First release of the IPHAS catalogue of new extended planetary nebulae

Date of Acceptance: 30/06/2014We present the first results of our search for new, extended planetary nebulae (PNe) based on careful, systematic, visual scrutiny of the imaging data from the Isaac Newton Telescope Photometric Ha Survey of the Northern Galactic plane (IPHAS). The newly uncovered PNe will help to improve the census of this important population of Galactic objects that serve as key windows into the late-stage evolution of low- to intermediate-mass stars. They will also facilitate study of the faint end of the ensemble Galactic PN luminosity function. The sensitivity and coverage of IPHAS allows PNe to be found in regions of greater extinction in the Galactic plane and/or those PNe in a more advanced evolutionary state and at larger distances compared to the general Galactic PN population. Using a set of newly revised optical diagnostic diagrams in combination with access to a powerful, new, multiwavelength imaging data base, we have identified 159 true, likely and possible PNe for this first catalogue release. The ability of IPHAS to unveil PNe at low Galactic latitudes and towards the Galactic Anticentre, compared to previous surveys, makes this survey an ideal tool to contribute to the improvement of our knowledge of the whole Galactic PN population.Peer reviewe

Instability of LBV-stars against radial oscillations

In this study we consider the nonlinear radial oscillations exciting in LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <= 49.1 M_odot. Hydrodynamic computations were carried out with initial conditions obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show instability against radial oscillations with amplitude growth time comparable with dynamical time scale of the star. Radial oscillations exist in the form of nonlinear running waves propagating from the boundary of the compact core to the upper boundary of the hydrodynamical model. The velocity amplitude of outer layers is of several hundreds of km/s while the bolometric light amplitude does not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism and are due to the instability of the gas with adiabatic exponent close to the critical value Gamma_1 = 4/3 due to the large contribution of radiation in the total pressure. The range of the light variation periods (6 day <= P <= 31 day) of hydrodynamical models agrees with periods of microvariability observed in LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter

Iron uptake and physiological response of phytoplankton during a mesoscale Southern Ocean Iron enrichment.

Iron supply is thought to regulate primary production in high nitrate, low chlorophyll (HNLC) regions of the sea in both the past and the present. A critical aspect of this relationship is acquisition of iron (Fe) by phytoplankton, which occurs through a complex series of extracellular reactions that are influenced by Fe chemistry and speciation. During the first in situ mesoscale Fe-enrichment experiment in the Southern Ocean (Southern Ocean iron release experiment [SOIREE]), we monitored the uptake of Fe by three size classes of plankton and their ensuing physiological response to the Fe enrichment. Rates of Fe uptake from both inorganic Fe (Fe') and organic Fe complexes (FeL) were initially fast, indicative of Fe-limitation. After Fe enrichment phytoplankton down-regulated Fe uptake and optimized physiological performance, but by day 12 they had greatly increased their capacity to acquire Fe from FeL. The increase in Fe uptake from FeL coincided with a sixfold decrease in Fe' that followed the production of Fe-binding organic ligands. Phytoplankton were able to use organically bound Fe at rates sufficient to maintain net growth for more than 42 d. Adaptation to such shifts in Fe chemistry may contribute to bloom longevity in these polar HNLC waters

Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption

Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky Way. It became the second-brightest star in the sky during its mid-19th century "Great Eruption," but then faded from view (with only naked-eye estimates of brightness). Its eruption is unique among known astronomical transients in that it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3 kpc away, spatially resolved studies of the nebula have constrained the ejected mass and velocity, indicating that in its 19th century eruption, Eta Car ejected more than 10 M_solar in an event that had 10% of the energy of a typical core-collapse supernova without destroying the star. Here we report the discovery of light echoes of Eta Carinae which appear to be from the 1838-1858 Great Eruption. Spectra of these light echoes show only absorption lines, which are blueshifted by -210 km/s, in good agreement with predicted expansion speeds. The light-echo spectra correlate best with those of G2-G5 supergiant spectra, which have effective temperatures of ~5000 K. In contrast to the class of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption, the effective temperature of its outburst is significantly cooler than allowed by standard opaque wind models. This indicates that other physical mechanisms like an energetic blast wave may have triggered and influenced the eruption.Comment: Accepted for publication by Nature; 4 pages, 4 figures, SI: 6 pages, 3 figures, 5 table

The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions

We present an overview of the initial results from the Chandra Planetary Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of X-ray-luminous point sources at the central stars (CSPNe) of these objects. Combining these results with those obtained from Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date, we find an overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence of X-ray sources appears correlated with PN density structure, in that molecule-poor, elliptical nebulae are more likely to display X-ray emission (either point-like or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100 kK) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback. Most PNe detected as diffuse X-ray sources are elliptical nebulae that display a nested shell/halo structure and bright ansae; the diffuse X-ray emission regions are confined within inner, sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe.Comment: 41 pages, 6 figures; submitted to the Astronomical Journa

New frontiers in belowground ecology for plant protection from root-feeding insects

