arXiv subjects

A. Henden

Publications and source records attributed to A. Henden.

At least 37 records · Page 2Linked to original sources

arXiv2009

An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: II. Changes in the physical parameters

The analysis of the multicolour photometric observations of MW Lyr, a large modulation amplitude Blazhko variable, shows for the first time how the mean global physical parameters vary during the Blazhko cycle. About 1-2 percent changes in the mean radius, luminosity and surface effective temperature are detected. The mean radius and temperature changes are in good accordance with pulsation model results, which show that these parameters do indeed vary within this order of magnitude if the amplitude of the pulsation changes significantly. We interpret the phase modulation of the pulsation to be a consequence of period changes. Its magnitude corresponds exactly what one expects from the detected changes of the mean radius assuming that the pulsation constant remains the same during the modulation. Our results indicate that during the modulation the pulsation remains purely radial, and the underlying mechanism is most probably a periodic perturbation of the stellar luminosity with the modulation period.

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arXiv2008

The nature and evolution of Nova Cygni 2006

AIMS: Nova Cyg 2006 has been intensively observed throughout its full outburst. We investigate the energetics and evolution of the central source and of the expanding ejecta, their chemical abundances and ionization structure, and the formation of dust. METHOD: We recorded low, medium, and/or high-resolution spectra (calibrated into accurate absolute fluxes) on 39 nights, along with 2353 photometric UBVRcIc measures on 313 nights, and complemented them with IR data from the literature. RESULTS: The nova displayed initially the normal photometric and spectroscopic evolution of a fast nova of the FeII-type. Pre-maximum, principal, diffuse-enhanced, and Orion absorption systems developed in a normal way. After the initial outburst, the nova progressively slowed its fading pace until the decline reversed and a second maximum was reached (eight months later), accompanied by large spectroscopic changes. Following the rapid decline from second maximum, the nova finally entered the nebular phase and formed optically thin dust. We computed the amount of formed dust and performed a photo-ionization analysis of the emission-line spectrum during the nebular phase, which showed a strong enrichment of the ejecta in nitrogen and oxygen, and none in neon, in agreement with theoretical predictions for the estimated 1.0 Msun white dwarf in Nova Cyg 2006. The similarities with the poorly investigated V1493 Nova Aql 1999a are discussed.

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arXiv2008

An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: I. Light curve solution

We have obtained the most extensive and most accurate photometric data of a Blazhko variable MW Lyr during the 2006-2007 observing seasons. The data within each 0.05 phase bin of the modulation period (P_m=1/f_m) cover the entire light cycle of the primary pulsation period (P_0=1/f_0), making possible a very rigorous and complete analysis. The modulation period is found to be 16.5462 d, which is about half of that was reported earlier from visual observations. Previously unknown features of the modulation have been detected. Besides the main modulation frequency f_m, sidelobe modulation frequencies around the pulsation frequency and its harmonics appear at +/- 2 f_m, +/- 4 f_m, and +/- 12.5 f_m separations as well. Residual signals in the prewhitened light curve larger than the observational noise appear at the minimum-rising branch-maximum phase of the pulsation, which most probably arise from some stochastic/chaotic behaviour of the pulsation/modulation. The Fourier parameters of the mean light curve differ significantly from the averages of the Fourier parameters of the observed light curves in the different phases of the Blazhko cycle. Consequently, the mean light curve of MW Lyrae never matches its actual light variation. The Phi_21, Phi_31 phase differences in different phases of the modulation show unexpected stability during the Blazhko cycle. A new phenomenological description of the light curve variation is defined that separates the amplitude and phase (period) modulations utilising the phase coherency of the lower order Fourier phases.

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arXiv2008

Optical evolution of Nova Ophiuchi 2007 = V2615 Oph

The moderately fast Nova Oph 2007 reached maximum brightness on March 28, 2007 at V=8.52, B-V=+1.12, V-Rc=+0.76, V-Ic=+1.59 and Rc-Ic=+0.83, after fast initial rise and a pre-maximum halt lasting a week. Decline times were t(V,2)=26.5, t(B,2)=30, t(V,3)=48.5 and t(B,3)=56.5 days. The distance to the nova is d=3.7 kpc, the height above the galactic plane z=215 pc, the reddening E(B-V)=0.90 and the absolute magnitude at maximum M(V,max)=-7.2 and M(B,max)=-7.0. The spectrum four days before maximum resembled a F6 super-giant, in agreement with broad-band colors. It later developed into that of a standard 'FeII'-class nova. Nine days past maximum, the expansion velocity estimated from the width of H$α$ emission component was 730 km/s, and the displacement from it of the principal and diffuse enhanced absorption systems were 650 and 1380 km/s, respectively. Dust probably formed and disappeared during the period from 82 to 100 days past maximum, causing (at peak dust concentration) an extinction of Delta B=1.8 mag and an extra Delta E(B-V)=0.44 reddening.

