact set of 82 grating photographic spectrograms obtained at Merate by means of the 137 cm reflector with an inverse dispersion of about 35 Å/mm between Jan 17 and Jan 24, 1983. Forty four of these spectra cover the range between ˜ 4000 and ˜ 5000 Å, the other ones being centered on Hα.

Rotation and H? emission activity in F8-G3 dwarfs

A study of the dependence of chromospheric Hα emission strength on rotation period in F8-G3 dwarfs is made from the published data. The emission strength shows strong correlation with rotation irrespective of age and spectral type. Furthermore, the measurement of Hα emission in late-type dwarfs can be used for the determination of rotation period.

A SEARCH FOR SHORT-TIME-SCALE VARIATIONS IN H ALF EMISSION IN THE SPECTRA OF FOUR BRIGHT BE STARS.

ABSTRACT Simultaneous Hα observations of four bright Be stars with a photoelectric scanner and an interference filter photometer during seven consecutive nights in May 1978 show (1) no evidence for real changes larger than about 5% in Hα emission strengths during any given night; (2) possible night-to-night changes for two of the stars, though the evidence is uncertain; and (3) definite changes on time scales of months for all four stars.