The mysterious eruption of V838 Mon

V838 Mon is marking one of the most mysterious stellar outbursts on record. The spectral energy distribution of the progenitor resembles an under-luminous F main sequence star (at V=15.6 mag), that erupted into a cool supergiant following a complex and multi-maxima lightcurve (peaking at V=6.7 mag). The outburst spectrum show BaII, LiI and lines of several s-elements, with wide P-Cyg profiles and a moderate and retracing emission in the Balmer lines. A light-echo discovered expanding around the object helped to constrain the distance (d=790+/-30 pc), providing M_V=+4.45 in quiescence and M_V=-4.35 at optical maximum (somewhat dependent on the still uncertain E(B-V)=0.5 reddening). The general outburst trend is toward lower temperatures and larger luminosities, and continuing so at the time of writing. The object properties conflict with a classification within already existing categories: the progenitor was not on a post-AGB track and thus the similarities with the born-again AGB stars FG Sge, V605 Aql and Sakurai's object are limited to the cool giant spectrum at maximum; the cool spectrum, the moderate wind velocity (500 km/sec and progressively reducing) and the monotonic decreasing of the low ionization condition argues against a classical nova scenario. The closest similarity is with a star that erupted into an M-type supergiant discovered in M31 by Rich et al. (1989), that became however much brighter by peaking at M_V=-9.95, and with V4332 Sgr that too erupted into an M-type giant (Martini et al. 1999) and that attained a lower luminosity, closer to that of V838 Mon. M31-RedVar, V4332 Sgr and V838 Mon could be manifestations of the same and new class of astronomical objects.Comment: A&A, in pres

Line Evolution of the Nova V5587 Sgr from Early to Nebula Phase

The spectral evolution of the nova V5587 Sgr has been monitored at Koyama Astronomical Observatory and Higashi-Hiroshima Observatory, Japan, from the early to nebula phase. The nova rebrightened several times. The spectra during the early phase showed emission lines of H α, H β, O I, He I, He II, N II, Fe II. Nova V5587 Sgr is classified into the Fe II type. The helium abundance of the nova is estimated as N(He)/N(H) = 0.134 ± 0.09. The light curve, the spectral evolution, and the helium abundance in V5587 Sgr are similar to those of the nova PW Vul

Deformation and cyclic strength characteristics of loose and medium-dense clean sand under sloping ground conditions: insights from cyclic undrained torsional shear tests with static shear

The effects of liquefaction on sloping ground often include the development of extremely large deformation. Although such phenomenon has been repeatedly observed following major earthquakes, the triggering conditions are not fully understood yet. To provide new insights into this issue, in this paper, results of two series of large-strain undrained cyclic torsional shear tests with initial static shear conducted on loose and medium-dense Toyoura sand specimens (relative density of 25-30% and 44-48%) are presented and analyzed. The post-liquefaction response of Toyoura sand is assessed in terms of failure modes and cyclic resistance up to 50% single amplitude shear strain. It is shown that, depending on the combined magnitude of static and cyclic shear stresses, a sand in sloping ground will likely experience a sudden development of large shear deformation (flow deformation) if initial liquefaction takes place, or a more progressive accumulation of large residual deformation, which yet may bring sand to failure, when the onset of initial liquefaction is not achieved

Liquefaction potential of sand-gravel mixtures: experimental observations

Case histories from at least 27 earthquakes worldwide (including three from New Zealand: 1929 Mw7.6 Murchison earthquake; 2010 Mw 7.1 Darfield earthquake; and 2016 Mw 7.8 Kaikoura earthquake) have indicated that liquefaction can occur in gravelly soils (both in natural deposits and manmade reclamations) inducing large ground deformation and causing severe damage to civil infrastructures. However, the evaluation of the liquefaction potential and deformation characteristics of gravelly soils remains to be a major challenge in geotechnical earthquake engineering. Aimed at providing new and useful insights on this important topic, in this study, a series of undrained cyclic triaxial tests were conducted on selected sand-gravel mixtures (SGMs), which were attained by varying the proportion by weight of a fine sand (New Brighton sand), a coarse sand (washed river sand) and a rounded pea gravel. Reconstituted specimens (height = 130 mm and diameter = 61 mm) were prepared at two relative density states of 25% and 45% by wet tamping method. Fully saturated specimens were then isotopically consolidated at 100 kPa confining pressure and subjected to cyclic stress ratio (CSR) levels ranging between 0.15 and 0.45. In this paper, preliminary results are presented and discussed in terms of effects density state – i.e., relative density (Dr), and inter-granular void ratios – and gravel content ( ), on the liquefaction potential of SGMs. It is shown that while the liquefaction potential tends to increase with both increasing Dr and , it can be more uniquely described by the equivalent void ratio that accounts simultaneously for both the density state and gravel content effects

Photometric study of new southern SU UMa-type dwarf novae and candidates: V877 Ara, KK Tel and PU CMa

