arXiv subjects

W. Herbst

Publications and source records attributed to W. Herbst.

At least 19 recordsLinked to original sources

arXiv2013

The spectral type of CHS7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster

We present the spectroscopic characterization of the unusual high-amplitude very low mass pre-main-sequence periodic variable CHS7797. This study is based on optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range, carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have been carried out at two distinct phases of the 17.8d period, namely at maximum and four days before maximum. Four different spectral indices were used for the spectral classification at these two phases, all of them well-suited for spectral classification of young and obscured late M dwarfs. In addition, the gravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines were used to confirm the young age of CHS7797. From the spectrum obtained at maximum light we derived a spectral type (SpT) of M6.05, while for the spectrum taken four days before maximum the derived SpT is M5.75. The derived SpTs confirm that CHS7797 has a mass in the stellar-substellar boundary mass range. In addition, the small differences in the derived SpTs at the two observed phases may provide indirect hints that CHS7797 is a binary system of similar mass components surrounded by a tilted circumbinary disk, a system similar to KH15D.

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arXiv2010

Gas Absorption in the KH 15D System: Further Evidence for Dust Settling in the Circumbinary Disk

Na I D lines in the spectrum of the young binary KH 15D have been analyzed in detail. We find an excess absorption component that may be attributed to foreground interstellar absorption, and to gas possibly associated with the solids in the circumbinary disk. The derived column density is log N_NaI = 12.5 cm^-2, centered on a radial velocity that is consistent with the systemic velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~ 12.3 cm^-2. There is no detectable change in the gas column density across the "knife edge" formed by the opaque grain disk, indicating that the gas and solids have very different scale heights, with the solids being highly settled. Our data support a picture of this circumbinary disk as being composed of a very thin particulate grain layer composed of millimeter-sized or larger objects that are settled within whatever remaining gas may be present. This phase of disk evolution has been hypothesized to exist as a prelude to the formation of planetesimals through gravitational fragmentation, and is expected to be short-lived if much gas were still present in such a disk. Our analysis also reveals the presence of excess Na I emission relative to the comparison spectrum at the radial velocity of the currently visible star that plausibly arises within the magnetosphere of this still-accreting young star.

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arXiv2008

Results of the ROTOR-program. II. The long-term photometric variability of weak-line T Tauri stars

T Tauri stars exhibit variability on all timescales, whose origin is still debated. On WTTS the variability is fairly simple and attributed to long-lived, ubiquitous cool spots. We investigate the long term variability of WTTS, extending up to 20 years in some cases, characterize it statistically and discuss its implications for our understanding of these stars. We have obtained a unique, homogeneous database of photometric measurements for WTTS extending up to 20 years. It contains more than 9 000 UBV R observations of 48 WTTS. All the data were collected at Mount Maidanak Observatory (Uzbekistan) and they constitute the longest homogeneous record of accurate WTTS photometry ever assembled. Definitive rotation periods for 35 of the 48 stars are obtained. Phased light curves over 5 to 20 seasons are now available for analysis. Light curve shapes, amplitudes and colour variations are obtained for this sample and various behaviors exhibited, discussed and interpreted. Our main conclusion is that most WTTS have very stable long term variability with relatively small changes of amplitude or mean light level. The long term variability seen reflects modulation in the cold spot distributions. Photometric periods are stable over many years, and the phase of minimum light can be stable as well for several years. On the long term, spot properties do change in subtle ways, leading to secular variations in the shape and amplitudes of the light curves.

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arXiv2006

Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars

We present a unique, homogeneous database of photometric measurements for Classical T Tauri stars extending up to 20 years. The database contains more than 21,000 UBVR observations of 72 CTTs. All the data were collected within the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan) and together they constitute the longest homogeneous, accurate record of TTS variability ever assembled. We characterize the long term photometric variations of 49 CTTs with sufficient data to allow a robust statistical analysis and propose an empirical classification scheme. Several patterns of long term photometric variability are identified. The most common pattern, exhibited by a group of 15 stars which includes T Tau itself, consists of low level variability (Delta(V)<=0.4mag) with no significant changes occurring from season to season over many years. A related subgroup of 22 stars exhibits a similar stable long term variability pattern, though with larger amplitudes (up to Delta(V)~1.6 mag). Besides these representative groups, we identify three smaller groups of 3-5 stars each which have distinctive photometric properties. The long term variability of most CTTs is fairly stable and merely reflects shorter term variability due to cold and hot surface spots. Only a small fraction of CTTs undergo significant brightness changes on the long term (months, years), which probably arise from slowly varying circumstellar extinction.

