Persei Cluster Research Articles

Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the α Persei Open Cluster

We have identified twenty-seven candidate very low mass members of the relatively young Alpha Persei open cluster from a six square degree CCD imaging survey. Based on their I magnitudes and the nominal age and distance to the cluster, these objects should have masses less than 0.1 Msunif they are cluster members. We have subsequently obtained intermediate resolution spectra of seventeen of these objects using the Keck II telescope and LRIS spectrograph. We have also obtained near-IR photometry for many of the stars. Our primary goal was to determine the location of the "lithium depletion boundary" and hence to derive a precise age for the cluster. We detect lithium with equivalent widths greater than or equal to 0.4 \AA in five of the program objects. We have constructed a color-magnitude diagram for the faint end of the Alpha Persei main sequence. These data allow us to accurately determine the Alpha Persei single-star lithium depletion boundary at M(I$_C$) = 11.47, M(Bol) = 11.42, (R-I)$_{C0}$ = 2.12, spectral type M6.0. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Alpha Persei cluster of 90 $\pm$ 10 Myr. At this age, the two faintest of our spectroscopically confirmed members should be sub-stellar (i.e., brown dwarfs) according to theoretical models.

The Lithium-Depletion Boundary and the Age of the Young Open Cluster IC 2391

We have obtained new photometry and intermediate resolution ($\Delta \lambda = 2.7$ \AA\ ) spectra of 19 of these objects (14.9 $\le$ $I_c$ $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their $VRI$ photometry, spectral types, radial velocity, and H$\alpha$ emission strengths. Higher S/N spectra were obtained for 8 of these probable cluster members in order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain an estimate of the cluster's age. One of these 8 stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium depletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical model predictions, we derive an age for IC~2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster ($\sim$35 Myr) this result for IC~2391 is comparable those recently derived for the Pleiades and Alpha Persei clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.

The Pleiades and α Persei Clusters

The upper–main-sequence members of the Pleiades and α Persei clusters, considered as members of the Local Association, yield mean parallaxes that are only 4% larger than the mean values from Hipparcos observations. The (log Teff, MV) diagram reveals that in the temperature range from 6000 to 8000 K, the Hyades and α Persei main-sequence members are nearly identical and several tenths of a magnitude brighter than similar stars on the Pleiades main sequence. The departure of the Pleiades main sequence cannot be traced to either age or heavy-element abundance differences in the range thought to apply to these clusters. A 50% increase in the helium abundance of Pleiades over Hyades stars could account for the luminosity difference. Alternative explanations are that the Pleiades cluster is rejected from supercluster membership and/or that the Hipparcos parallax results for the Pleiades are in error by some 10%.

Latitude distributions and lifetimes of star-spots on G dwarfs in the α Persei cluster

Latitude distributions and lifetimes of star-spots on G dwarfs in the α Persei cluster

Light Element Depletion in Contracting Brown Dwarfs and Pre–Main‐Sequence Stars

We present an analytic calculation of the thermonuclear depletion of the light elements lithium, beryllium, and boron in fully convective, low-mass stars. Under the presumption that the pre--main-sequence star is always fully mixed during contraction, we find that the burning of these rare light elements can be computed analytically, even when the star is degenerate. Using the effective temperature as a free parameter, we constrain the properties of low-mass stars from observational data, independently of the uncertainties associated with modeling their atmospheres and convection. Our analytic solution explains the dependence of the age at a given level of elemental depletion on the stellar effective temperature, nuclear cross sections, and chemical composition. Most importantly, our results allow observers to translate lithium non-detections in young cluster members into a model-independent minimum age for that cluster. Using this procedure, we have found lower limits to the ages of the Pleiades (100 Myr) and Alpha Persei (60 Myr) clusters. Recent experimental work on the low energy resonance in the ^10B(p,\alpha)^7Be reaction has greatly enhanced estimates of the destruction rate of ^10B, making it possible for stars with M>0.1 M_sun to deplete both ^10B and ^11B before reaching the main sequence. Moreover, there is an interesting range of masses, 0.085 M_sun < M < 0.13 M_sun, where boron depletion occurs on the main sequence in less than a Hubble time, providing a potential ``clock'' for dating low-mass stars.

Prominences on Rapidly-Rotating Solar-Type Stars

AbstractWe present Hα observations of two rapidly-rotating G2 dwarfs in the Alpha Persei cluster and of AB Dor, a young K0 dwarf. All three stars have projected rotational velocities of about 90 km s−1 and axial rotation periods ranging from 11 to 15 hours. He 520 and AB Dor show very clear transient features that move through the profile. The most likely explanation for these features is that they are huge prominences in co-rotation with the star. We also use the recently developed technique of least squares deconvolution to present simultaneous dynamic profiles of the stars’ photospheric lines which show the locations of the surface spots. This technique has allowed us to extend our studies to a greater range of stars.