We photometrically observed three dwarf novae V877 Ara, KK Tel and PU CMa. We discovered undisputed presence of superhumps in V877 Ara and KK Tel, with mean periods of 0.08411(2) d and 0.08808(3) d, respectively. Both V877 Ara and KK Tel are confirmed to belong to long-period SU UMa-type dwarf novae. In V877 Ara, we found a large decrease of the superhump period (dot(P)/P = -14.5 +/- 2.1 x 10^(-5)). There is evidence that the period of KK Tel decreased at a similar or a more exceptional rate. Coupled with the past studies of superhump period changes, these findings suggest that a previously neglected diversity of phenomena is present in long-period SU UMa-type dwarf novae. The present discovery of a diversity in long-period SU UMa-type systems would become an additional step toward a full understanding the dwarf nova phenomenon. PU CMa is shown to be an excellent candidate for an SU UMa-type dwarf nova. We examined the outburst properties of these dwarf novae, and derived characteristic outburst recurrence times. Combined with the recently published measurement of the orbital period of PU CMa, we propose that PU CMa is the first object filling the gap between the extreme WZ Sge-type and ER UMa-type stars.Comment: 12 pages, 14 figures, accepted for publication in MNRA

Grazing Eclipsing Dwarf Nova CW Monocerotis: Dwarf Nova-Type Outburst in a Possible Intermediate Polar?

We observed the 2002 October-November outburst of the dwarf nova CW Mon. The outburst showed a clear signature of a premaximum halt, and a more rapid decline after reaching the outburst maximum. On two separate occasions, during the premaximum stage and near the outburst maximum, shallow eclipses were recorded. This finding confirms the previously suggested possibility of the grazing eclipsing nature of this system. The separate occurrence of the eclipses and the premaximum halt can be understood as a result of a combination of two-step ignition of an outburst and the inside-out propagation of the heating wave. We detected a coherent short-period (0.02549 d) signal on two subsequent nights around the optical maximum. This signal was likely present during the maximum phase of the 2000 January outburst. We interpret this signal as a signature of the intermediate polar (IP) type pulses. The rather strange outburst properties, strong and hard X-ray emission, and the low luminosity of the outburst maximum might be understood as consequences of the supposed IP nature. The ratio between the suggested spin period and the orbital period, however, is rather unusual for a system having an orbital period of ~0.176 d.Comment: 11 pages, 11 figures, to appear in PAS

Investigation into the multiple recent sinkholes in Pokhara, Nepal

Since November 2013, numerous sinkholes have been forming in the Armala area of Pokhara Valley, Central Nepal, posing serious threat to local residents. In order to provide countermeasures for reducing sinkhole risk, detailed investigations into the cause and the formation mechanism of the sinkholes are crucial. Preliminary surveys were conducted in June 2014 and November 2014. Comparison of photos, taken in the two surveys, clearly indicates not only the formation of new sinkholes, but also the re-activation of filled sinkholes. By means of dynamic cone penetration tests and surface wave investigations, qualitative characterization of the soil profile was attained, and shallow weak soil layers which are believed to be the location for future sinkholes could be identified. On the basis of the preliminary field investigation, possible sinkhole formation mechanisms are considered. A risk of sinkhole does not seem to disappear as white turbid water continuously springs. It indicates that the internal erosion of white clayey silt layer is still in progress. In August 2015, a boring was carried out beside one of the largest sinkholes. The overall structure of ground layers was first revealed and a 2.5m high cavity at 7.5-10m deep from the ground surface was found within a thick white clayey silt layer. Further ground investigations including surface wave exploration were conducted in December 2015 and the results are reported

Constraining jet theories using nova outbursts

It is commonly accepted that jets have not been observed in CVs so far. This absence was recently explained by their low mass transfer rates compared with objects with jets. A mass accretion limit for jets in CVs was proposed to be about 1e-7-1e-6 Mo/yr. There was, however, a report of evidence for jets in V1494 Aql=Nova Aql 1999/2. We estimate the mass transfer rate of this system around the reported event and show that it is consistent with the theoretical limit for jets. We further propose that the X-ray flare that was observed in this object may be connected with a jet as well. The appearance of jets in novae is actually expected since during the early decline from outburst some are alike supersoft X-ray sources, in which jets have been found. The detection of jets in novae also fits the suggestion that in addition to the presence of an accretion disk, a hot central source is required for the formation of jets. The observations of jets during the transition phase in V1494 Aql can be regarded as evidence for the early existence of the accretion disk in the system. This conclusion supports our previous suggestion for a link between the transition phase in novae and the re-establishment of the accretion disk. We further speculate that jets may be restricted to transient novae. If our ideas are confirmed, jets should be common in transient novae and may be formed several times during the transition phase and perhaps even long after it ends. In classical novae jets may be launched and observed in real time. As binary systems, these objects are easy to study. Novae may, therefore, be key systems in understanding the formation and evolution of jets and ideal targets to test and constrain jet theories.Comment: 4 pages, 1 .eps figure, 2 .sty files, ApJL accepted, abstract was truncated to the 24 lines limi