Lisa A. Crause

V838 MONOCEROTIS: A GEOMETRIC DISTANCE FROM HUBBLE SPACE TELESCOPE POLARIMETRIC IMAGING OF ITS LIGHT ECHO*

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys on board the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a distance of 6.1 ± 0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be θmax = 90° ± 5°. The polarimetric distance agrees remarkably well with a distance of 6.2 ± 1.2 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this distance, V838 Mon at maximum light had MV −9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes.

Infrared study of the η Chamaeleontis cluster and the longevity of circumstellar discs

We have analysed JHKL observations of the stellar population of the ≈9 Myr-old η Chamaeleontis cluster. Using infrared (IR) colour‐colour and colour-excess diagrams, we find that the fraction of stellar systems with near-IR excess emission is 0.60 ± 0.13 (2σ ). This result implies considerably longer disc lifetimes than found in some recent studies of other young stellar clusters. For the classical T Tauri (CTT) and weak-lined T Tauri (WTT) star population, we also find a strong correlation between the IR excess and Hα emission. The IR excesses of these stars indicate a wide range of star‐disc activity: from a CTT star showing high levels of accretion to CTT‐WTT transition objects with evidence for some on-going accretion and WTT stars with weak or absent IR excesses. Of the 15 known cluster members, four stars with IR excesses � (K − L) > 0.4 mag are likely experiencing ongoing accretion owing to strong or variable optical emission. The resulting accretion fraction (0.27 ± 0.13; 2σ ) shows that the accretion phase, in addition to the discs themselves, can endure for at least ∼10 Myr.

The post-outburst photometric behaviour of V838 Mon

The unusual eruptive variable discovered in Monoceros in 2002 January underwent dramatic photometric and spectroscopic changes in the months prior to its 2002 June‐August conjunction with the Sun. Optical and infrared (IR) photometry obtained at the South African Astronomical Observatory (SAAO) between 2002 January and June (JD 245 2280‐440) is presented here in an analysis of the post-outburst behaviour of the star. The light curve indicated that three eruptions took place in 2002 January, February and March. SAAO echelle spectra obtained in the week prior to the March maximum indicated the ejection of a new shell of material. JHKL photometry obtained during 2002 April showed the development of an IR excess owing to the formation of a dust shell. The shell appears to be largely responsible for the rapid fade in the optical flux during 2002 April‐May (� V > 6 mag within 3 weeks). Blueing of the optical colours during the decline is probably due either to the revealing of an emissionline region surrounding V838 Mon, or to the unveiling of the progenitor or a spatially close early-type star.

Population and dynamical state of the η Chamaeleontis sparse young open cluster

We report new results in our continuing study of the unique compact (1 pc extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the B9 star η Chamaeleontis. An optical photometric survey spanning 1.3 × 1.3 pc adds two M5‐M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses M > 0.15 M� . The cluster now consists of 17 primaries and �9 secondaries lying within 100 au of their primaries. The apparent distribution of 9 :7:1 single : binary : triple systems shows 2‐4 times higher multiplicity than in the field main-sequence stars, and is comparable to that seen in other pre-main-sequence populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10 14

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.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 2002 outbursting event that produced a most spectacular light-echo. The reddening affecting V838 Mon is found to obey the RV = 3.1 law and amounts to (i) EB−V = 0.86 from the interstellar NaI and KI lines; (ii) EB−V = 0.88 from the energy distribution of the B3 V component; and (iii) EB−V = 0.87 from the progression of extinction along the line of sight. The adopted EB−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 B3 V 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 B3 V companion. The best fit is obtained for a 50 000 K source, 0.5 mag brighter than the B3 V companion. The latter passed unaffected through the outburst, which implies an orbital separation wide enough to avoid mass exchange during the evolution of the binary system, and to allow a safe comparison with theoretical isochrones for single stars. Such a comparison suggests that the progenitor of the outbursting component had an initial mass ∼65 M� ,t hat it was approaching the carbon ignition stage in its core at the time it erupted in 2002 and that the age of the V838 Mon binary system is close to 4 million yr. The 2002 event is probably just a shell thermonuclear event in the outer envelope of the star.

Rotation periods for stars of the TW Hydrae association: the evidence for two spatially and rotationally distinct pre-main-sequence populations

We have conducted a photometric study of late-type members of the TW Hydrae association (TWA) and measured the rotation periods for 16 stars in 12 systems. For TWA stars listed by Webb et al. and Sterzik et al. (TWA 1–13; led by TW Hya = TWA 1) we find a median period of 4.7 d. However, for stars that we measured in the TWA 14–19 group identified by Zuckerman et al., we find a median period of only 0.7 d. The period distributions of the two groups cannot be reconciled at the 3σ significance level. Using photometric arguments supported by the Hipparcos distance to HD 102458 (= TWA 19A), we find that TWA 14–19 reside at an average distance of d≈ 90 pc, spatially at the near boundary of the Lower Centaurus–Crux (LCC) subgroup of the Ophiuchus–Scorpius–Centaurus OB-star association. Proper motions for HD 102458, TWA 14, 18 and 19B link these stars to the LCC subgroup. From Hertzsprung–Russell diagram placement, we derive an age of ≈17 Myr for the HD 102458 system that may be the representative age for the TWA 14–19 group. Merging various lines of evidence, we conclude that these stars form a spatially and rotationally distinct population of older pre-main-sequence (PMS) stars, rather than being an extension of the TWA beyond those stars associated with TW Hya that have an age of ∼10 Myr and reside at d≈ 55 pc. Instead, TWA 14–19 likely represent the population of low-mass stars still physically associated with the LCC subgroup.

