Padmakar Parihar

Magnetic activity in the photosphere of CoRoT-Exo-2a - Active longitudes and short-term spot cycle in a young Sun-like star

Context. The space experiment CoRoT has recently detected transits by a hot Jupiter across the disk of an active G7V star (CoRoTExo-2a) that can be considered as a good proxy for the Sun at an age of approximately 0.5 Gyr. Aims. We present a spot modelling of the optical variability of the star during 142 days of uninterrupted observations performed by CoRoT with unprecedented photometric precision. Methods. We apply spot modelling approaches previously tested in the case of the Sun by modelling total solar irradiance variatio ns, a good proxy for the optical flux variations of the Sun as a star . The best results in terms of mapping of the surface brightness inhomogeneities are obtained by means of maximum entropy regularized models. To model the light curve of CoRoT-Exo-2a, we take into account both the photometric effects of cool spots as well as those of solar-like faculae, ado pting solar analogy. Results. Two active longitudes initially on opposite hemispheres are found on the photosphere of CoRoT-Exo-2a with a rotation period of 4.522± 0.024 days. Their separation changes by≈ 80 ◦ during the time span of the observations. From this variation, a relative amplitude of the surface differential rotation lower than ∼ 1 percent is estimated. Individual spots form within the act ive longitudes and show an angular velocity about∼ 1 percent smaller than that of the longitude pattern. The tot al spotted area shows a cyclic oscillation with a period of 28.9± 4.3 days, which is close to 10 times the synodic period of the planet as seen by the rotating active longitudes. We discuss the effects of solar-like faculae on our models, finding indication of a facular contribution to the optical flux variations of CoRoT-Exo-2a being significantly smaller than in the present Sun. Conclusions. The implications of such results for the internal rotation o f CoRoT-Exo-2a are discussed on the basis of solar analogy. A possible magnetic star-planet interaction is suggested by the cyclic variation of the spotted area. Alternatively, t he 28.9-d cycle may be related to Rossby-type waves propagating in the subphotospheric layers of the star.

Exploring pre-main-sequence variables of the ONC: the new variables

Since 2004, we have been engaged in a long-term observing programme to monitor young stellar objects (YSOs) in the Orion Nebula Cluster (ONC). We have collected about 2000 frames in V, R and I broad-band filters on more than 200 nights distributed over five consecutive observing seasons. The high-quality and time-extended photometric data give us an opportunity to address various phenomena associated with young stars. The prime motivations of this project are (i) to explore various manifestations of stellar magnetic activity in very young low-mass stars, (ii) to search for new pre-main-sequence eclipsing binaries and (iii) to look for any EXor and FUor-like transient activities associated with YSOs. Since this is the first paper on this programme, we give a detailed description of the science drivers, the observation and the data reduction strategies as well. In addition to these, we also present a large number of new periodic variables detected from our first 5 yr of time-series photometric data. Our study reveals that about 72 per cent of classical T Tauri stars (CTTS) in our field of view are periodic, whereas only 32 per cent of weak-lined T Tauri stars (WTTS) are periodic. This indicates that inhomogeneity patterns on the surface of CTTS of the ONC stars are much more stable than on WTTS. From our multiyear monitoring campaign, we found that the photometric surveys based on single season are incapable of identifying all periodic variables. And any study on evolution of angular momentum based on single-season surveys must be carried out with caution.

RACE-OC project: rotation and variability in the open cluster NGC 2099 (M 37)