THE ANGULAR MOMENTUM OF MAIN SEQUENCE STARS AND ITS RELATION TO STELLAR ACTIVITY

ABSTRACT Rotational velocities are reported for intermediate-mass main sequence stars it the field. The measurements are based on new, high S/N CCD spectra from the Coudé Feed Telescope of the Kitt Peak National Observatory. We analyze these rotation rates for a dependence on both mass and age. We compare the average rotation speeds of the field stars with mean velocities for young stars in Orion, the Alpha Persei cluster, the Pleiades, and the Hyades. The average rotation speeds of stars more massive than ∼1.6 M⊙\\experience little or no change during the evolutionary lifetimes of these stars on the zero age main sequence or within the main sequence band. Less massive stars in the range betwee n 1.6M⊙\\ and 1.3M⊙\\ also show little decline in mean rotation rate while they are on the main sequence, and at most a factor of 2 decrease in velocity as they evolve off the main sequence. The {e}-folding time for the loss of angular momentum b y the latter group of stars is at least 1--2 billion years. This inferred characteristic time scale for spindown is far longer than the established rotational braking time for solar-type stars with masses below ∼1.3 M⊙. We conclude from a comparison of the trends in rotation with trends in chromospheric and coronal activity that the overall decline in mean rotation speed along the main sequence, from ∼2 M⊙\\ down to ∼1.3 M⊙, is imposed during the pre-main sequence phase of evolution, and that this pattern changes little thereafter while the star resides on the main sequence. The magnetic activity implicated in the rotational spindown of the Sun and of similar stars during their main sequence lifetimes mus t therefore play only a minor role in determining the rotation rates of the intermediate mass stars, either because a solar-like dynamo is weak or absent, or else the geometry of the magnetic field is appreciably less effective in removing angular momentu m from these stars.

Rotation periods of selected members of the Persei cluster -- II

Rotation periods of selected members of the Persei cluster -- II

UBVRI photometric monitoring of 7 rapidly rotating late-type dwarfs in the Alpha Persei cluster

The UBVRI light curves of 7 late-type dwarfs amongst the fastest rotators of the Alpha Per cluster ( v sin i ≥ 140 km s -1 ) are presented. The shape of the light curves suggests that the photometric variations are most often dominated by a single group of cool spots located at intermediate or high latitude on the stellar surface. Assuming that starspots are good tracers of the stellar magnetic field, the smooth light curves indicate the existence of a large scale, slowly varying magnetic structure at the surface of these ultrafast rotators.

ROSAT Pointed Observations of the Alpha Persei Cluster

ROSAT Pointed Observations of the Alpha Persei Cluster

Chromospherically active stars. 12: ADS 11060 C: A double lined K dwarf binary in a quintuple system

ADS 11060 C is a double lined spectroscopic binary with a period of 25.7631 days and an eccentricity of 0.565. Spectral types of the two stars are estimated as K7 V and MO V with a magnitude difference of about 0.55 mag in V. The stars appear to be somewhat metal rich with respect to the Sun. Despite the relatively large masses of 0.53 and 0.51 solar mass, our photometric observations find no evidence for eclipses and we estimate an inclination of 77 deg plus or minus 11 deg. ADS 11060 C is, however, photometrically variable with a period of 9 plus or minus 1 day and an amplitude of 0.05 mag in V. Thus, it is a newly identified BY Draconis variable. The center-of-mass velocity of ADS 11060 C and an estimated parallax of 0.030 sec support its physical association with ADS 11060 AB, making this a quintuple system. The projected separation of the AB-C system is nearly 1200 AU. Although the log lithium abundances of the two components of ADS 11060 C are only upper limits, less than or equal to -0.14, lithium abundances of the AB-C components appear to be consistent with those of similar stars in the alpha Persei and Pleiades clusters, suggesting an age of about 70 Myr for ADS 11060 AB-C. The system is a possible member of the Pleiades moving group. Listed as an optical counterpart to a source in the ROSAT Wide Field Camera extreme-ultraviolet bright source catalog, both ADS 11060 AB and C may contribute to the observed flux.