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arXiv2006

The Recurrent Eclipse of an Unusual Pre--Main-Sequence Star in IC 348

The recurrence of a previously documented eclipse of a solar-like pre--main-sequence star in the young cluster IC 348 has been observed. The recurrence interval is 4.7 $\pm 0.1$ yr and portions of 4 cycles have now been seen. The duration of each eclipse is at least 3.5 years, or $\sim 75$% of a cycle, verifying that this is not an eclipse by a stellar companion. The light curve is generally symmetric and approximately flat-bottomed. Brightness at maximum and minimum have been rather stable over the years but the light curve is not perfectly repetitive or smooth and small variations exist at all phases. We confirm that the star is redder when fainter. Models are discussed and it is proposed that this could be a system similar to KH 15D in NGC 2264. Specifically, it may be an eccentric binary in which a portion of the orbit of one member is currently occulted during some binary phases by a circumbinary disk. The star deserves sustained observational attention for what it may reveal about the circumstellar environment of low-mass stars of planet-forming age.

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arXiv2006

The Variability and Rotation of Pre-main Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?

A variability study of the young cluster IC 348 at Van Vleck Observatory has been extended to a total of seven years. Twelve new periodic stars have been found in the last two years, bringing the total discovered by this program to 40. In addition, we confirm 16 of the periods reported by others and resolve some discrepancies. The total number of known rotation periods in the cluster, from all studies has now reached 70. This is sufficient to demonstrate that the parent population of K5-M2 stars is rotationally indistinguishable from that in the Orion Nebula Cluster even though their radii are 20% smaller and they would be expected to spin about twice as fast if angular momentum were conserved. The median radius and, therefore, inferred age of the IC 348 stars actually closely matches that of NGC 2264, but the stars spin significantly more slowly. This suggests that another factor besides mass and age plays a role in establishing the rotation properties within a cluster and we suggest that it is environment. If disk locking were to persist for longer times in less harsh environments, because the disks themselves persist for longer times, it could explain the generally slower rotation rates observed for stars in this cluster, whose earliest type star is of class B5. We have also obtained radial velocities, the first for PMS stars in IC348, and v sin i measurements for 30 cluster stars to assist in the study of rotation and as an independent check on stellar radii. Several unusual variable stars are discussed; in some or all cases their behavior may be linked to occultations by circumstellar material. A strong correlation exists between the range of photometric variability and the slope of the spectral energy distribution in the infrared. Nineteen of the 21 stars with I ranges exceeding 0.4 mag show infrared evidence for circumstellar disks.

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arXiv2006

Evidence for Differential Rotation on a T Tauri Star

Five years of photometric monitoring of the T Tauri star HBC 338 in NGC 1333 has revealed that it is a periodic variable, but the period has changed significantly with time. From 2000-2003, a period near 5.6 days was observed, while in the last two seasons, the dominant period is near 4.6 days. No other T Tauri star has been seen to change its period by such a large percentage. We propose a model in which a differentially rotating star is seen nearly equator-on and a high latitude spot has gradually been replaced by a low latitude spot. We show that this model provides an excellent fit to the observed shapes of the light curves at each epoch. The amplitude and sense of the inferred differential rotation is similar to what is seen on the Sun. This may be surprising given the likely high degree of magnetic surface activity on the star relative to the Sun but we note that HBC 338 is clearly an exceptional T Tauri star.

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arXiv2006

The Rotation of Young Low-Mass Stars and Brown Dwarfs

We review the current state of our knowledge concerning the rotation and angular momentum evolution of young stellar objects and brown dwarfs from a primarily observational view point. Periods are typically accurate to 1% and available for about 1700 stars and 30 brown dwarfs in young clusters. Discussion of angular momentum evolution also requires knowledge of stellar radii, which are poorly known for pre-main sequence stars. It is clear that rotation rates at a given age depend strongly on mass; higher mass stars (0.4-1.2 M$_\odot$) have longer periods than lower mass stars and brown dwarfs. On the other hand, specific angular momentum is approximately independent of mass for low mass pre-main sequence stars and young brown dwarfs. A spread of about a factor of 30 is seen at any given mass and age. The evolution of rotation of solar-like stars during the first 100 Myr is discussed. A broad, bimodal distribution exists at the earliest observable phases ($\sim$1 Myr) for stars more massive than 0.4 M$_\odot$. The rapid rotators (50-60% of the sample) evolve to the ZAMS with little or no angular momentum loss. The slow rotators continue to lose substantial amounts of angular momentum for up to 5 Myr, creating the even broader bimodal distribution characteristic of 30-120 Myr old clusters. Accretion disk signatures are more prevalent among slowly rotating PMS stars, indicating a connection between accretion and rotation. Disks appear to influence rotation for, at most, $\sim$5 Myr, and considerably less than that for the majority of stars. If the dense clusters studied so far are an accurate guide, then the typical solar-like star may have only $\sim$1 Myr for this task. It appears that both disk interactions and stellar winds are less efficient at braking these objects.