Context. Rotation and solar-type m agnetic activity are closely related to each other in m ain-sequence stars ofG or later spectraltypes.Presence and levelofm agnetic activity depend on stars rotation and rotation itselfis strongly inuenced bystrength and topologyofthem agneticelds.O pen clustersrepresentespecially usefultargetstoinvestigate theconnection between rotation and activity. Aim s. The open clusterNG C 2099 hasbeen studied asa partofthe RACE-O C project,which isaim ed atexploring theevolution ofrotation and m agnetic activity in thelate-typem em bersofopen clustersofdierentages. M ethods. Tim eseriesCCD photom etricobservationsofthisclusterwerecollected duringJanuary 2004 and thepresence ofperiodicities in the ux variation related to the stellar rotation are determ ined by Fourier analysis.The relations between activity m anifestations,such asthelightcurveam plitude,and globalstellarparam etersare investigated. Results. W e have discovered 135 periodic variables,122 ofwhich are candidate cluster m em bers.Determ ination of rotation periods ofG -and K -type stars has allowed us to better explore evolution ofangular m om entum at an age ofabout 500 M yr.In our analysis we have also identied 3 new detached eclipsing binary candidates am ong cluster m em bers. Conclusions. A com parison with the older Hyades cluster (� 625 M yr) shows that the newly determ ined distribution ofrotation periods is consistent with the scenario ofrotationalbraking ofm ain-sequence spotted stars as they age. However,acom parison with theyoungerM 34 cluster(� 200 M yr)showsthattheG 8-K 5m em bersoftheseclustershave the sam e rotation period distribution,that is G 8-K 5 m em bers in NG C 2099 seem to have experienced no signicant braking in the age range from � 200 to � 500 M yr.Finally,NG C 2099 m em bershave a levelofphotospheric m agnetic activity,as m easured by light curve am plitude,sm aller than in younger stars ofsam e m ass and rotation,suggesting thatthe activity levelalso dependson som e otherage-dependentparam eters.

RACE-OC project: rotation and variability in the open cluster M 11 (NGC 6705)

Context. Rotation and magnetic activity are intimately linked in main-sequence stars of G or later spectral types. The presence and level of magnetic activity depend on stellar rotation, and rotation itself is strongly influenced by the strength and topology of the magnetic fields. Open clusters represent especially useful targets for investigating the connection among rotation, activity, and age. Over time, stellar activity and rotation evolve, providing us with a promising diagnostic tool to determine the age of the field stars. Aims. The open cluster M11 has been studied as a part of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters), which aims to explore the evolution of rotation and magnetic activity in the late-type members of open clusters with different ages. Methods. Photometric observations of the open cluster M11 were carried out in June 2004 using the LOAO 1 m telescope. The rotation periods of the cluster members were determined by Fourier analysis of photometric data time series. We investigated the relations between the surface activity, characterized by the light curve amplitude, and rotation. Results. We have discovered a total of 75 periodic variables in the M11 FoV, of which 38 are candidate cluster members. Specifically, among cluster members we discovered 6 early-type periodic variables, 2 eclipsing binaries, and 30 bona-fide single periodic late-type variables. Considering the rotation periods of 16 G-type members of the almost coeval 200-Myr M34 cluster, we could determine the rotation period distribution from a more numerous sample of 46 single G stars at an age of about 200-230 Myr and determine a median rotation period of P = 4. 8d . Conclusions. A comparison with the younger M35 cluster (∼150 Myr) and with the older M37 cluster (∼550 Myr) shows that G stars rotate more slowly than younger M35 stars and more rapidly than older M37 stars. The measured variation in the median rotation period is consistent with the scenario of rotational braking of main-sequence spotted stars as they age. Finally, G-type M11 members have a level of photospheric magnetic activity, as measured by light curve amplitude, comparable to what is observed in the younger 110-Myr Pleiades stars of similar mass and rotation.

Post-outburst phase of McNeil's nebula (V1647 Orionis)

We present a detailed study of the post-outburst phase of McNeils nebula (V1647 Orionis) using optical B, V, R, I and near-infrared (NIR) J, H, K photometric and low-resolution optical spectroscopic observations. The observations were carried out with the Himalaya Faint Object Spectrograph Camera (HFOSC), NIR camera (NIRCAM), the Tata Institute of Fundamental Research (TIFR) Near-Infrared Camera (TIRCAM) and NICMOS cameras on the 2-m Himalayan Chandra Telescope (HCT) and 1.2-m Physical Research Laboratory (PRL) telescopes during the period 2004 February-2005 December. The optical and NIR observations show a general decline in the brightness of the exciting source of McNeils nebula (V1647 Ori). Our recent optical images show that V1647 Ori has faded by more than 3 mag since February 2004. McNeils nebula has also faded considerably. The optical/NIR photometric data also show a significant variation in the magnitudes (ΔV = 0.78, ΔR = 0.44, ΔI = 0.21, ΔJ = 0.24 and ΔH = 0.20 mag) of V1647 Ori within a period of one month, which is possibly undergoing a phase similar to eruptive variables, like EXors or FUors. The optical spectra show a few features such as strong Ha emission with blue-shifted absorption and the Ca II IR triplet (8498, 8542 and 8662 A) in emission. As compared to the period just after outburst, there is a decrease in the depth and extent of the blue-shifted absorption component, indicating a weakening in the powerful stellar wind. The presence of the Ca II IR triplet in emission confirms that V1647 Ori is a pre-main-sequence star. The long-term, post-outburst photometric observations of V1647 Ori suggest an EXor rather than an FUor event. An optical/IR comparison of the region surrounding McNeils nebula shows that the optical nebula is more widely and predominantly extended to the north, whereas the IR nebula is relatively confined (diameter ∼60 arcsec), but definitely extended, to the south, too. The large colour gradient from north to south and the sudden absence of an optical nebula to the south are suggestive of a large-scale disc-like structure (or envelope) surrounding the central source that hides the southern nebula.

