Anomalous extinction behaviour towards the Type Ia SN 2003cg

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. The observations cover a period between −8.5 and +414 d post-maximum. SN 2003cg is a normal but highly reddened Type Ia event. Its B magnitude at maximum   B _max= 15.94 ± 0.04  and  Δ m ₁₅( B )_obs= 1.12 ± 0.04 [Δ m ₁₅( B )_intrinsic= 1.25 ± 0.05]  . Allowing   R_V   to become a free parameter within the Cardelli et al. extinction law, simultaneous matches to a range of colour curves of normal SNe Ia yielded   E ( B − V ) = 1.33 ± 0.11  , and   R_V = 1.80 ± 0.19  . While the value obtained for   R_V   is small, such values have been invoked in the past, and may imply a grain size which is small compared with the average value for the local interstellar medium.     

Globular cluster calibration of the peak brightness of the Type Ia supernova 1992A and the value of H0

We have determined the absolute magnitude at maximum light of SN 1992A by using the turnover magnitude of the globular cluster luminosity function of its parent galaxy, NGC 1380. A recalibration of the peak of the turnover magnitude of the Milky Way clusters using the latest Hipparcos results has been made with an assessment of the complete random and systematic error budget. The following results emerge: a distance to NGC 1380 of 18.6 ± 1.4 Mpc, corresponding to ( m  −  M )  31.35 ± 0.16, and an absolute magnitude of SN 1992A at maximum of M _B (max)  −18.79 ± 0.16. Taken at face value, SN 1992A seems to be more than half a magnitude fainter than the other SNe Ia for which accurate distances exist. We discuss the implications of this result and present possible explanations. We also discuss the Phillips relationship between rate of decline and the absolute magnitude at maximum, on the basis of nine SNe Ia, the individual distances of which have been obtained with Cepheids and the globular cluster luminosity function. The new calibration of this relationship, applied to the most distant SNe of the Calan–Tololo survey, yields H ₀ = 62 ± 6 km s⁻¹ Mpc⁻¹.     

A search for the progenitors of two Type Ia Supernovae in NGC 1316

Recent evidence of a young progenitor population for many Type Ia supernovae (SNe Ia) raises the possibility that evolved intermediate-mass progenitor stars may be detected in pre-explosion images. NGC 1316, a radio galaxy in the Fornax cluster, is a prolific producer of SNe Ia, with four detected since 1980. We analyse Hubble Space Telescope ( HST ) pre-explosion images of the sites of two of the SNe Ia that exploded in this galaxy, SN2006dd (a normal Type Ia) and SN2006mr (likely a subluminous, 1991bg-like, SN Ia). Astrometric positions are obtained from optical and near-infrared ground-based images of the events. We find no candidate point sources at either location, and set upper limits on the flux in B, V and I from any such progenitors. We also estimate the amount of extinction that could be present, based on analysis of the surface-brightness inhomogeneities in the HST images themselves. At the distance of NGC 1316, the limits correspond to absolute magnitudes of  ∼−5.5, −5.4  and −6.0 mag in   M _B , M _V   and   M _I   , respectively. Comparison to stellar evolution models argues against the presence at the supernova sites, 3 yr prior to the explosion, of normal stars with initial masses  ≳6 M_⊙  at the tip of their asymptotic-giant branch (AGB) evolution, young post-AGB stars that had initial masses  ≳4 M_⊙  and post-red giant stars of initial masses  ≳9 M_⊙  .     

A study of the Type II-P supernova 2003gd in M74

We present photometric and spectroscopic data of the Type II-P supernova (SN II-P) 2003gd, which was discovered in M74 close to the end of its plateau phase. SN 2003gd is the first Type II supernova (SN) to have a directly confirmed red supergiant (RSG) progenitor. We compare SN 2003gd to SN 1999em, a similar SN II-P, and estimate an explosion date of 2003 March 18. We determine a reddening towards the SN of   E ( B − V ) = 0.14 ± 0.06  , using three different methods. We also calculate three new distances to M74 of  9.6 ± 2.8, 7.7 ± 1.7  and  9.6 ± 2.2 Mpc  . The former was estimated using the standard candle method (SCM), for Type II supernovae (SNe II), and the latter two using the brightest supergiants method (BSM). When combined with existing kinematic and BSM distance estimates, we derive a mean value of  9.3 ± 1.8 Mpc  . SN 2003gd was found to have a lower tail luminosity compared with other normal Type II-P supernovae (SNe II-P) bringing into question the nature of this SN. We present a discussion concluding that this is a normal SN II-P, which is consistent with the observed progenitor mass of  8⁺⁴₋₂ M_⊙  .     