Photometry of 2006 RH120: an asteroid temporary captured into a geocentric orbit

Aims. From July 2006 to July 2007 a very small asteroid orbited the Earth within its Hill sphere. We used this opportunity to study its rotation and estimate its diameter and shape. Methods. Due to its faintness, 2006 RH120 was observed photometrically with the new 10-m SALT telescope at the SAAO (South Africa). We obtained data on four nights: 11, 15, 16, and 17 March 2007 when the solar phase angle remained almost constant at 74 ◦ . The observations lasted about an hour each night and the object was exposed for 7−10 s through the “clear” filter. Results. From the lightcurves obtained on three nights we derived two solutions for a synodical period of rotation: P1 = 1.375 ± 0.001 min and P2 = 2.750 ± 0.002 min. The available data are not sufficient to choose between them. The absolute magnitude of the object was found to be H = 29.9 ± 0.3 mag (with the assumed slope parameter G = 0.25) and its effective diameter D = 2− 7m , depending on the geometric albedo pV (with the most typical near-Earth asteroids albedo pV = 0.18 its diameter would be D = 3.3 ± 0.4 m). The body has an elongated shape with the a/b ratio greater than 1.4. It probably originates in low-eccentricity Amor or Apollo orbits. There is still a possibility, which needs further investigation, that it is a typical near-Earth asteroid that survived the aerobraking in the Earth’s atmosphere and returned to a heliocentric orbit similar to that of the Earth.

Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)

We report optical spectroscopy and high-speed photometry and polarimetry of the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry, polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1) h. Orbital circularly polarized modulations are seen from ∼0 to ∼−18 per cent, unambiguously identifying IGRJ14536-5522 as a polar. The negative circular polarization is seen over ∼95 per cent of the orbit, which is consistent (as viewed from the Earth) with a single-pole accretor. We estimate some of the system parameters by modelling the polarimetric observations. Some of the high-speed photometric data show modulations that are consistent with quasi-periodic oscillations (QPOs) on the order of 5–6 min. Furthermore, for the first time, we detect the (5–6) min QPOs in the circular polarimetry. We discuss the possible origins of these QPOs. In addition, we note that the source undergoes frequent changes between different accretion states. We also include details of HIgh-speed Photo-POlarimeter (HIPPO), a new high-speed photo-polarimeter, used for some of our observations. This instrument is capable of high-speed, multi-filtered, simultaneous all-Stokes observations. It is therefore ideal for investigating rapidly varying astronomical sources such as magnetic cataclysmic variables.

Abell 48 – a rare WN-type central star of a planetary nebula

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogendeficient. Almost all of these H-deficient central stars (CSs) display spectra with strong carbon and helium lines. Most of them exhibit emission-line spectra resembling those of massive WC stars. Therefore these stars are classed as CSPNe of spectral type [WC]. Recently, quantitative spectral analysis of two emission-line CSs, PB 8 and IC 4663, revealed that these stars do not belong to the [WC] class. Instead PB 8 has been classified as [WN/WC] type and IC 4663 as [WN] type. In this work we report the spectroscopic identification of another rare [WN] star, the CS of Abell 48. We performed a spectral analysis of Abell 48 with the Potsdam Wolf– Rayet (PoWR) models for expanding atmospheres. We find that the expanding atmosphere of Abell 48 is mainly composed of helium (85 per cent by mass), hydrogen (10 per cent) and nitrogen (5 per cent). The residual hydrogen and the enhanced nitrogen abundance make this object different from the other [WN] star IC 4663. We discuss the possible origin of this atmospheric composition.

Discovery of two new Galactic candidate luminous blue variables with Wide-field Infrared Survey Explorer

We report the discovery of two new Galactic candidate luminous blue variable (cLBV) stars via detection of circular shells (typical of known confirmed and cLBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 um in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered cLBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R- and I-bands by 0.68\pm0.10 mag and 0.61\pm0.04 mag, respectively, during the last 13-18 years, while the second one (known as Hen 3-1383) varies its B,V,R,I and K_s brightnesses by \simeq 0.5-0.9 mag on time-scales from 10 days to decades. We also found significant changes in the spectrum of Hen 3-1383 on a timescale of \simeq 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered cLBVs might be directly related to their possible runaway status.