ABSTRACT
Fr 2-30 = PN? G126.8−15.5 is a faint emission nebula, hosting a 14th-mag central star that we identify here for the first time. Deep Hα and O iii images reveal a roughly elliptical nebula ...with dimensions of at least 22 arcmin × 14 arcmin, fading into a surrounding network of even fainter emission. Optical spectrograms of the central star show it to have a subdwarf O spectral type, with a Gaia parallax distance of 890 pc. A model-atmosphere analysis gives parameters of $T_{\rm eff}=60\, 000$ K, log g = 6.0, and a low helium content of nHe/nH = 0.0017. The location of the central star in the log g–Teff plane is inconsistent with a post-asymptotic-giant-branch evolutionary status. Two alternatives are that it is a helium-burning post-extreme-horizontal-branch object, or a hydrogen-burning post-red-giant-branch star. In either case, the evolutionary ages are so long that a detectable planetary nebula (PN) should not be present. We find evidence for a variable radial velocity (RV), suggesting that the star is a close binary. However, there are no photometric variations, and the spectral-energy distribution rules out a companion earlier than M2 V. The RVs of the star and surrounding nebula are discordant, and the nebula lacks typical PN morphology. We suggest that Fr 2-30 is a ‘PN mimic’ – the result of a chance encounter between the hot sdO star and an interstellar cloud. However, we note the puzzling fact that there are several nuclei of genuine PNe that are known to be in evolutionary states similar to that of the Fr 2-30 central star.
Abstract
We present Spitzer/InfraRed Array Camera observations of dust formation from six extragalactic carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (
Z
≲ 0.65
Z
⊙
) environments ...using multiepoch mid-infrared (IR) imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS). Optical follow-up spectroscopy of SPIRITS 16ln, 19q, 16df, 18hb, and 14apu reveals emission features from C
iv
λ
5801−12 and/or the C
iii–iv
λ
4650/He
ii
λ
4686 blend that are consistent with early-type WC stars. We identify SPIRITS 16ln as the variable mid-IR counterpart of the recently discovered colliding-wind WC4 + O binary candidate, N604-WRXc, located in the subsolar metallicity NGC 604 H
ii
region in M33. We interpret the mid-IR variability from SPIRITS 16ln as a dust-formation episode in an eccentric colliding-wind WC binary. SPIRITS 19q, 16df, 14apu, and 18hb exhibit absolute 3.6 magnitudes exceeding that of one of the most IR-luminous dust-forming WC systems known, WR 104 (
M
3.6
≲ −12.3). An analysis of dust formation in the mid-IR outburst from SPIRITS 19q reveals a high dust production rate of
M
⊙
yr
−1
, which may therefore exceed that of the most efficient dust-forming WC systems known. We demonstrate that efficient dust formation is feasible from early-type WC binaries in the theoretical framework of colliding-wind binary dust formation if the systems host an O-type companion with high mass-loss rates (
M
⊙
yr
−1
). This efficient dust formation from early-type WC binaries highlights their potential role as significant sources of dust in low-metallicity environments.
Spectroscopy of V341 Arae Bond, Howard E.; Miszalski, Brent
Publications of the Astronomical Society of the Pacific,
09/2018, Letnik:
130, Številka:
991
Journal Article
Recenzirano
Odprti dostop
V341 Arae is a 10th-magnitude variable star in the southern hemisphere, discovered over a century ago by Henrietta Leavitt, but relatively little studied since then. Although historically considered ...to be a Cepheid, it is actually blue and coincides with an X-ray source. The star lies near the edge of the large, faint Hα nebula Fr 2–11, discovered by D. Frew, who showed that V341 Ara is actually a cataclysmic variable (CV). His deep imaging of the nebula revealed a bow-shock morphology in the immediate vicinity of the star. We have carried out spectroscopic monitoring of V341 Ara, and we confirm that it is a nova-like CV, with an orbital period of 0.15216 days (3.652 hr). We show that V341 Ara is remarkably similar to the previously known BZ Cam, a nova-like CV with a nearly identical orbital period, associated with the bow shock nebula EGB 4. Archival sky-survey photometry shows that V341 Ara normally varies between V ≃ 10.5 and 11, with a characteristic timescale ranging from about 10 to 16 days. V341 Ara lies well off-center within Fr 2–11. We speculate that either the star is undergoing a chance high-speed encounter with a small interstellar cloud, or that the nebula was ejected from the star itself in a nova outburst in the fairly distant past. At a distance of only 156 pc, V341 Ara is one of the nearest and brightest known nova-like variables, and we encourage further studies.