The template type Ia supernova 1996X

UBVRIJ photometry and optical spectra of the type Ia SN 1996X obtained at the European Southern Observatory (ESO) during a 1-yr-long observational campaign are presented, and supplemented by late-time Hubble Space Telescope (HST) photometry. Spectroscopically, SN 1996X appears to be a 'normal' SN Ia. The apparent magnitude at maximum was     and the colour     The luminosity decline rate,     is close to average for a SN Ia. The best estimate of the galactic extinction is     and there is evidence that reddening within the parent galaxy is negligible.
Detailed comparison of the light and colour curves of various 'normal' SNe Ia shows that the assumption that multicolour light curves can be described simply as a one-parameter family is not perfect. Together with problems in the calibration of the templates, this may explain the discrepancies in the distance modulus derived adopting different calibrations of the absolute magnitude versus light-curve shape relations. Indeed, we found that M _B ranges from −19.08 to −19.48 and μ ranges from 32.02 to 32.48 depending on the method used.
Computations of model light-curve and synthetic spectra for both early and late times confirm that 1996X is a normal type Ia SN and that a satisfactory fit can be obtained using a W7 progenitor structure only if we adopt the short distance. A larger distance would imply too large a Ni mass for this fainter than average SN Ia.     

Supernova 1994aj: a probe for pre-supernova evolution and mass-loss from the progenitor

Extensive photometric and spectroscopic observations of SN 1994aj until 540 d after maximum light have been obtained. The photometry around maximum suggests that the SN belongs to the Type II Linear class, with a peak absolute magnitude of M _V∼−17.8 (assuming H ₀=75 km s⁻¹ Mpc⁻¹). The spectra of SN 1994aj were unusual, with the presence of a narrow line with a P Cygni profile on top of the broad Balmer line emission. This narrow feature is attributed to the presence of a dense superwind surrounding the SN. At 100–120 d after maximum light the SN ejecta start to interact with this circumstellar material. The SN luminosity decline rates slowed down [γ _R =0.46 mag (100 d)⁻¹], becoming less steep than the average late luminosity decline of normal SN II [∼1 mag (100 d)⁻¹]. This dense ( ˙M / u _W∼10¹⁵ g cm⁻¹) wind was confined to a short distance from the progenitor ( R _out=∼5×10¹⁶ cm), and results from a very strong mass-loss episode ( ˙M =10⁻³ M⊙ yr⁻¹), which terminated shortly before explosion (∼5–10 yr).     

Orbital solution and fundamental parameters of σ Scorpii

The first orbital solution for the spectroscopic pair in the multiple star system σ Scorpii, determined from measurements with the Sydney University Stellar Interferometer, is presented. The primary component is of β Cephei variable type and has been one of the most intensively studied examples of its class. The orbital solution, when combined with radial velocity results found in the literature, yields a distance of  174⁺²³₋₁₈ pc  , which is consistent with, but more accurate than the Hipparcos value. For the primary component we determine  18.4 ± 5.4 M_⊙, −4.12 ± 0.34 mag  and  12.7 ± 1.8 R_⊙  for the mass, absolute visual magnitude and radius, respectively. A B1 dwarf spectral type and luminosity class for the secondary is proposed from the mass determination of  11.9 ± 3.1 M_⊙  and the estimated system age of 10 Myr.     

Characteristics of the supernova remnant G351.7+0.8 and a distance argument against its association with PSR J1721−3532

New images of the supernova remnant (SNR) G351.7+0.8 are presented based on 21-cm H  i -line emission and continuum emission data from the Southern Galactic Plane Survey. SNR G351.7+0.8 has a flux density of 8.4 ± 0.7 Jy at 1420 MHz. Its spectral index is 0.52 ± 0.25 ( S = v ^−α) between 1420 and 843 MHz, typical of adiabatically expanding shell-like remnants. H  i observations show structures possibly associated with the SNR in the radial velocity range of −10 to −18 km s⁻¹, and suggest a distance of 13.2 kpc and a radius of 30.7 pc. The estimated Sedov age for G351.7+0.8 is less than  6.8×10⁴ yr  . A young radio pulsar PSR J1721−3532 lies close to SNR G351.7+0.8 on the sky. The new distance and age of G351.7+0.8 and recent proper motion measurements of the pulsar strongly argue against an association between SNR G351.7+0.8 and PSR J1721−3532. There is an unidentified, faint X-ray point source 1RXS J172055.3−353937 which is close to G351.7+0.8. This may be a neutron star potentially associated with G351.7+0.8.     

Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster ω Centauri as standard candles

New photometry of RRab and RRc stars in ω Centauri is used to calibrate their absolute magnitudes M_V as a function of (a) metallicity and (b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the Cluster AgeS Experiment (CASE) project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables, we obtained a relation of   M_V = (0.26 ± 0.08)[ Fe/H ] + (0.91 ± 0.13)  . A dereddened distance modulus to the Large Magellanic Cloud (LMC) based on that formula is  μ₀= 18.56 ± 0.14 mag  . The second calibration of M_V , which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of  μ₀= 18.51 ± 0.07 mag  .     

Optical and infrared observations of the Centaur 1997 CU26

Minor planet 1997 CU₂₆ is a Centaur, and is probably undergoing dynamical evolution inwards from the Kuiper Belt. We present optical and infrared ( VRIJHK ) photometry which gives mean colours of V − R =0.46±0.02, V − I =1.02±0.02, V − J =1.74±0.02, V − H =2.15±0.02 and V − K =2.25±0.02. The resulting relative reflectance spectrum lies between those of Chiron and Pholus (although closer to that of Chiron). A 1.6–2.6 μm spectrum confirms the broad absorption feature at 2.05 μm associated with water ice reported by Brown et al. 1997 CU₂₆ displays no significant light curve variation and (unlike Chiron) has no observable coma. We place an upper limit to the dust production rate of 1.5 kg s⁻¹. J -band data taken at phase angles of 1.°7 to 4.°0 give a phase parameter of G _J =0.36±0.1, and are consistent with a phase parameter of G =0.15 in the V band (a value often assigned to low-albedo objects when no other information is available) if we assume a phase reddening of 0.017 mag deg⁻¹ in the J band. We find V (1, α =4.°1) =7.022±0.02, from which we deduce, by assuming G =0.15±0.1, an absolute visual magnitude of H _V =6.64±0.04.     

Supernova 2002ap: the first month

Supernova (SN) 2002ap in M74 was discovered on 2002 January 29. Being one of the nearest (10 Mpc) SN events in the last decades, and spectroscopically similar to the so-called 'hypernovae' 1997ef and 1998bw, both possibly associated with gamma-ray bursts (GRBs), it is of great interest. Shortly after its discovery, we launched an intensive photometric and spectroscopic monitoring campaign of this event, and here we report the results of the first month of observations. We use our UBVRI photometry to estimate the magnitudes at, and dates of, peak brightness. Our data suggest that this object reached its peak B -band luminosity on February     . Based on its similarity to SN 1998bw, we estimate the range of possible dates for a GRB that may have been associated with SN 2002ap. We find that it may include dates outside the time frame for which all available gamma-ray data have been intensively scanned, according to recent reports. The absolute magnitude at peak brightness of SN 2002ap  ( M _B =-16.9)  shows that it was significantly fainter than SN 1998bw, or normal Type Ia SNe, but similar to SN 1997ef. Our spectroscopic observations confirm that SN 2002ap is strikingly similar to SNe 1998bw and 1997ef. We briefly describe the spectral evolution of this object. To assist other observers and to stimulate theoretical models, we make our entire data set publicly available in digital form (http://wise-obs.tau.ac.il/∼avishay/local.html).     

The Cepheid distance to M96 and the Hubble constant

HST WFPC2 observations of Cepheids in M96 (NGC 3368) are used to find a distance to that galaxy of 11.2±1.0  Mpc. This estimate is based on a calibration of the Cepheid period–luminosity relation in the Large Magellanic Cloud, and includes a correction for the difference in metallicity between the two systems. There are good reasons for believing M96 is at the same distance as four E/S0 galaxies in the Leo-I group, and hence we calibrate secondary distance indicators based on the early-type galaxies, namely the fundamental plane and surface-brightness-fluctuation method. Also the Type Ia supernova 1998bu occurred in M96 itself and is used to calibrate the SN Ia distance scale. These methods reach to recession velocities of greater than 5000 km s⁻¹ and can therefore allow us to evaluate the Hubble constant without reference to the peculiar velocity of M96 itself. In fact, these indicators agree well between themselves and hence we find H ₀=67±7 km s⁻¹ Mpc⁻¹ where the quoted error includes estimates of potential systematic effects.     

Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio

We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as   L _X∝ t ^−0.4  . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of  ∼2 × 10⁻⁶ M_⊙ yr⁻¹  . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of ⁵⁶Ni to be  0.06 ± 0.03 M_⊙  . Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of  (0.98 ± 0.25) × 10⁵¹ erg  . We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of  ∼20 M_⊙  for SN 2004et.     

The absolute infrared magnitudes of type Ia supernovae

The absolute luminosities and homogeneity of early-time infrared (IR) light curves of type Ia supernovae are examined. Eight supernovae are considered. These are selected to have accurately known epochs of maximum blue light as well as having reliable distance estimates and/or good light curve coverage. Two approaches to extinction correction are considered. Owing to the low extinction in the IR, the differences in the corrections via the two methods are small. Absolute magnitude light curves in the J , H and K bands are derived. Six of the events, including five established 'branch-normal' supernovae, show similar coeval magnitudes. Two of these, supernovae (SNe) 1989B and 1998bu, were observed near maximum infrared light. This occurs about 5 d before maximum blue light. Absolute peak magnitudes of about −19.0, −18.7 and −18.8 in J , H and K respectively were obtained. The two spectroscopically peculiar supernovae in the sample, SNe 1986G and 1991T, also show atypical IR behaviour. The light curves of the six similar supernovae can be represented fairly consistently with a single light curve in each of the three bands. In all three IR bands the dispersion in absolute magnitude is about 0.15 mag, and this can be accounted for within the uncertainties of the individual light curves. No significant variation of absolute IR magnitude with B -band light curve decline rate, Δ m ₁₅( B ), is seen over the range 0.87<Δ m ₁₅( B )<1.31. However, the data are insufficient to allow us to decide whether or not the decline rate relation is weaker in the IR than in the optical region. IR light curves of type Ia supernovae should eventually provide cosmological distance estimates that are of equal, or even superior, quality to those obtained in optical studies.     

Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627

Charge-coupled device (CCD) photometry in the Johnson V , Kron–Cousins I and Washington CMT ₁ systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of  [Fe/H]=−0.12 ± 0.08  , which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour–magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the ( V , V − I ) and  ( T ₁, C − T ₁)  diagrams, we estimate   E ( V − I ) = 0.25 ± 0.05  and   V − M_V = 11.80 ± 0.25  for  log  t = 9.15  , and   E ( C − T ₁) = 0.23 ± 0.07  and   T ₁− M _{T 1}= 11.85 ± 0.25  for  log  t = 9.10  , respectively, assuming solar metal content. The derived reddening value   E ( C − T ₁)  implies   E ( B − V ) = 0.12 ± 0.07  and a distance from the Sun of  2.0 ± 0.4 kpc  . Using the WEBDA data base and the available literature, we re-examined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7–0.8, 1.0–1.1, 1.6–1.7 and 2.0–2.1 Gyr, separated by relative quiescent epochs of ∼0.2–0.3 Gyr. We also estimated a radial abundance gradient of  −0.08 ± 0.02  , which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found.     

Optical and infrared photometry of the Type IIn SN 1998S: days 11–146

We present contemporaneous optical and infrared (IR) photometric observations of the Type IIn SN 1998S covering the period between 11 and 146 d after discovery. The IR data constitute the first ever IR light curves of a Type IIn supernova. We use blackbody and spline fits to the photometry to examine the luminosity evolution. During the first 2–3 months, the luminosity is dominated by the release of shock-deposited energy in the ejecta. After ∼100 d the luminosity is powered mostly by the deposition of radioactive decay energy from 0.15±0.05 M_⊙ of ⁵⁶Ni which was produced in the explosion. We also report the discovery of an astonishingly high IR excess, K − L '=2.5, that was present at day 130. We interpret this as being due to thermal emission from dust grains in the vicinity of the supernova. We argue that to produce such a high IR luminosity so soon after the explosion, the dust must be pre-existing and so is located in the circumstellar medium of the progenitor. The dust could be heated either by the UV/optical flash (IR echo) or by the X-rays from the interaction of the ejecta with the circumstellar material.     

