Flare events are mainly due to magnetic reconnection and thus are indicative of stellar activity. TheKepler Space Observatoryrecords numerous stellar activities with unprecedented high photometric precision in flux measurements. It is perfectly suitable for carrying out a statistical study of flares. Here we present 540 M dwarfs with flare events discovered usingKeplerlong-cadence data. The normalized flare energy, as defined by the ratio to bolometric stellar luminosity, Lflare/Lbol, is used to indicate the flare activity. We find that, similar to the X-ray luminosity relation, the Lflare/Lbolversus Protrelation can also be described with three phases, supersaturation, saturation, and exponential decay, corresponding to an ultra-short period, a short period, and a long period. The flare activity and the number fraction of flaring stars in M dwarfs rise steeply near M4, which is consistent with the prediction of a turbulent dynamo. The size of starspots are positively correlated with flare activity. The Lflare/Lbolratio has a power-law dependence on LHα/Lbol, a parameter indicative of stellar chromosphere activity. According to this relation, a small enhancement in chromosphere activity may cause a huge rise in flare energy, which suggests that superflares or hyperflares may not need an extra excitation mechanism. Through a comparison study, we suggest that flare activity is a more suitable indicator for stellar activity, especially in the boundary region. However, contrary to what is expected, some M dwarfs with strong flares do not show any light variation caused by starspots. Follow-up observations are needed to investigate this problem.

We present deep optical spectroscopy of eight H ii regions located in the anti-centre of the Milky Way. The spectra were obtained at the 10.4 m GTC and 8.2 m VLT. We determined T_e([N ii]) for all objects and T_e([O iii]) for six of them. We also included in our analysis an additional sample of 13 inner-disc Galactic H ii regions from the literature that have excellent T_e determinations. We adopted the same methodology and atomic data set to determine the physical conditions and ionic abundances for both samples. We also detected the C ii and O ii optical recombination lines in Sh 2-100, which enables the determination of the abundance discrepancy factor for this object. We found that the slopes of the radial oxygen gradients defined by the H ii regions from R₂₅ (=11.5 kpc) to 17 kpc and those within R₂₅ are similar within the uncertainties, indicating the absence of flattening in the radial oxygen gradient in the outer Milky Way. In general, we found that the scatter of the O/H ratios of H ii regions is not substantially larger than the observational uncertainties. The largest possible local inhomogeneities of the oxygen abundances are of the order of 0.1 dex. We also found positive radial gradients in T_e([O iii]) and T_e([N ii]) across the Galactic disc. The shapes of these temperature gradients are similar and also consistent with the absence of flattening of the metallicity distribution in the outer Galactic disc.

We present narrowband near-infrared images of a sample of 11 Galactic planetary nebulae (PNe) obtained in the H₂ 2.122 μm and Brγ 2.166 μm emission lines and the K _c 2.218 μm continuum. These images were collected with the Wide-field Infrared Camera on the 3.6 m Canada–France–Hawaii Telescope (CFHT); their unprecedented depth and wide field of view allow us to find extended nebular structures in H₂ emission in several PNe, some of these being the first detection. The nebular morphologies in H₂ emission are studied in analogy with the optical images, and indication of stellar wind interactions is discussed. In particular, the complete structure of the highly asymmetric halo in NGC 6772 is witnessed in H₂, which strongly suggests interaction with the interstellar medium. Our sample confirms the general correlation between H₂ emission and the bipolarity of PNe. The knotty or filamentary fine structures of the H₂ gas are resolved in the inner regions of several ring-like PNe, also confirming the previous argument that H₂ emission mostly comes from knots or clumps embedded within fully ionized material at the equatorial regions. Moreover, the H₂ image of the butterfly-shaped Sh 1-89, after removal of field stars, clearly reveals a tilted ring structure at the waist. These high-quality CFHT images justify follow-up detailed morphokinematic studies that are desired in order to deduce the true physical structures of a few PNe in the sample.

Jets (fast collimated outflows) are claimed to be the main shaping agent of the most asymmetric planetary nebulae (PNs), as they impinge on the circumstellar material at late stages of the asymptotic giant branch phase. The first jet detected in a PN was that of NGC 2392, yet there is no available image because of its low surface brightness contrast with the bright nebular emission. Here we take advantage of the tomographic capabilities of Gran Telescopio de Canarias Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía high-dispersion integral field spectroscopic observations of the jet in NGC 2392 to gain unprecedented details of its morphology and kinematics. The jet of NGC 2392 is found to emanate from the central star, break through the walls of the inner shell of this iconic PN and extend outside the nebula’s outermost regions with an S-shaped morphology suggestive of precession. At odds with the fossil jets found in mature PNs, the jet in NGC 2392 is currently being collimated and launched. The high nebular excitation of NGC 2392, which implies an He⁺⁺/He ionization fraction too high to be attributed to the known effective temperature of the star, has been proposed in the past to hint at the presence of a hot white dwarf companion. In conjunction with the hard X-ray emission from the central star, the present-day jet collimation would support the presence of such a double-degenerate system where one component undergoes accretion from a remnant circumbinary disk of the common envelope phase.

We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T ≈ 1.0 × 10⁶ K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L _{X, CSPN} = (1.2 ± 0.3) × 10³¹ erg s^–1 and L _{X, DIFF} = (9.2 ± 2.3) × 10³⁰ erg s^–1 for the CSPN and diffuse components, respectively.