Pleiades Members Research Articles

New Brown Dwarfs in the Pleiades Cluster

We present intermediate- and low-resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20>I≥17.8, I-Z≥ 0.5) discovered in a CCD-based IZ survey covering an area of 1 deg2 in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs having spectral types in the range M6-M9. Five out of the seven objects can be considered Pleiades members on the basis of their radial velocities, Hα emissions, and other gravity-sensitive atomic features like the Na I doublet at 818.3 and 819.5 nm. According to current evolutionary models, the masses of these new objects range from roughly 80 MJup for the hottest in the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed member. These observations prove that the cloud fragmentation process extends well into the brown dwarf realm, suggesting a rise in the initial mass function below the substellar limit.

Spectral and Temporal Characteristics of X-Ray--bright Stars in the Pleiades

We follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond &amp; Smith (1977) one-and two-temperature models have been used to fit the position-sensitive proportional counter (PS PC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L<SUB>X</SUB>/L<SUB>bol</SUB>. A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of ∼10<SUP>30</SUP> ergs s<SUP>-1</SUP>. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of ∼18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.

Radial velocities of very low mass stars and candidate brown dwarf members of the Hyades and Pleiades, 2

view Abstract Citations (35) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radial Velocities of Very Low Mass Stars and Candidate Brown Dwarf Members of the Hyades and Pleiades. II. Stauffer, John R. ; Liebert, James ; Giampapa, Mark Abstract We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km/s for approximately 20 candidate very low mass members of the Pleiades cluster and for a few proposed very low mass members of the Hyades. Most of the Pleiades targets were selected from the recent Hambly, Hawkins, and Jameson proper motion survey, where they were identified as probable Pleiades brown dwarfs with an age spread from 3 to 70 Myr. Our spectroscopic data and a reinterpretation of the photometric data confirm that these objects are indeed likely Pleiades members; however, we believe that they more likely have masses slightly above the hydrogen burning mass limit and that there is no firm evidence for an age spread amongst these stars. All of the very low mass Pleiades and Hyades members show H alpha in emission. However, the ratio of H alpha flux to biometric flux in the Pleiades shows a maximum near MBol approximately equal to 9.5 (M approximately equal to 0.3 solar mass) and a sharp decrease to lower masses. This break occurs at the approximate mass where low mass stars are expected to become fully convective, and it is tempting to assume that the decrease in H alpha flux is caused by some change in the behavior of stellar dynamos at this mass. We do not see a similar break in activity at this mass in the Hyades. We discuss possible evolutionary explanations for this difference in the H alpha activity between the two clusters. Publication: The Astronomical Journal Pub Date: January 1995 DOI: 10.1086/117273 Bibcode: 1995AJ....109..298S Keywords: Brown Dwarf Stars; H Alpha Line; Pleiades Cluster; Radial Velocity; Stellar Mass; Stellar Evolution; Stellar Spectrophotometry; Astrophysics; OPEN CLUSTERS AND ASSOCIATIONS: GENERAL; STARS: LOW MASS; STARS: BROWN DWARFS full text sources ADS | data products SIMBAD (35)

A CCD-based search for very low mass members of the Pleiades cluster

We have obtained deep charge coupled device (CCD)V and I images of a number of fields near the center of the Pleiades open cluster. We have also obtained imaging data for Praesepe, a very similar cluster in terms of distance and richness but nearly 10 times older than the Pleiades. Because brown dwarfs are predicted to become much fainter and cooler between Pleiades and Praesepe ages, this provides a powerful differential technique for placing constraints on the brown dwarf population in open clusters. Combined with our previously reported observations, we now have data for about 0.4 sq deg in the Pleiades, corresponding roughly to 5% of the area of that cluster. We have searched the new CCD frames for additional Pleiades brown dwarf candidates. Two possible candidates are present, the faintest of which has V approximately equal to 22.5, (V-I)(sub K) approximately equal to 4.6. Because we do not have proper motion data and the colors of these objects are not redder than the reddest known field stars, it is possible that some or all of our candidates are somewhat higher mass field stars rather than Pleiades-age brown dwarfs. Even if all six of the proposed brown dwarf candidates in our 0.4 sq deg field are Pleiades members, the relatively small number found suggests that low mass stars or brown dwarfs do not contribute significantly to the total mass of the cluster.

