We present an analysis of the evolution of circumstellar dust and molecules in the environment of the very late thermal pulse object V4334 Sgr (Sakurai's Object) over a $\sim20$-year period, drawing on ground-, airborne- and space-based infrared photometry and spectroscopy. The dust emission, which started in 1997, resembles a blackbody that cooled from $\sim1200$K in 1998 August to $\sim180$K in 2016 July. The dust mass, assuming amorphous carbon, was $\sim5\times10^{-10}$M$_\odot$ in 1998 August, and we estimate that the total dust mass was $\sim2\times10^{-5}$M$_\odot$ by $\sim2016$. The appearance of a near infrared excess in 2008 suggests a new episode of (or renewed) mass loss began then. We infer lower limits on the bolometric luminosity of the embedded star from that of the dust shell, which rose to $\sim16000$L$_\odot$ before declining to $\sim3000$L$_\odot$. There is evidence for weak 6-7$μ$m absorption, which we attribute to hydrogenated amorphous carbon formed in material ejected by Sakurai's Object during a mass ejection phase that preceded the 1997 event. We detect small hydrocarbon and other molecules in the spectra, and trace the column densities in hydrogen cyanide (HCN) and acetylene (C$_2$H$_2$). We use the former to determine the $^{12}$C/$^{13}$C ratio to be $6.4\pm0.7$, 14 times smaller than the Solar System value.
Near-infrared (NIR) and optical photometry and spectroscopy are presented for the nova V1831 Aquilae, covering the early decline and dust-forming phases during the first ∼90 d after its discovery. The nova is highly reddened due to interstellar extinction. Based solely on the nature of the NIR spectrum, we are able to classify the nova to be of the Fe ii class. The distance and extinction to the nova are estimated to be 6.1 ± 0.5 kpc and Av ∼ 9.02, respectively. Lower limits of the electron density, emission measure and ionized ejecta mass are made from a Case B analysis of the NIR Brackett lines, while the neutral gas mass is estimated from the optical [O i] lines. We discuss the cause of the rapid strengthening of the He i 1.0830-μm line during the early stages. V1831 Aql formed a modest amount of dust fairly early (∼19.2 d after discovery); the dust shell is not seen to be optically thick. Estimates of the dust temperature, dust mass and grain size are made. Dust formation commences around day 19.2 at a condensation temperature of 1461 ± 15 K, suggestive of a carbon composition, following which the temperature is seen to decrease gradually to 950 K. The dust mass shows a rapid initial increase, which we interpret as being due to an increase in the number of grains, followed by a period of constancy, suggesting the absence of grain destruction processes during this latter time. A discussion of the evolution of these parameters is made, including certain peculiarities seen in the grain radius evolution.
Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from near-infrared (near-IR) monitoring in 2010–2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X-ray and near-IR behaviour. Comparison of the star’s present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06, respectively. The JHK spectra are dominated by emission lines of He i and Paschen and Brackett lines of H i . Lines of O i 1.1287 and 1.3165 μm are also present and their relative strength indicates, since O i 1.1287 is the stronger among the two lines, that Lyman β fluorescence plays an important role in their excitation. Recombination analysis of the H i lines is done, which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick, and this supposition is verified by calculating the line centre optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick, which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an IR excess. Based on the magnitude of the IR excess, which is modelled using a free–free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs.
We present near-infrared spectroscopy of the NGC 5128 supernova SN 2016adj in the first 2 months following discovery. We report the detection of first overtone carbon monoxide emission at $\sim58.2$ d after discovery, one of the earliest detections of CO in an erupting supernova. We model the CO emission to derive the CO mass, temperature and velocity, assuming both pure $^{12}$CO and a composition that includes $^{13}$CO; the case for the latter is the isotopic analyses of meteoritic grains, which suggest that core collapse supernovae can synthesise significant amounts of $^{13}$C. Our models show that, while the CO data are adequately explained by pure $^{12}$CO, they do not preclude the presence of $^{13}$CO, to a limit of $^{12}$C/$^{13}$C $> 3$, the first constraint on the $^{12}$C/$^{13}$C ratio determined from near-infrared observations. We estimate the reddening to the object, and the effective temperature from the energy distribution at outburst. We discuss whether the ejecta of SN 2016adj may be carbon-rich, what the infrared data tell us about the classification of this supernova, and what implications the early formation of CO in supernovae may have for CO formation in supernovae in general.
Near-infrared spectroscopic and photometric observations are presented for the recent 2011 outburst of recurrent nova T Pyxidis. Spectra, commencing from one day after outburst, are reported for a total of 18 epochs spread over the first 50 d of the eruption. A major finding is the transition of the nova from the He/N to the Fe ii class within a few days after the outburst. The transition is inferred from the appearance of prominent He i lines soon after the onset of the eruption which then fade rapidly in strength within the next few days. Certain lines of carbon which are near-IR hallmarks of the Fe ii class of novae are also used to establish and trace the transition. The evolution in the strength and shape of the line profiles is presented and discussed. A Case B recombination analysis is done which shows that the Brackett lines are affected by optical depth effects from the start to the end of the observations. Based on the observation that the Br γ line is clearly optically thick, we are able to place lower limits on the emission measure |$n_{\rm e}^2$|L and show that the electron density of the ejecta ne during the first 50 d must be in the range 109–1010 cm−3.