Observation of R-Band Variability of L Dwarfs

We report, for the first time, photometric variability of L dwarfs in the R band. Out of three L1 dwarfs (2MASS 1300+19, 2MASS 1439+19, and 2MASS 1658+70) observed, we have detected R-band variability in 2MASS 1300+19 and 2MASS 1439+19. The objects exhibit variability of amplitude ranging from 0.01 to 0.02 mag. Object 2MASS 1658+70 turns out to be nonvariable in both the R and I bands. However, more observations are needed to infer its variability. No periodic behavior in the variability is found from the two L1 dwarfs that are variable. All three L1 dwarfs have either negligible or no Hα activity. In the absence of any direct evidence for the presence of a sufficiently strong magnetic field, the detection of polarization at the optical favors the presence of dust in the atmosphere of L dwarfs. We suggest that the observed R-band photometric variability is most likely due to atmospheric dust activity.

An unusual very low-mass high-amplitude pre-main sequence periodic variable

Aims. We have investigated the nature of the variability of CHS 7797, an unusual periodic variable in the Orion Nebula Cluster. Methods. An extensive I-band photometric data set of CHS 7797 was compiled between 2004-2010 using various telescopes. Further optical data have been collected in R and zbands. In addition, simultaneous observations of the ONC region including CHS 7797 were performed in the I, J, Ks & IRAC 3.6 and 4.5µm bands over a time interval of≈ 40 d. Results. CHS 7797 shows an unusual large-amplitude variation of≈ 1.7 mag in the R, I, and zbands with a period 17.786 ± 0.03 d (FAP = 1x10 −15 %). The amplitude of the brightness modulation decreases only slightly at longer wavelengths. The star is faint during≈ 2/3 of the period and the shape of the phased light-curves for the seven different observing seasons shows minor changes and small-amplitude variations. Interestingly, there are no significant colour-flux correlations for �. 2µm, while the object becomes redder when fainter at longer wavelengths. CHS 7797 has a spectral type of M6 and an estimated mass between 0.04-0.1 M⊙. Conclusions. The analysis of the data suggests that the periodic variabil ity of CHS 7797 is most probably caused by an orbital motion. Variability as a result of rotational brightness modulatio n by a hot spot is excluded by the lack of any colour-brightness correlation in the optical. The latter indicates that CHS 7797 is most probably occulted by circumstellar matter in which grains have grown from typical 0.1 µm to≈ 1-2µm sizes. We discuss two possible scenarios in which CHS 7797 is periodically eclipsed by structures in a disc, namely that CHS 7797 is a single object with a circumstellar disc, or that CHS 7797 is a binary system, similar to KH 15D, in which an inclined circumbinary disc is responsible of the variability. Possible reasons for the typical 0.3 mag variati ons in I-band at a given phase are discussed.