A deep imaging survey of the Pleiades with ROSAT

view Abstract Citations (276) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Deep Imaging Survey of the Pleiades with ROSAT Stauffer, J. R. ; Caillault, J. -P. ; Gagne, M. ; Prosser, C. F. ; Hartmann, L. W. Abstract We have obtained deep ROSAT images of three regions within the Pleiades open cluster. We have detected 317 X-ray sources in these ROSAT PSPC images, 171 of which we associate with certain or probable members of the Pleiades cluster. We detect nearly all Pleiades members with spectral types later than G0 and within 25 arcminutes of our three field centers where our sensitivity is highest. This has allowed us to derive for the first time the luminosity function for the G, K, and M dwarfs of an open cluster without the need to use statistical techniques to account for the presence of upper limits in the data sample. Because of our high X-ray detection frequency down to the faint limit of the optical catalog, we suspect that some of our unidentified X-ray sources are previously unknown, very low-mass members of the Pleiades. A large fraction of the Pleiades members detected with ROSAT have published rotational velocities. Plots of L_x_/L_Bol_ versus spectroscopic rotational velocity show tightly correlated "saturation" type relations for stars with (B - V)_0_ >= 0.60. For each of several color ranges, X-ray luminosities rise rapidly with increasing rotation rate until v sin i = 15 km s^-1^ and then remain essentially flat for rotation rates up to at least v sin i ~ 100 km s^-1^. The dispersion in rotation among low-mass stars in the Pleiades is by far the dominant contributor to the dispersion in L_x_ at a given mass. Only about 35% of the B, A, and early F stars in the Pleiades are detected as X-ray sources in our survey. There is no correlation between X-ray flux and rotation for these stars. The X-ray luminosity function for the early-type Pleiades stars appears to be bimodal-with only a few exceptions, we either detect these stars at fluxes in the range found for low-mass stars or we derive X-ray limits below the level found for most Pleiades dwarfs. The X-ray spectra for the early-type Pleiades stars detected by ROSAT are indistinguishable from the spectra of the low-mass Pleiades members. We believe that the simplest explanation for this behavior is that the early-type Pleiades stars are not themselves intrinsic X-ray sources and that the X-ray emission actually arises from low-mass companions to these stars. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1994 DOI: 10.1086/191951 Bibcode: 1994ApJS...91..625S Keywords: A Stars; B Stars; Dwarf Stars; F Stars; G Stars; K Stars; M Stars; Pleiades Cluster; Sky Surveys (Astronomy); X Ray Astronomy; X Ray Imagery; X Ray Sources; Error Analysis; Heao 2; Rosat Mission; Stellar Luminosity; Stellar Mass; Stellar Models; Stellar Rotation; X Ray Spectroscopy; Astronomy full text sources ADS | data products SIMBAD (406) NED (4) CDS (2) HEASARC (1) Related Materials (1) Catalog: 1996yCat..20910625S

PHOTOMETRIC LIGHT CURVES FOR TEN RAPIDLY ROTATING STARS IN ALPHA PERSEI, THE PLEIADES, AND THE FIELD

ABSTRACT We present the results from a photometric monitoring program of 10 rapidly rotating stars observed during 1991 using the Fred Lawrence Whipple Observatory (FLWO) 48-inch telescope. Brightness variations for an additional six cluster stars observed with the Lick 40-inch telescope are also given. The periods and light curves for seven alpha Persei members, two Pleiades members, and one nake T Tauri field star are report. The individual photometric measurements have been deposited with the NSSDC.

The search for faint members of the Pleiades. I - A proper motion membership study of the Pleiades to M(V) of about 12.5

view Abstract Citations (69) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Search for Faint Members of the Pleiades. I. A Proper Motion Membership Study of the Pleiades to MV = 12.5 Stauffer, J. ; Klemola, A. ; Prosser, C. ; Probst, R. Abstract New probable Pleiades cluster members are identified, based on an unbiased survey of stars within 4 x 4-deg region centered near Alcyone. The faint magnitude limit of the survey is about V = 18, and the bright limit is about V = 14. In order to accurately define the luminosity function of the Pleiades, new proper motions are derived for a large number of stars previously suggested as possible Pleiades members. Accurate coordinates, V magnitudes, and membership probabilities are provided for these stars. Publication: The Astronomical Journal Pub Date: March 1991 DOI: 10.1086/115741 Bibcode: 1991AJ....101..980S Keywords: Faint Objects; Pleiades Cluster; Sky Surveys (Astronomy); Star Distribution; Stellar Mass; Stellar Motions; Open Clusters; Star Formation; Stellar Luminosity; Stellar Magnitude; Astrophysics; CLUSTERS: OPEN full text sources ADS | data products SIMBAD (692) Related Materials (1) Part 2: 1991AJ....101.1361P

X-ray studies of coeval star samples. II - The Pleiades cluster as observed with the Einstein Observatory

Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that of Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solarlike Pleiades stars is about two orders of magnitude more intense than for the nearby solarlike stars.