We present near-infrared (1 - 2.5 micron) spectroscopic and photometric results of Nova V2615 Ophiuchi which was discovered in outburst in 2007 March. Our observations span a period of ~ 80 days starting from 2007 March 28 when the nova was at its maximum light. The evolution of the spectra are shown from the initial P-Cygni phase to an emission-line phase and finally to a dust formation stage. The characteristics of the JHK spectra are very similar to those observed in a nova outburst occurring on a carbon-oxygen white dwarf. We analyse an observed line at 2.088 micron and suggest it could be due to FeII excited by Lyman alpha fluorescence. The highlight of the observations is the detection of the first overtone bands of carbon monoxide (CO) in the 2.29 - 2.40 micron region. The CO bands are modeled to estimate the temperature and mass of the emitting CO gas and also to place limits on the 12C/13C ratio. The CO bands are recorded over several epochs thereby allowing a rare opportunity to study its evolution from a phase of constant strength through a stage when the CO is destroyed fairly rapidly. We compare the observed timescales involved in the evolution of the CO emission and find a good agreement with model predictions that investigate the chemistry in a nova outflow during the early stages.
We present multi-epoch near-infrared observations of Nova Ophiuchi 2015 which was discovered during outburst in March 2015. This nova showed a few special properties viz. (i) it displayed an unusual prolonged flat-top light curve which does not easily fit into known classes of nova light curves, (ii) it showed evidence for possessing an evolved secondary in the outbursting binary system, and (iii) it made a rare reverse hybrid transition from the He/N class to Fe ii class early during its outburst. The present studies focus on the spectroscopic evolution of the object around maximum light and early decline. We show that there was a unique, rapid strengthening and decline in the He 1.0831, 2.0581 μm line strengths during this stage, wherein the nova combined the traits of both the He/N and Fe ii classes. Possible causes for this behaviour are discussed. The relative strengths of the Lyβ fluoresced O i 0.8446, 1.1287 μm lines are used to estimate the reddening to the nova. A recombination Case B analysis of the early spectra is used to set constraints on the electron density and emission measure, and a later time spectrum when the ejecta were optically thin is used to estimate the ejecta mass to be (0.95–1.9) × 10−4 M⊙. Power-law fits made to study the evolution of the continuum, show a fairly constant slope which differs from the trend generally expected during a nova's evolution viz. beginning with a blackbody and evolving to a free–free distribution at later stages.
V4643 Sagittarii or Nova Sagittarii 2001 was discovered in outburst at 7.7 mag. on 2001 February 24. Here, we present near-infrared results of this fast classical nova obtained in the early decline phase in 2001 March followed by optical observations about one month later. Subsequently we also present near-infrared spectra taken later in the nova's evolution, about four months after the outburst, when V4643 Sgr had entered the coronal phase. The spectra in the early decline phase are dominated by emission lines of the HI Brackett series and also the Paschen beta and gamma lines. We study the cause of the excitation of the the OI line at 1.128 micron and discuss the variation in its strength with time after outburst. We discuss the role of optical depth effects on the observed strengths of the hydrogen Brackett and Paschen lines and discuss possible reasons for the puzzling behavior of the Br gamma line strength and whether it is correlated with the OI 1.128 micron line behavior. An optical spectrum is presented which shows that HeII lines are the most prominent features - after HI - to be seen in early 2001 April. We present and also discuss spectra taken in 2001 June and August which prominently show coronal lines of [Si VI] and [Si VII] at 1.9641 micron and 2.4807 micron respectively.
We present near-infrared (1-2.5 micron) JHK photo-spectroscopic results of the unusually slow nova V5558 Sgr (2007). V5558 Sgr showed a slow climb to maximum that lasted for about 60 days and then underwent at least five strong secondary outbursts. We have analyzed the optical light curve to derive large t2 and t3 values of 281 +/- 3 and 473 +/- 3 days respectively. An alternate approach is adopted to derive a distance estimate of 1.55 +/- 0.25 kpc as conventional MMRD relation may not be applicable for a slow nova. In the pre-maxima stage the spectra showed narrow (FWHM ~ 400 - 550 km/s and strong emission lines of Paschen and Brackett series with prominent P-Cygni components. In the later phase the spectra show significant changes with the development of strong and broad ~ 1000 km/s emission lines of HI, HeI, OI, and NI and some uncommon Fe II emission lines. No evidence of dust formation is seen. V5558 Sgr has been shown to be a rare hybrid nova showing a transition from He/N to Fe II type from optical spectra. However the near-infrared data do not show such a transition and we discuss this anomalous behavior. A recombination analysis of the Brackett lines allows us to constrain the electron density and emission measure during the early optically thick phase and to estimate the mass of the ejecta to be (6.0 +/- 1.5) x 10^(-4) Msun, assuming a filling factor of unity, from later observations.
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