Herbivorous insect pests living in the soil represent a significant challenge to food security given their persistence, the acute damage they cause to plants and the difficulties associated with managing their populations. Ecological research effort into rhizosphere interactions has increased dramatically in the last decade and we are beginning to understand, in particular, the ecology of how plants defend themselves against soil-dwelling pests. In this review, we synthesise information about four key ecological mechanisms occurring in the rhizosphere or surrounding soil that confer plant protection against root herbivores. We focus on root tolerance, root resistance via direct physical and chemical defences, particularly via acquisition of silicon-based plant defences, integration of plant mutualists (microbes and entomopathogenic nematodes, EPNs) and the influence of soil history and feedbacks. Their suitability as management tools, current limitations for their application, and the opportunities for development are evaluated. We identify opportunities for synergy between these aspects of rhizosphere ecology, such as mycorrhizal fungi negatively affecting pests at the root-interface but also increasing plant uptake of silicon, which is also known to reduce herbivory. Finally, we set out research priorities for developing potential novel management strategies

Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period

The periodicity of 5.5 years for some observational events occurring in Eta Carinae manifests itself across a large wavelength range and has been associated with its binary nature. These events are supposed to occur when the binary components are close to periastron. To detect the previous periastron passage of Eta Car in 2003, we started an intensive, ground-based, optical, photometric observing campaign. We continued observing the object to monitor its photometric behavior and variability across the entire orbital cycle. Our observation program consisted of daily differential photometry from CCD images, which were acquired using a 0.8 m telescope and a standard BVRI filter set at La Plata Observatory. The photometry includes the central object and the surrounding Homunculus nebula. We present up-to-date results of our observing program, including homogeneous photometric data collected between 2003 and 2008. Our observations demonstrated that Eta Car has continued increasing in brightness at a constant rate since 1998. In 2006, it reached its brightest magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its brightening trend, fading to V = 5.0 at the beginning of 2007, and has maintained a quite steady state since then. We continue the photometric monitoring of Eta Car in anticipation of the next "periastron passage", predicted to occur at the beginning of 2009.Comment: Accepted by A&A. The paper contains 3 figures and 2 table

SPITZER SAGE Observations of Large Magellanic Cloud Planetary Nebulae

We present IRAC and MIPS images and photometry of a sample of previously known planetary nebulae (PNe) from the SAGE survey of the Large Magellanic Cloud (LMC) performed with the Spitzer Space Telescope. Of the 233 known PNe in the survey field, 185 objects were detected in at least two of the IRAC bands, and 161 detected in the MIPS 24 micron images. Color-color and color-magnitude diagrams are presented using several combinations of IRAC, MIPS, and 2MASS magnitudes. The location of an individual PN in the color-color diagrams is seen to depend on the relative contributions of the spectral components which include molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), infrared forbidden line emission from the ionized gas, warm dust continuum, and emission directly from the central star. The sample of LMC PNe is compared to a number of Galactic PNe and found to not significantly differ in their position in color-color space. We also explore the potential value of IR PNe luminosity functions (LFs) in the LMC. IRAC LFs appear to follow the same functional form as the well-established [O III] LFs although there are several PNe with observed IR magnitudes brighter than the cut-offs in these LFs.Comment: 18 pages, 10 figures, 3 tables, to be published in the Astronomical Journal. Additional online data available at http://www.cfa.harvard.edu/irac/publications

Observations of red-giant variable stars by Aboriginal Australians

Aboriginal Australians carefully observe the properties and positions of stars, including both overt and subtle changes in their brightness, for subsistence and social application. These observations are encoded in oral tradition. I examine two Aboriginal oral traditions from South Australia that describe the periodic changing brightness in three pulsating, red-giant variable stars: Betelgeuse (Alpha Orionis), Aldebaran (Alpha Tauri), and Antares (Alpha Scorpii). The Australian Aboriginal accounts stand as the only known descriptions of pulsating variable stars in any Indigenous oral tradition in the world. Researchers examining these oral traditions over the last century, including anthropologists and astronomers, missed the description of these stars as being variable in nature as the ethnographic record contained several misidentifications of stars and celestial objects. Arguably, ethnographers working on Indigenous Knowledge Systems should have academic training in both the natural and social sciences.Comment: The Australian Journal of Anthropology (2018

Silicon-induced root nodulation and synthesis of essential amino acids in a legume is associated with higher herbivore abundance

Ecologists have become increasingly aware that silicon uptake by plants, especially the Poaceae, can have beneficial effects on both plant growth and herbivore defence. The effects of silicon on other plant functional groups, such as nitrogen-fixing legumes, have been less well studied. Silicon could, however, indirectly promote herbivore performance in this group if reported increases in N2 fixation caused improvements in host plant quality for herbivores. We tested how silicon supplementation in the legume (Medicago sativa) affected plant growth rates, root nodulation and foliage quality (silicon content and amino acid profiles) for an insect herbivore (Acyrthosiphon pisum). Plants supplemented with silicon (Si+) grew three times as quickly as those without supplementation (Si−), almost entirely in shoot mass. While root growth was unaffected by silicon uptake, root nodules containing nitrogen-fixing bacteria were 44% more abundant on Si+ plants. Aphid abundance was twice as high on Si+ plants compared to Si− plants and was positively correlated with silicon-stimulated plant growth. Si+ plants accumulated more than twice as much silicon as Si− plants, but did not have higher silicon concentrations because of dilution effects linked to the rapid growth of Si+ plants. Si+ plants showed a 65% increase in synthesis of essential foliar amino acids, probably due to increased levels of root nodulation. These results suggest that increased silicon supply makes M. sativa more susceptible to A. pisum, mainly because of increased plant growth and resource availability (i.e. essential amino acids). While silicon augmentation of the Poaceae frequently improves herbivore defence, the current study illustrates that this cannot be assumed for other plant families where the beneficial effects of silicon on plant growth and nutrition may promote herbivore performance in some instances