Possible Pleiades members with M of about 0.07 solar mass - Identification of brown dwarf candidates of known age, distance, and metallicity

A small number of very faint very red stars have been discovered in CCD frames taken near the center of the Pleiades cluster. The V, I, and K photometry for these stars is consistent with the expected luminosity and temperatures for brown dwarfs of mass about 0.07 solar mass at the distance and age of the Pleiades. It is concluded that these are the first identified single brown dwarfs of known age, distance, and metallicity.

The distribution of rotational velocities for low-mass stars in the Pleiades

The available spectral type and color data for late-type Pleiades members have been reanalyzed, and new reddening estimates are obtained. New photometry for a small number of stars and a compilation of H-alpha equivalent widths for Pleiades dwarfs are presented. These data are used to examine the location of the rapid rotators in color-magnitude diagrams and the correlation between chromospheric activity and rotation. It is shown that the wide range of angular momenta exhibited by Pleiades K and M dwarfs is not necessarily produced by a combination of main-sequence spin-downs and a large age spread; it can also result from a plausible spread in initial angular momenta, coupled with initial main-sequence spin-down rates that are only weakly dependent on rotation. The new reddening estimates confirm Breger's (1985) finding of large extinctions confined to a small region in the southern portion of the Merope nebula.

A REDDENING-FREE MAIN SEQUENCE FOR THE PLEIADES CLUSTER.

ABSTRACT The effects of differential reddening on the UBV colors of Pleiades members have been removed for stars earlier than (B-V)0 = 1.00m, and the dereddened colors have been found to exhibit an ultraviolet excess of delta(U-B) = +0.03m with respect to Hyades stars. A correction of +0.15m in mV at constant (B-V) has been derived for the Hyades main sequence to adjust it to Pleiades metallicity, and this has been fit to Pleiades late-type dwarfs in order to define a new photometric ZAMS for typical solar neighborhood stars of spectral types A, F, and G. A zero-point of 3.15 was adopted for the Hyades distance modulus based upon a review of available astrometric data, and the corresponding Pleiades distance modulus is 5.56.

Studies of Stellar Rotation. III. a Redetermination of Rotational Velocities in the Pleiades

view Abstract Citations (72) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies of Stellar Rotation. III. a Redetermination of Rotational Velocities in the Pleiades Anderson, Christopher M. ; Stoeckly, Robert ; Kraft, Robert P. Abstract A redetermination of the rotational velocities of fifty-seven Pleiades members down to spectral type F5 V has been made. The function (V sin i for the Pleiades is shown to be always larger than that for the field stars regardless of absolute magnitude. This conclusion appears to support the Abt-llunter hypothesis that high rotation is accompanied by a low frequency of spectroscopic binaries with P < 100 days. Publication: The Astrophysical Journal Pub Date: February 1966 DOI: 10.1086/148512 Bibcode: 1966ApJ...143..299A full text sources ADS | data products SIMBAD (58) Related Materials (5) Part 1: 1965ApJ...142..681K Part 2: 1965ApJ...142.1699K Part 3: 1965ApJ...142..703K Part 5: 1967ApJ...148..129K Part 6: 1967ApJ...150..551K

Studies in Stellar Rotation. III. A Redertimination of Rotational Velocities in the Pleiades.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies in Stellar Rotation. III. A Redertimination of Rotational Velocities in the Pleiades. Anderson, Christopher M. ; Stoeckly, Robert ; Kraft, Robert P. Abstract A redetermination of the rotational velocities of 57 Pleiades members down to spectral type HP5V has been made. The function ~ V sin i> for the Pleiades is shown to be always larger than that for the field stars regardless of absolute magnitude. This conclusion appears to support the Abt-Hunter hypothesis that high rotation is accomplished by a low frequency of spectroscope binaries with F~ 100 days. Publication: The Astronomical Journal Pub Date: February 1966 DOI: 10.1086/109999 Bibcode: 1966AJ.....71Q.154A full text sources ADS |