FR Cnc revisited: photometry, polarimetry and spectroscopy

This paper is part of a multiwavelength study aimed at using complementary photometric, polarimetric and spectroscopic data to achieve an understanding of the activity process in late-type stars. Here, we present the study of FR Cnc, a young, active and spotted star. We performed analysis of All Sky Automated Survey 3 (ASAS-3) data for the years 2002–08 and amended the value of the rotational period to be 0.826518 d. The amplitude of photometric variations decreased abruptly in the year 2005, while the mean brightness remained the same, which was interpreted as a quick redistribution of spots. BVR_C and I_C broad-band photometric calibration was performed for 166 stars in FR Cnc vicinity. The photometry at Terskol Observatory shows two brightening episodes, one of which occurred at the same phase as the flare of 2006 November 23. Polarimetric BVR observations indicate the probable presence of a supplementary source of polarization. We monitored FR Cnc spectroscopically during the years 2004–08. We concluded that the radial velocity changes cannot be explained by the binary nature of FR Cnc. We determined the spectral type of FR Cnc as K7V. Calculated galactic space-velocity components (U, V, W) indicate that FR Cnc belongs to the young disc population and might also belong to the IC 2391 moving group. Based on Li Iλ6707.8 measurement, we estimated the age of FR Cnc to be between 10 and 120 Myr. Doppler tomography was applied to create a starspot image of FR Cnc. We optimized the goodness of fit to the deconvolved profiles for axial inclination, equivalent width and v sin i, finding v sin  i=46.2 km s^−1 and i= 55°. We also generated a syntheticV-band light curve based on Doppler imaging that makes simultaneous use of spectroscopic and photometric data. This synthetic light curve displays the same morphology and amplitude as the observed one. The starspot distribution of FR Cnc is also of interest since it is one of the latest spectral types to have been imaged. No polar spot was detected on FR Cnc.

A soft actuator for prototype segmented mirror telescope

The Segmented Mirror Telescopes (SMT) are built using small hexagonal mirror segments placed side by side to form a monolithic primary mirror of very large size. The effective figure of such a segmented primary mirror is maintained against external disturbances introduced by gravity, temperature, wind and vibration with the help of primary mirror active control system. This active control system consists of two levels of control – global and local level. At the global scale, three actuators per segment and two edge sensors per intersegment sides are used to maintain the shape of the primary mirror. At the local level, actuator control system executes the commands generated by the global control loop. Every mirror segment is controlled with the help of three actuators, where the major role of these actuators is to provide a tip, tilt, and piston to the mirror segments. In this paper, we describe the actuator developed for 1.5m diameter Prototype Segmented Mirror Telescope (PSMT). The actuator for this telescope is a soft actuator based on the voice coil mechanism. This actuator is designed for with the range of travel of ±1.5mm and the force range of 25N along with an offloading capability to reduce the power consumption. The prototype actuator is undergoing different tests at Indian Institute of Astrophysics (IIA), Bangalore. The tracking rate of 324nm/s is achieved with the tracking error of 22.5 nm RMS.

Primary mirror active control system simulation of Prototype Segmented Mirror Telescope

The upcoming large astronomical telescopes are trending towards the Segmented Mirror Telescope (SMT) technology, initially developed at the W M Keck Observatory in Hawaii, where two largest SMTs in the world are in use. SMT uses large number of smaller hexagonal mirror segments aligned and positioned by the use of three position/force actuators and six intersegment edge sensors. This positioning needs to be done within nanometer range to make them act like a monolithic primary mirror in the presence of different disturbances like wind, vibration & thermal effects. The primary mirror active control system of SMT does this important task at two levels. First at a global scale, by measuring edge sensor information continuously and commanding actuators to correct for any departure from the reference surface. And second at local actuator level, where all the actuators maintain their position to the reference control inputs. The paper describes our novel approach of primary mirror active control simulation of Prototype Segmented Mirror Telescope (PSMT) under design and development at Indian Institute of Astrophysics (IIA), Bangalore. The PSMT is a 1.5m segmented mirror telescope with seven hexagonal segments, 24 inductive edge sensors, and 21 soft actuators. The state space model of the soft actuator with Multiple-Input Multiple-Output (MIMO) capability is developed to incorporate dynamic wind disturbances. Further, a segment model was developed using three such actuators which accept actuator position commands from the global controller and telescope control system and yields tip-tilt-piston (TTP) of a single segment. A dynamic wind disturbance model is developed and used to disturb the mirror in a more realistic way. A feed forward PID controller is implemented, and gains are appropriately tuned to get the good wind rejection. In the last phase, a global controller is implemented based on SVD algorithm to command all the actuators of seven segments combined to act as a single monolithic mirror telescope. In this paper, we present the progress of PSMT active control system simulation along with the simulation results.