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arXiv2005

Rotational Evolution of Solar-Like Stars in Clusters from Pre-Main Sequence to Main Sequence: Empirical Results

Rotation periods are now available for ~500 pre-main sequence and recently arrived main sequence stars of solar-like mass (0.4-1.2 M_sun) in five nearby young clusters: the Orion Nebula Cluster, NGC 2264, alpha Per, IC 2602 and the Pleiades. In combination with estimates of stellar radii these data allow us to construct distributions of surface angular momentum per unit mass at three different epochs: nominally, 1, 2 and 50 My. Our main result is illustrated in Fig. 18 and may be summarized as follows: (1) 50-60% of the stars on convective tracks in this mass range are released from any locking mechanism very early on and are free to conserve angular momentum throughout most of their PMS evolution, i.e. to spin up and account for the rapidly rotating young main sequence stars. (2) The other 40-50% lose substantial amounts of angular momentum during the first few million years, and end up as slowly rotating main sequence stars. The duration of the rapid angular momentum loss phase is ~5-6 My, which is roughly consistent with the lifetimes of disks estimated from infrared surveys of young clusters. The rapid rotators of Orion age lose less than 10% of their (surface) specific angular momentum during the next 50 My while the slow rotators lose about two-thirds of theirs. A detectable part of this loss occurs even during the ~1 My interval between the ONC and NGC 2264. The data support the view that interaction between an accretion disk and star is the primary mechanism for evolving the broad, bimodal distribution of rotation rates seen for solar-like stars in the ONC into the even broader distributions seen in the young MS clusters.

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arXiv2005

The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004

We present results from the most recent (2002-2004) observing campaigns of the eclipsing system KH 15D, in addition to re-reduced data obtained at Van Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of photometric data shows substantial evolution in the width and depth of the eclipses. The most recent data indicate that the eclipses are now approximately 24 days in length, or half the orbital period. These results are interpreted and discussed in the context of the recent models for this system put forward by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et al. Another significant peak, at 9.6 days, was found in the periodogram of the out-of-eclipse data at two different epochs. We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion. If so, application of Hut's theory implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i) and the measured rotation period of the star, we calculate the lower limit on the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value obtained by Hamilton et al. from its luminosity and effective temperature. Here we assume that i = 90 degrees since it is likely that the spin and orbital angular momenta vectors are nearly aligned.

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arXiv2005

Rotational modulation of X-ray emission in Orion Nebula young stars

We investigate the spatial distribution of X-ray emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light-curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17x17 arcmin field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period while in seven cases it is about half that value, possibly due to the presence of X-ray emitting structures at opposite stellar longitudes. These results constitute the largest sample of low mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the stellar surface with X-ray emitting regions.

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arXiv2004

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

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arXiv2004

A rotational and variability study of a large sample of PMS stars in NGC 2264

We present the results of an extensive search for periodic and irregular variable pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC 2664, based on photometric monitoring using the Wide Field Imager (WFI) on the 2.2m telescope on La Silla (Chile). In total, about 10600 stars with I_c magnitudes between 9.8mag and 21mag have been monitored in our 34x33 arcmin field. Time series data were obtained in the I_c band in 44 nights between Dec. 2000 and March 2001; altogether we obtained 88 data points per star. Using two different time series analysis techniques (Scargle and CLEAN) we found 543 periodically variable stars with periods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified using a chi-squared-test. In addition we have carried out nearly simultaneous observations in V, R_c and a narrow-band Halpha filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the I_c vs. (R_c-I_c) colour-magnitude and (R_c-Halpha) vs. (R_c-I_c) colour-colour diagram. We could in the end identify 405 periodically variable and 184 irregular variable PMS stars as cluster members using these two different tests. In addition 35 PMS stars for which no significant variablilty were detected could be identified as members using an Halpha emission index criterion. This yields to a total of 624 PMS stars in NGC 2264, of which only 182 were known before. Most of the newly found PMS stars are fainter than I_c = 16 and of late spectral type (>M2). (abridged)

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arXiv2003

A Multi-Year Photometric Study of IC 348

The extremely young cluster IC 348 has been monitored photometrically over 5 observing seasons from Dec 1998 to March 2003 in Cousins I with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16 irregular variables have been identified. Among the brighter stars, 14 of the 16 known K or M-type WTTS were found to be periodic variables, while all 5 of the known CTTS were found to be irregular variables. In the full sample, which includes 150 stars with I mag as faint as 18, we find that 40% of the 63 WTTS are detected as variables, nearly all of them periodic, while 55% of the 20 CTTS are also detected as variable, with none of them periodic. Our study suggests that 80-90% of all WTTS in young clusters will be detected as periodic variables given sufficiently precise and extended monitoring, whereas CTTS will reveal themselves primarily or solely as irregular variables. This has clear consequences for PMS rotational studies based on photometric periods. We examine the stability of the periodic light curves from season to season. All periodic stars show modulations of their amplitude, mean brightness and light curve shape on time scales of less than 1 yr, presumably due to changes in spot configurations and/or physical characteristics. In no case, however, can we find definitive evidence of a change in period, indicating that differential rotation is probably much less on WTTS than it is on the Sun. Among the non-periodic stars, we report the detection of two possible UXors as well as a pre-main sequence star, HMW 15, which apparently undergoes an eclipse with a duration exceeding three years.