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arXiv2006

V1647 Ori: The X-ray Evolution of a Pre-main Sequence Accretion Burst

We present Chandra X-ray Observatory monitoring observations of the recent accretion outburst displayed by the pre-main sequence (pre-MS) star V1647 Ori. The X-ray observations were obtained over a period beginning prior to outburst onset in late 2003 and continuing through its apparent cessation in late 2005, and demonstrate that the mean flux of the spatially coincident X-ray source closely tracked the near-infrared luminosity of V1647 Ori throughout its eruption. We find negligible likelihood that the correspondence between X-ray and infrared light curves over this period was the result of multiple X-ray flares unrelated to the accretion burst. The recent Chandra data confirm that the X-ray spectrum of V1647 Ori hardened during outburst, relative both to its pre-outburst state and to the X-ray spectra of nearby pre-MS stars in the L1630 cloud. We conclude that the observed changes in the X-ray emission from V1647 Ori over the course of its 2003-2005 eruption were generated by a sudden increase and subsequent decline in its accretion rate. These results for V1647 Ori indicate that the flux of hard X-ray emission from erupting low-mass, pre-MS stars, and the duration and intensity of such eruptions, reflect the degree to which star-disk magnetic fields are reorganized before and during major accretion events.

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arXiv2006

The Recently-Discovered Dwarf Nova System ASAS J002511+1217.2: A New WZ Sagittae Star

The cataclysmic variable ASAS J002511+1217.2 was discovered in outburst by the All-Sky Automated Survey in September 2004, and intensively monitored by AAVSO observers through the following two months. Both photometry and spectroscopy indicate that this is a very short-period system. Clearly defined superhumps with a period of 0.05687 +/- 0.00001 days (1-sigma) are present during the superoutburst, 5 to 18 days following the ASAS detection. We observe a change in superhump profile similar to the transition to ``late superhumps'' observed in other short-period systems; the superhump period appears to increase slightly for a time before returning to the original value, with the resulting superhump phase offset by approximately half a period. We detect variations with a period of 0.05666 +/- 0.00003 days (1-sigma) during the four-day quiescent phase between the end of the main outburst and the single echo outburst. Weak variations having the original superhump period reappear during the echo and its rapid decline. Time-resolved spectroscopy conducted nearly 30 days after detection and well into the decline yields an orbital period measurement of 82 +/- 5 minutes. Both narrow and broad components are present in the emission line spectra, indicating the presence of multiple emission regions. The weight of the observational evidence suggests that ASAS J002511+1217.2 is a WZ Sge-type dwarf nova, and we discuss how this system fits into the WZ classification scheme.

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arXiv2005

The triple-mode pulsating variable V823 Cas

Based on extended multicolour CCD photometry of the triple-mode radial pulsator V823 Cas we studied the properties of the coupling frequencies invoked by nonlinear processes. Our results support that a resonance connection as suggested by Antonello & Aikawa (1998) affects the mode coupling behaviour. The P1/P0 period ratio of V823 Cas has an "out of range" value if compared with the period ratios of the known double mode pulsators, while the P2/P1 period ratio is normal. The periods and period ratios cannot be consistently interpret without conflict with pulsation and/or evolution models. We attempt to interpret this failure by the suggestion that at present, the periods of V823 Cas are in a transient, resonance affected state, thus do not reflect the true parameters of the object. The anomalous period change behaviour of the fundamental and second overtone modes supports this idea. We have also raised the possibility that a f0 + f2 = 2f1 resonance may act in triple mode pulsators.

