Marcello Rodono

Multiband modelling of the Sun as a variable star from VIRGO/SoHO data

The time series of total solar irradiance (TSI) and optical spectral irradiance at 402, 500 and 862 nm (SSIs) obtained by the VIRGO experiment on board the satellite SoHO are analysed in order to model their variability in the framework of a purely stellar-like approach. The different time scales of variability are estimated by means of the pooled variance method revealing the growth and decay of sunspots and faculae in active regions, as well as their rotationally modulated visibility. The determination of the rotation period of the Sun from the time modulation of the TSI and SSIs is made difficult by the short lifetimes of photospheric brightness inhomogeneities in comparison to the rotation period. Only during the phases with the lowest level of activity of solar cycle 23, when the variability is dominated by long-lived faculae, is it possible to determine the truly solar synodic period. The simultaneous modelling of the rotational modulation of the TSI and SSIs can be performed by means of a simple stellar-like approach which extends the model we previously applied to the TSI modulation alone (Lanza et al. 2003). Our model yields residuals about 20-30 times smaller than the amplitudes of the TSI and SSI variations in all the phases of the 11-yr activity cycle. The determination of the model parameters, including the temperature of the surface brightness inhomogeneities and the trade-off among them are discussed and compared with the results obtained with different modelling approaches. The advantages and the drawbacks of applying the model to other stars are also considered.

Long-term starspot evolution, activity cycle, and orbital period variation of V711 Tauri (HR 1099)

The long-term optical photometry of V711 Tau (HR 1099) from 1975 to 2001 is analysed with different techniques to provide information on the activity of this widely studied RS CVn-type binary. Pooled variance analysis gives a characteristic timescale of evolution for the photospheric pattern of starspots of the order of 100–200 days, while periodogram analysis of seasonal photometric data reveals that the rotational modulation period is 2.837 ± 0.094 d, without significant changes during the 26 analysed years. Considering subsets of the entire data sequence along which the rotational modulation of the optical flux stays stable, a set of 66 light curves is obtained. They are modelled by means of inversion techniques based on Maximum Entropy and Tikhonov regularizations to derive the distributions in longitude and the total amount of the spotted area on the more luminous K1 IV component that dominates the optical variability of the system. An extensive comparison with published Doppler Imaging maps shows the advantages and drawbacks of inversion techniques based solely on optical photometry. The large-scale structure of the longitudinal distribution of the spotted area is correctly reproduced, although individual starspots revealed by Doppler Imaging cannot be detected because of the limited spatial resolution of the photometric imaging. On the other hand, the variation of the total spotted area is best reconstructed from optical photometry and shows an activity cycle with a period of 19.5 ± 2.0 yr, upon which a short-term modulation might be superposed having variable amplitude and phase and a mean duration of approximately 3–5 years. The distributions of the spotted area versus longitude show the presence of one dominant preferential longitude from 1975 to 1989; afterwards, two preferential longitudes with comparable spotted areas are detected. Their migration rates with respect to the orbital reference frame are variable versus time, possibly as a consequence of a variable drift rate of the respective non-axisymmetric dynamo modes. The variation of the orbital period of the system appears to be correlated with the magnetic activity of the K1 IV component. Specifically, the decrease of the orbital period that occurred around 1990 coincides with a re-configuration of the stellar magnetic field, as indicated by the simultaneous variation of its different proxies. In combination with previous studies, these results allow us to discriminate among different theoretical models proposed to explain the connection between magnetic activity and orbital dynamics.

REM: a fully robotic telescope for GRB observations

The Rapid Eye Mount (REM) telescope is an ambitious project devoted to the prompt observations, in the optical and Near Infrared (NIR), of Gamma-Ray Bursts (GRBs) whose high energy emission is mainly detected by the Swift satellite. The system is able to immediately react to a GRB alert and perform observations, data reduction and analyses, distributing GRB counterparts in a timescale of tens of seconds. Apart from GRB observations, REM can also drive autonomous observations of a variety of targets as X-ray transients, flare stars, etc. We describe here how REM can manage all these tasks robotically, taking into account environmental and scientific parameters as seeing, visibility, target priority, etc.

The REM telescope: a robotic multiwavelength facility

The REM Observatory, recently installed and commissioned at la Silla Observatory Chile, is the first moderate aperture robotic telescope able to cover simultaneously the visible-NIR (0.45-2.3 microns) wavelength range. His very fast pointing and his full robotization makes it an ideal observing facility for fast transients. The high throughput Infrared Camera and the Visible imaging spectrograph simultaneously fed by a dichroic allows to collect high S/N data in an unprecedented large spectral range on a telescope of this size. The REM observatory is an example of a versatile and agile facility necessary complement to large telescopes in fileds in which rapid response and/or target pre-screening are necessary. We give in this paper an overview of the Observatory and its performances with emphasis to the innovative technical solution adopted to reach such performances.