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arXiv2003

An Unusual Eclipse of a Pre-Main Sequence Star in IC 348

A solar-like pre-main sequence star (TJ 108 = H 187 = LRLL 35 = HMW 15) in the extremely young cluster IC 348 has been found, which apparently experienced an eclipse lasting ~3.5 years, much longer than has ever been detected for any normal eclipsing binary. The light curve is flat-bottomed and rather symmetric, with a depth of 0.66 mag in Cousins I. During eclipse, the system reddened by \~0.17 mag in R-I. We argue that the eclipsing body is not a star because of the small probability of detecting an eclipse in what would be a very widely separated binary. Instead, it appears that the eclipse was caused by a circumstellar or circumbinary cloud or disk feature which occulted the star, or one of its components, if it is a binary system. We emphasize the importance of more detailed study of this object, which appears to be a new member of a small class of pre-main sequence stars whose variability can be firmly linked to occultation by circumstellar (or circumbinary) matter.

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arXiv2001

Rotational Velocities and Radii of Pre-Main-Sequence Stars in the Orion Nebula Cluster

We have obtained high-dispersion spectra for 256 pre-main-sequence stars in the Orion Nebula Cluster in order to measure their projected rotational velocities and study the rotational evolution and physical properties of young, low-mass stars. Half the stars were chosen because they had known photometric periods and half were selected as a control sample of similar objects without known periods. More than 90% of the spectra yielded vsin(i) measurements, although one-third are upper limits. We find strong evidence confirming the assumption that the periodic light variations of T Tauri stars are caused by rotation of surface spots. We find no significant difference between the vsin(i) distributions of the periodic and control samples, indicating that there is no strong bias in studying the rotation of young stars using periodic variables. Likewise, the classical and weak T Tauri stars exhibit vsin(i) distributions that are statistically the same. For stars with known period and vsin(i), the mean value of sin(i) is lower than expected for a random distribution of rotation axes. This could be due to errors in the quantities that contribute to the sin(i) calculation or to a real physical effect. We find that mean sin(i) has the expected value if we increase the stars' Teff values by 400-600 K. Finally, we have calculated minimum radii for stars with vsin(i) and period, and average radii for stars grouped by location in the H-R diagram. We find evidence at the 3-sigma level that the radii of stars on similar mass tracks are decreasing as the stars move closer to the ZAMS. [abridged]

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arXiv2001

The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster

We have determined new rotation periods for 404 stars in the Orion Nebula Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on La Silla, Chile. Mass estimates are available for 335 of these and most have M < 0.3 M_sun. We confirm the existence of a bimodal period distribution for the higher mass stars in our sample and show that the median rotation rate decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While the spread in angular momentum (J) at any given mass is more than a factor of 10, the majority of lower mass stars in the ONC rotate at rates approaching 30% of their critical break-up velocity, as opposed to 5-10% for solar-like stars. This is a consequence of both a small increase in observed specific angular momentum (j=J/M) and a larger decrease in the critical value of j with decreasing mass. Perhaps the most striking fact, however, is that j varies by so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in nature to what is found in the Pleiades (age ~ 100 My). These observations provide a significant new guide and test for models of stellar angular momentum evolution during the proto-stellar and pre-main sequence phases.

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arXiv2000

A Variability Study of Pre-Main Sequence Stars in the Extremely Young Cluster IC 348

The extremely young cluster IC 348 has been monitored in the Cousins I band with a 0.6 m telescope at Wesleyan's Van Vleck Observatory. Photometry of 150 stars was obtained on 76 images taken on 27 separate nights during the period December, 1998, through March, 1999. As expected, spectral characteristics largely determine the nature of a star's variability in this cluster. None of the stars with H-alpha in absorption were found to be variables. On the other hand, all 16 stars identified as CTTS by their H-alpha emission equivalent widths and the majority of the 49 WTTS in the part of the cluster we monitored showed evidence of variability. Nineteen stars were found to be periodic, with periods ranging from 2.24 to 16.2 days and masses ranging from 0.35 to 1.1 M-sun. Seventeen of these are WTTS and the other 2 are of unknown spectral class. The period distribution is remarkably similar to what is found in the Orion Nebula Cluster for stars in the same mass range. Namely, it is bimodal with peaks at 2-3 days and 7-8 days, although there are not enough periods known to define these features significantly by the IC 348 data alone. The three fastest rotators are also the three most massive stars in the periodic sample.

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