Abstract
We present a systematic study of the most luminous (
M
IR
Vega magnitudes brighter than −14) infrared (IR) transients discovered by the
SPitzer
InfraRed Intensive Transients Survey (SPIRITS) ...between 2014 and 2018 in nearby galaxies (
D
< 35 Mpc). The sample consists of nine events that span peak IR luminosities of
M
4.5,peak
between −14 and −18.2, show IR colors between 0.2 < (3.6–4.5) < 3.0, and fade on timescales between 55 days <
t
fade
< 480 days. The two reddest events (
A
V
> 12) show multiple, luminous IR outbursts over several years and have directly detected, massive progenitors in archival imaging. With analyses of extensive, multiwavelength follow-up, we suggest the following possible classifications: five obscured core-collapse supernovae (CCSNe), two erupting massive stars, one luminous red nova, and one intermediate-luminosity red transient. We define a control sample of all optically discovered transients recovered in SPIRITS galaxies and satisfying the same selection criteria. The control sample consists of eight CCSNe and one Type Iax SN. We find that 7 of the 13 CCSNe in the SPIRITS sample have lower bounds on their extinction of 2 <
A
V
< 8. We estimate a nominal fraction of CCSNe in nearby galaxies that are missed by optical surveys as high as
(90% confidence). This study suggests that a significant fraction of CCSNe may be heavily obscured by dust and therefore undercounted in the census of nearby CCSNe from optical searches.
Single stars in ancient globular clusters (GCs) are believed incapable of producing planetary nebulae (PNs), because their post-asymptotic-giant-branch evolutionary timescales are slower than the ...dissipation timescales for PNs. Nevertheless, four PNs are known in Galactic GCs. I carried out a snapshot imaging search with the Hubble Space Telescope (HST) for PNs in bright Local Group GCs outside the Milky Way. I used a filter covering the 5007 A nebular emission line of OIII, and another one in the nearby continuum, to image 66 GCs. Inclusion of archival HST frames brought the total number of extragalactic GCs imaged at 5007 A to 75, whose total luminosity slightly exceeds that of the entire Galactic GC system. I found no convincing PNs in these clusters, aside from one PN in a young M31 cluster misclassified as a GC, and two PNs at such large angular separations from an M31 GC that membership is doubtful.
The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD's mass. Until recently, however, the mass depended on orbital elements ...determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to 0.573 0.018 M☉. In this paper, we use model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then compare these results with WD interior models. Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass-radius relation (MRR) is consistent with the star's location in the mass-radius plane. This consistency is, however, achieved only if we assume a "thin" outer hydrogen layer, with qH = MH/MWD 10−10. We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of the expectation from canonical stellar-evolution theory of "thick" H layers with qH 10−4. The cooling age of 40 Eri B is ∼122 Myr, and its total age is ∼1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observations is excellent in all cases.
Abstract
We have carried out a search for above-horizontal-branch (AHB) stars—objects lying above the horizontal branch (HB) and blueward of the asymptotic giant branch (AGB) in the color–magnitude ...diagram—in 97 Galactic and seven Magellanic Cloud globular clusters (GCs). We selected AHB candidates based on photometry in the
uBVI
system, which is optimized for detection of low-surface-gravity stars with large Balmer jumps, in the color range −0.05 ≤ (
B
−
V
)
0
≤1.0. We then used Gaia astrometry and Gaussian-mixture modeling to confirm cluster membership and remove field interlopers. Our final catalog contains 438 AHB stars, classified and interpreted in the context of post-HB evolution as follows: (1) AHB1: 280 stars fainter than
M
V
= −0.8, evolving redward from the blue HB (BHB) toward the base of the AGB. (2) Post-AGB (PAGB): 13 stars brighter than
M
V
≃ −2.75, departing from the top of the AGB and evolving rapidly blueward. (3) AHB2: 145 stars, with absolute magnitudes between those of the AHB1 and PAGB groups. This last category includes a mixture of objects leaving the extreme BHB and evolving toward the AGB, and brighter ones moving back from the AGB toward higher temperatures. Among the AHB1 stars are 59 RR Lyrae interlopers, observed by chance in our survey near maximum light. PAGB and AHB2 stars (including W Virginis Cepheids) overwhelmingly belong to GCs containing BHB stars, in accordance with predictions of post-HB evolutionary tracks. We suggest that most W Vir variables are evolving toward lower temperatures and are in their first crossings of the instability strip. Nonvariable yellow PAGB stars show promise as a Population II standard candle for distance measurement.