The commissioning of the REM-IR camera at La Silla

During the early Summer 2003, the REM telescope has been installed at La Silla, together with the near infrared camera REM-IR and the optical spectrograph. ROSS. The REM project is a fully automated instrument to follow-up Gamma Ray Burst, triggered mainly by satellites, such as HETE II, INTEGRAL, AGILE and SWIFT. REM-IR will perform high efficiency imaging of the prompt infrared afterglow of GRB and, together with the optical spectrograph ROSS, will cover simultaneously a wide wavelength range, allowing a better understanding of the intriguing scientific case of GRB. In this paper we present the result of the commissioning phase of the near infrared camera REM-IR, lasted for an extended period of time and currently under the final fine tuning.

Coordinated observations of the red dwarf flare star EV LAC in 1994 and 1995

Abstract The results of photometric, spectral and radio studies of the flare star EV Lac we obtained during the course of cooperative observations in 1994 and 1995 are presented. A quantitative analysis of the radiation emitted by two powerful flares using the colour-colour diagram confirms the previous conclusion on the essential heterogeneity of matter radiating in optical flares. From simultaneous observations of the star in UBVRI, K and H bands no significant brightness variations in IR were found in coincidence with observed small-amplitude optical flares, except a gradual decrease in the K-band following a 1.0 mag flare in the U-band. A differential spottedness of bright and dark hemispheres of EV Lac is estimated using observations in separate seasons and the total stellar spottedness is determined within the framework of the zonal spottedness model. Spectra of quiet and active states of the star in the blue-green region in 1994 and in the red region in 1995 are described: the characteristics of th...

The REM optical slitless spectrograph (ROSS)

Fast ground based simultaneous optical-near infrared observation of gamma-ray bursts (GRBs) is a mandatory priority to understand the physical mechanisms at work in these objects. The REM (Rapid Eye Mount) telescope, recently installed at La Silla (ESO, Chile), is an example of a new generation of small robotic telescopes having the capability to allow simultaneous optical and near infrared photometry and low resolution spectroscopy. The REM Optical Slitless Spectrograph (ROSS) is the optical instrument mounted on REM. ROSS has been attached, in one of the two Nasmyth foci, orthogonally to the optical axis and receives the optical light deflected by a beam splitter (dichroic), which leaves the infrared beam to continue along the optical axis where the infrared camera (REM-IR) is installed. Low resolution optical spectroscopy is obtained using an Amici prism mounted on the same filter wheel where are also mounted the broad-band V, R, I photometric filters. The detector head is a commercial camera hosting a Marconi 1024×1024 CCD chip.

Goddard High Resolution Spectrograph Observations of Variability in the RS Canum Venaticorum System V711 Tauri (HR 1099)

Goddard High Resolution Spectrograph (GHRS) observations of the RS CVn-type binary V711 Tau (Kl IV+G5 IV) were obtained at several phases over two consecutive stellar orbital cycles in order to study ultraviolet emission-line profile and flux variability. Spectra cover the Mg II h and k lines, C IV doublet, and Si IV region, as well as the density-sensitive lines of C III] (1909 A) and Si III] (1892 A). IUE spectra, Extreme Ultra Violet (EUV) data, and Ultraviolet, Blue, Visual (UBV) photometry were obtained contemporaneously with the GHRS data. Variable extended wings were detected in the Mg II lines. We discuss the Mg II line profile variability using various Gaussian emission profile models. No rotational modulation of the line profiles was observed, but there were several large flares. These flares produced enhanced emission in the extended line wings, radial velocity shifts, and asymmetries in some line profiles. Nearly continuous flaring for more than 24 hr, as indicated in the IUE data, represents the most energetic and long-lived chromospheric and transition region flare ever observed with a total energy much greater than 5 x 10(exp 35) ergs. The C III] to Si III] line ratio is used to estimate the plasma density during the flares.

The REM-IR camera: High quality near infrared imaging with a small robotic telescope

We present the near infrared camera REM-IR that will operate aboard the REM telescope, intended as a fully automated instrument to follow-up Gamma Ray Burst, triggered mainly by satellites, such as HETE II, INTEGRAL, AGILE and SWIFT. REM-IR will perform high efficiency imaging of the prompt infrared afterglow of GRB and, together with the optical spectrograph ROSS, will cover simultaneously a wide wavelength range, allowing a better understanding of the intriguing scientific case of GRB. Due to the scientific and technological requirements of the REM project, some innovative solutions has been adopted in REM-IR.

Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars

We derive the properties of spotted regions in late-type active stars from V - and B -band photometric data. Specifically, we compare the amplitudes of spot-induced light and color variations with model amplitudes, which depend on the area, temperature and distribution of spotted regions over the stellar surface. Our analysis is applied to three well-known chromospherically active K dwarf stars: DX Leonis, AB Doradus and LQ Hydrae, and indicates that all three stars show significant variations of the average spot temperature from epoch to epoch. The observed temporal variations of the starspot temperatures are best explained as arising from the contemporary presence on the stellar photosphere of two-component (two-temperature) active regions. These consist of either dark spots and hot faculae and/or dark umbrae and penumbrae and the variations over time arise from change of their areal ratio.