New distance measurements to the Pleiades and Formula Persei clusters

We apply the new distance measuring technique of Hendry, O'Dell and Collier- Cameron, 1993, to the Pleiades and $\alpha$ Persei clusters. The method relies on knowledge of the periods, rotation velocities and angular diameters of a sample of late-type stars within each cluster. The angular diameters were found by recalibrating the Barnes-Evans surface brightness relation. Applying our new distance method we derive a Pleiades distance in very good agreement with previous ZAMS fitting determinations, whereas our $\alpha$ Persei distance appears substantially greater. In response to this greater distance, we apply a recent semi-empirical main-seqeunce fitting technique to $\alpha$ Persei and confirm the findings of our new distance indicator. (Figures / preprint available on request)

Lithium and chromospheric activity in the Alpha Persei cluster

We have obtained high resolution spectra of a set of proposed members of the Alpha Persei open cluster which had previously been shown to have anomalously low lithium abundances. We have used the new spectra to derive Ca II emission strengths, radial velocities, and reddening estimates in order to assess whether these stars truly are members of the Alpha Per cluster. We conclude that most of the weak lithium stars are indeed bonafide cluster members, and thus the problem of explaining their anomalous lithium abundances remains. We have also obtained spectra for another sample of stars for which the previous membership data were relatively limited. Most of these stars do not appear to be members of the cluster

Rotation periods of selected members of the Persei cluster *

We present a photometric study of 11 G-K dwarf members of the young open cluster surrounding α Persei, whose projected rotational velocities have indicated them to be rapid rotators. We detect significant rotational modulation due to surface inhomogeneities in 10 of the targets. We determine reliable axial rotation periods for six of these stars, using power spectrum analysis augmented with the CLEAN algorithm. Similar period analysis is also given for a further member whose power spectrum yielded ambiguous results

Prominence activity in G dwarfs of the Persei cluster

We have found evidence for prominence-like circumstellar material in four rapidly rotating G-K dwarfs of the α Persei cluster, He 373, 520, 622 and 699. Our time-series Hα spectra of these stars show transient absorption features similar to those seen in the southern field rapid rotator AB Doradus. The absorption transients have durations of order 1 hr, during which they drift across the rotationally broadened Hα profiles of the stars concerned. This behaviour is consistent with the model developed for AB Dor, in which the absorption transients are produced via scattering of Hα photons in clouds of mainly neutral material trapped in the summits of closed magnetic field structures extending several stellar radii above the surface of the star

Spectroscopic binaries in the Alpha Persei cluster

An average of 16 radial velocity measures for each of the 28 brightest (B3-A2) cluster members and found four binaries was obtained. The resulting binary frequency of 14 percent is, like the previous 20 percent for the B6-A1 stars in the Pleiades, unusually low compared with a typical 30 percent for early-type field stars or with 30 percent or more in other open clusters. These two clusters are the only known ones with unusually high mean rotational velocities. It is suspected that the mean rotational velocities are high either because these clusters lack short-period binaries or are not old enough for synchronization of rotational and orbital velocities to have occurred. The four Alpha Persei binaries are all relatively wide ones (greater than 20 d) and with small mass ratios (0.1-0.5). The same results apply to the young Orion Nebula cluster. These results can be explained in terms of the formation of binaries by capture in that during the first free-fall time, capture will produce wide binaries with small mass ratios, as in the Orion Nebula and Alpha Persei clusters, but repeated captures and disruptions will produce more closely spaced binaries with many mass ratios near 1.0, as in IC 4665.

Main-sequence broadening in the double clusterh and Ξ persei

Precise photometric observations of stars in the double cluster h and Ξ Persei reveal a large spread in the colours and magnitudes of the upper Main-Sequence; half of the stars are variables that are Be stars or related stars. The reported age difference between both clusters is found to be spurious. Rotation apparently affects both the intrinsic and the observed colours of the early-type stars in h and Ξ Persei. This result questions the validity of photometric calibrations that heavily rely on h and Ξ Persei or similar clusters.

Rotational velocities of newly discovered, low-mass members of the Alpha Persei cluster

About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.

Lithium abundances among solar-type pre-main-sequence stars

Measurements of Li I 6707 A line strengths were carried out for two samples of pre-main-sequence (PMS) stars (L 1641 and Taurus-Auriga), and the Li abundances estimated for PMS stars are compared with those deduced from observations of Li line strengths for main-sequence stars in the Alpha Persei cluster. It was found that the maximum Li abundances among the PMS stars with solar mass values greater than 1.0 exceed the maximum abundances for Alpha Per stars by at least 0.3 dex. Some PMS stars, including few apparently young stars, showed large (greater than 1.0 dex) Li depletion, and some apparently old PMS stars showed little or no depletion. 26 refs.

Lithium Abundances among Solar Type Pre Main Sequence Stars

Measurements of Li I 6707 A line strengths were carried out for two samples of pre-main-sequence (PMS) stars (L 1641 and Taurus-Auriga), and the Li abundances estimated for PMS stars are compared with those deduced from observations of Li line strengths for main-sequence stars in the Alpha Persei cluster. It was found that the maximum Li abundances among the PMS stars with solar mass values greater than 1.0 exceed the maximum abundances for Alpha Per stars by at least 0.3 dex. Some PMS stars, including few apparently young stars, showed large (greater than 1.0 dex) Li depletion, and some apparently old PMS stars showed little or no depletion.