Abstract
Nova Sagittarii 1943 (V1148 Sgr) was an eighth magnitude optical transient that was unusual in having a late-type spectrum during its outburst, in striking contrast to the normal ...high-excitation spectra seen in classical novae. Unfortunately, only an approximate position was given in the discovery announcement, hampering follow-up attempts to observe its remnant. We have identified the nova on two photographic plates in the Harvard archive, allowing us to determine a precise astrometric position. Apart from these two plates, obtained in 1943 and 1944, none of the photographs in the Harvard collection, from 1897 to 1950, show V1148 Sgr to limits as faint as
g
≃ 18.3. Modern deep images show a candidate remnant at
i
≃ 19.2, lying only 0.″26 from the site of the nova. V1148 Sgr may have been a luminous red nova (LRN), only the sixth one known in the Milky Way. However, it lacks the near- and mid-infrared excesses, and millimeter-wave emission, seen in other LRNe, leaving its nature uncertain. We urge spectroscopy of the candidate remnant.
Abstract During our spectroscopic survey of central stars of faint planetary nebulae (PNe), we found that the nucleus of Abell 57 exhibits strong nebular emission lines. Using synthetic narrowband ...images, we show that the emission arises from an unresolved compact emission knot (CEK) coinciding with the hot (90,000 K) central star. Thus Abell 57 belongs to the rare class of “EGB 6-type” PNe, characterized by dense emission cores. Photometric data show that the nucleus exhibits a near-infrared excess, due to a dusty companion body with the luminosity of an M0 dwarf but a temperature of ∼1800 K. Emission-line analysis reveals that the CEK is remarkably dense (electron density ∼ 1.6 × 10 7 cm −3 ), and has a radius of only ∼4.5 au. The CEK suffers considerably more reddening than the central star, which itself is more reddened than the surrounding PN. These puzzles may suggest an interaction between the knot and central star; however, Hubble Space Telescope imaging of EGB 6 itself shows that its CEK lies more than ∼125 au from the PN nucleus. We discuss a scenario in which a portion of the asymptotic giant branch wind that created the PN was captured into a dust cloud around a distant stellar companion; this cloud has survived to the present epoch, and has an atmosphere photoionized by radiation from the hot central star. However, in this picture EGB 6-type nuclei should be relatively common, yet they are actually extremely rare; thus they may arise from a different transitory phenomenon. We suggest future observations of Abell 57 that may help unravel its mysteries.
ABSTRACT
We report initial results from an ongoing spectroscopic survey of central stars of faint planetary nebulae (PNe), obtained with the Low-Resolution Spectrograph on the Hobby–Eberly Telescope. ...The six PN nuclei (PNNi) discussed here all have strong emission at the O vi 3811–3834 Å doublet, indicative of very high temperatures. Five of them – the nuclei of Ou 2, Kn 61, Kn 15, Abell 72, and Kn 130 – belong to the hydrogen-deficient PG 1159 class, showing a strong absorption feature of He ii and C iv at 4650–4690 Å. Based on exploratory comparisons with synthetic model-atmosphere spectra, and the presence of Ne viii emission lines, we estimate them to have effective temperatures of order 170 000 K. The central star of Kn 15 has a Wolf–Rayet-like spectrum, with strong and broad emission lines of He ii, C iv, N v, and O v–vi. We classify it WO2, but we note that the N v 4604–4620 Å emission doublet is extremely strong, indicating a relatively high nitrogen abundance. Several of the emission lines in Kn 15 vary in equivalent width by factors as large as 1.5 among our four observations from 2019 to 2022, implying significant variations in the stellar mass-loss rate. We encourage spectroscopic monitoring. Follow-up high-time-resolution photometry of these stars would be of interest, given the large fraction of pulsating variables seen among PG 1159 and WO PNNi.
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