Optical evolution of Nova Ophiuchi 2007 = V2615 Oph

The moderately fast Nova Oph 2007 reached maximum brightness on 2007 March 28 at   V = 8.52, B − V =+1.12, V − R _C=+0.76, V − I _C=+1.59  and   R _C− I _C=+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  d. The distance to the nova is   d = 3.7 ± 0.2 kpc  , the height above the Galactic plane is   z = 215 pc  , the reddening is   E ( B − V ) = 0.90  and the absolute magnitude at maximum is   M ^max _V =−7.2  and   M ^max _B =−7.0  . The spectrum four days before maximum resembled a F6 supergiant, in an agreement with broad-band colours. It later developed into that of a standard 'Fe  ii '-class nova. Nine days past maximum, the expansion velocity estimated from the width of Hα emission component was  ∼730 km s⁻¹  , and the displacement from it of the principal and diffuse-enhanced absorption systems was ∼650 and  1380 km s⁻¹  , respectively. Dust probably formed and disappeared during the period from 82 to 100 d past maximum, causing (at peak dust concentration) an extinction of  Δ B = 1.8  mag and an extra  Δ E ( B − V ) = 0.44  reddening.     

SN 1998A: explosion of a blue supergiant

We present spectroscopic and photometric observations of the peculiar Type II supernova (SN) 1998A. The light curves and spectra closely resemble those of SN 1987A, suggesting that the SN 1998A progenitor exploded when it was a compact blue supergiant. However, the comparison with SN 1987A also highlights some important differences: SN 1998A is more luminous and the spectra show bluer continua and larger expansion velocities at all epochs. These observational properties indicate that the explosion of SN 1998A is more energetic than SN 1987A and more typical of Type II supernovae. Comparing the observational data with simulations, we deduce that the progenitor of SN 1998A was a massive star  (∼25 M_⊙)  with a small pre-supernova radius  (≲6 × 10¹² cm)  . The Ba  ii lines, unusually strong in SN 1987A and some faint II-P events, are almost normal in the case of SN 1998A, indicating that the temperature plays a key role in determining their strength.     

NGC 2587: a sparse open cluster projected on to a populous star field

We present CCD photometry in the Johnson U , B and V and Kron–Cousins I passbands for the open cluster NGC 2587. The sample consists of 4406 stars reaching down to   V ∼ 21.0  . We developed a new method to clean statistically the colour–magnitude diagrams. NGC 2587 appears to be a sparse, relatively bright open cluster, with a few tens of members projected on to a populous star field. The comparatively bright F7/8 II type star HD 70927, located close to the cluster centre, seems not to be a member. Our analysis suggests that NGC 2587 is slightly younger than the Hyades and probably of solar metallicity. A cluster radius of roughly 8 arcmin was estimated from the radial stellar density profile. From 18 probable cluster members with measured proper motions, we derive the following mean values for NGC 2587:  μ_α=−4.3 ± 3.6 mas yr⁻¹  and  μ_δ=−2.5 ± 3.4 mas yr⁻¹  . Adopting the theoretical metal content   Z = 0.02  , which provides the best global fit, we derive a cluster age of  500⁺⁶⁰₋₅₀  . Simultaneously, colour excesses   E ( B − V ) = 0.10  and   E ( V − I ) = 0.15  and an apparent distance modulus of   V − M_V = 12.50  are obtained. The interstellar extinction in the cluster direction is found to follow the normal law. NGC 2587 is located at a distance of (2.70 ± 0.70) kpc from the Sun and ∼9.8 kpc from the Galactic centre.     

Orbital parameters, masses and distance to β Centauri determined with the Sydney University Stellar Interferometer and high-resolution spectroscopy

The bright southern binary star β Centauri (HR 5267) has been observed with the Sydney University Stellar Interferometer (SUSI) and spectroscopically with the European Southern Observatory Coude Auxiliary Telescope and Swiss Euler telescope at La Silla. The interferometric observations have confirmed the binary nature of the primary component and have enabled the determination of the orbital parameters of the system. At the observing wavelength of 442 nm the two components of the primary system have a magnitude difference of  0.15 ± 0.02  . The combination of interferometric and spectroscopic data gives the following results: orbital period  357.00 ± 0.07 d  , semimajor axis  25.30 ± 0.19 mas  , inclination  674 ± 03  , eccentricity  0.821 ± 0.003  , distance  102.3 ± 1.7 pc  , primary and secondary masses   M ₁= M ₂= 9.1 ± 0.3 M_⊙  and absolute visual magnitudes of the primary and secondary   M _{1 V} =−3.85 ± 0.05  and   M _{2 V} =−3.70 ± 0.05  , respectively. The high degree of accuracy of the results offers a fruitful starting point for future asteroseismic modelling of the pulsating binary components.