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arXiv2005

Bipolar jets growth and decline in Hen 3-1341: a direct link to fast wind and outburst evolution

The appearance and disappearance of collimated bipolar jets in the symbiotic star Hen 3-1341 is reported and investigated. From modeling of the emission line spectrum it turns out that the accreting white dwarf in quiescence has T_WD ~ 1.2 10(5) K and R_WD ~ 0.14 Rsun, for a luminosity of 3.8 10(3) Lsun, and it is stably burning hydrogen on the surface at a rate of 5 10(-8) Msun/yr, feeding ionizing photons to a radiation bounded circumstellar nebula extending for ~ 17 AU. The WD underwent a multi-maxima outburst lasting from 1998 to 2004 during which its H-burning envelope reacted to a probable small increase in the mass accretion by expanding and cooling to T_eff ~ 1 10(4) K and R ~ 20 Rsun, mimicking an A-type giant that radiated a total of 6 10(44) erg during the outburst, at an average rate of 1 10(3) Lsun. Bipolar jets developed at the time of outburst maximum and their strength declined in parallel with the demise of a fast wind from the WD, finally disappearing when the wind stopped halfway to quiescence, marking a 1:1 correspondence between jets presence and feeding action of the fast wind. The total mass in the jets was M_jet ~ 2.5 10(-7) Msun for a kinetic energy of E_jet ~ 1.7 10(42)/(sin i) erg, corresponding to 0.3/(sin i) percent of the energy radiated during the whole outburst.

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arXiv2005

On the distance, reddening and progenitor of V838 Mon

Extensive optical and infrared photometry as well as low and high resolution spectroscopy are used as inputs in deriving robust estimates of the reddening, distance and nature of the progenitor of V838 Mon. The reddening is found to obey the R_V=3.1 law and amounts to (i) E(B-V)=0.86 from the interstellar NaI and KI lines, (ii) E(B-V)=0.88 from the energy distribution of the B3V component and (iii) E(B-V)=0.87 from the progression of extinction along the line of sight. The adopted E(B-V)=0.87(+/-0.01) is also the amount required by fitting the progenitor with theoretical isochrones of appropriate metallicity. The distance is estimated from (a) the galactic kinematics of the three components of the interstellar lines, (b) the amount of extinction vs the HI column density and vs the dust emission through the whole Galaxy in that direction, from (c) spectrophotometric parallax to the B3V companion, from (d) comparison of the observed color-magnitude diagram of field stars with 3D stellar population models of the Galaxy, from (e) comparison of theoretical isochrones with the components of the binary system in quiescence and found to be around 10 kpc. Pre-outburst optical and IR energy distributions show that the component erupting in 2002 was brighter and hotter than the B3V companion. The best fit is obtained for a 50,000 K source, 0.5 mag brighter than the B3V companion. Comparison with theoretical isochrones suggests an age of 4 million year for the system and a mass around 65 M(sun) for the progenitor of the outbursting component, which at the time of the outburst was approaching the Carbon ignition stage in its core. The 2002 event is probably just a shell thermonuclear event in the outer envelope of the star.

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arXiv2005

The first three years of the outburst and light-echo evolution of V838 Mon and the nature of its progenitor

V838 Mon has undergone one of the most mysterious stellar outbursts on record, with (a) a large amplitude (Delta B ~ 10 mag) and multi-maxima photometric pattern, (b) a cool spectral type at maximum becoming cooler and cooler with time during the descent, until it reached the never-seen-before realm of L-type supergiants, never passing through optically thin or nebular stages, (c) the development of a spectacular, monotonically expanding light-echo in the circumstellar material, and (d) the identification of a massive and young B3V companion, unaffected by the outburst. In this talk we review the photometric and spectroscopic evolution during the first three full years of outburst, the light-echo development and infer the nature of the progenitor, which was brighter and hotter in quiescence than the B3V companion and with an inferred ZAMS mass of about 65 Msun.

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arXiv2004

Planetary Transits of TRES-1

Observations of TRES-1b transits made during the late summer and fall 2004 observing season reveal a statistically significant but low amplitude brightening event during egress.

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arXiv2004

A New Cataclysmic Variable in Hercules

We present time-series observations, spectra and archival outburst data of a newly-discovered variable star in Hercules, Var Her 04. Its orbital period, mass ratio, and outburst amplitude resemble those of the UGWZ-type subclass of UGSU dwarf novae. However, its supercycle and outburst light curve defy classification as a clear UGWZ. Var Her 04 is most similar to the small group of possible hydrogen-burning ``period bouncers'', dwarf novae that have passed beyond the period minimum and returned.

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arXiv2004

High-Resolution NIR Observations of the Circumstellar Disk System in the Bok Globule CB 26

We report on results of near-infrared and optical observations of the mm disk embedded in the Bok globule CB 26 (Launhardt & Sargent 2001). The near-infrared images show a bipolar reflection nebula with a central extinction lane which coincides with the mm disk. Imaging polarimetry of this object yielded a polarization pattern which is typical for a young stellar object surrounded by a large circumstellar disk and an envelope, seen almost edge-on. The strong linear polarization in the bipolar lobes is caused by single scattering at dust grains and allowed to locate the illuminating source which coincides with the center of the mm disk. The spectral energy distribution of the YSO embedded in CB 26 resembles that of a ClassI source with a luminosity of 0.5 L_sun.Using the pre-main-sequence evolutionary tracks and the stellar mass inferred from the rotation curve of the disk, we derive an age of the system of <10^6 yr. H_alpha and [SII] narrow-band imaging as well as optical spectroscopy revealed an Herbig-Haro object 6.15 arcmin northwest of CB 26 YSO 1, perfectly aligned with the symmetry axis of the bipolar nebula. This Herbig-Haro object (HH 494) indicates ongoing accretion and outflow activity in CB 26 YSO 1. Its excitation characteristics indicate that the Herbig-Haro flow is propagating into a low-density environment. We suggest that CB 26 YSO 1 represents the transition stage between embedded protostellar accretion disks and more evolved protoplanetary disks around T Tauri stars in an undisturbed environment.

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arXiv2004

An X-ray Outburst from the Rapidly Accreting Young Star That Illuminates McNeil's Nebula

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of such emission is uncertain. Although many or perhaps most recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor ~50 increase in the X-ray flux from a young star that is presently undergoing a spectacular optical/IR outburst. The outburst is thought to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/IR eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion- enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent accretion flood.

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arXiv2004

The host of GRB 030323 at z=3.372: a very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lya (DLA) absorption and low- and high-ionization lines at a redshift z=3.3718+-0.0005. The inferred neutral hydrogen column density, log N(HI)=21.90+-0.07, is larger than any (GRB- or QSO-) DLA HI column density inferred directly from Lya in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: A(V)<0.5 mag. The iron abundance is [Fe/H]=-1.47+-0.11, while the metallicity of the gas as measured from sulphur is [S/H]=-1.26+-0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2)+N(HI))<~10^-6. In the Lya trough, a Lya emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Msun per year. All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, SiII*, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the HI volume density to be n_HI=100-10000 cm^-3. HST/ACS imaging 4 months after the burst shows an extended AB(F606W)=28.0+-0.3 mag object at a distance of 0.14" (1kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.

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arXiv2003

The polarization evolution of the optical afterglow of GRB 030329

We report 31 polarimetric observations of the afterglow of GRB 030329 with high signal-to-noise and high sampling frequency. The data imply that the afterglow magnetic field has small coherence length and is mostly random, probably generated by turbulence.

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arXiv2003

Evolution of the polarization of the optical afterglow of the gamma-ray burst GRB 030329

We report 31 polarimetric observations of the afterglow of GRB 030329 with high signal-to-noise and high sampling frequency. We establish the polarization light curve, detect sustained polarization at the percent level, and find significant variability of polarization degree and angle. The data imply that the afterglow magnetic field has small coherence length and is mostly random, probably generated by turbulence.

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arXiv2003

Polarimetric evolution of V838 Mon

We present the results of our polarimetric and spectropolarimetric monitoring of V838 Monocerotis, performed at Asiago and Crimean observatories during and after the multiple outbursts that occurred in January-March 2002. The polarization of the object is mainly due to interstellar polarization (P=2.48%). Intrinsic polarization up to 0.7% at 5000 A is present during the second maximum of the object (February 2002). This intrinsic component increases toward shorter wavelengths but our limited spectral coverage (5000-7500 A) does not allow conclusive inferences about its origin. A strong depolarization across the Halpha profile is observed. The interstellar polarization gives a lower limit to the reddening of E(B-V)>0.28, with E(B-V)=0.5 being the most probable value. A normal ratio of total to selective absorption (R_V=3.22+-0.17) was derived from the wavelength of maximum interstellar polarization. This suggests a low (if any) contribution by circumstellar material with peculiar dust to gas ratio. A polarimetric map of a portion of the light echo shows a complex polarization distribution reaching P_max=45%.

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