The spectral Petersen diagram as a new tool to map pulsation modes in variable stars

Additional pulsation modes have been discovered in many Cepheids, RR Lyrae, and other variable stars. Fourier transforms are used to find, fit and subtract the main pulsation period and its harmonics to reveal additional modes. Commonly, for every star, the strongest of these modes is presented in a "Petersen diagram", where the shorter-to-longer period ratio is plotted against the longer period. This diagram discards the information about temporal variations, multiple pulsation modes, and signals which are below some chosen signal to noise threshold. I here present a new tool to map pulsation modes in variable stars, dubbed the "spectral Petersen diagram". It shows all signals, irrespective of their multiplicity or significance. Many (thousands) of light curves can be stacked to improve the signal to noise ratio and reveal unprecedented detail about the pulsation mode space. This tool is useful to constraint the parameters of variable star models.Comment: Accepted to MNRA

Interstellar communication. X. The colors of optical SETI

It has recently been argued from a laser engineering point of view that there are only a few magic colors for optical SETI. These are primarily the Nd:YAG line at 1064 nm and its second harmonic 532.1 nm. Next best choices would be the sum frequency and/or second harmonic generation of Nd:YAG and Nd:YLF laser lines, 393.8 nm (near Fraunhofer CaK), 656.5 nm (H$\alpha$) and 589.1 nm (NaD2). In this paper, we examine the interstellar extinction, atmospheric transparency and scintillation, as well as noise conditions for these laser lines. For strong signals, we find that optical wavelengths are optimal for distances $d\lesssim\,$kpc. Nd:YAG at $\lambda=1{,}064\,$nm is a similarly good choice, within a factor of two, under most conditions and out to $d\lesssim3\,$kpc. For weaker transmitters, where the signal-to-noise ratio with respect to the blended host star is relevant, the optimal wavelength depends on the background source, such as the stellar type. Fraunhofer spectral lines, while providing lower stellar background noise, are irrelevant in most use cases, as they are overpowered by other factors. Laser-pushed spaceflight concepts, such as "Breakthrough Starshot", would produce brighter and tighter beams than ever assumed for OSETI. Such beamers would appear as naked eye stars out to kpc distances. If laser physics has already matured and converged on the most efficient technology, the laser line of choice for a given scenario (e.g., Nd:YAG for strong signals) can be observed with a narrow filter to dramatically reduce background noise, allowing for large field-of-view observations in fast surveys.Comment: 17 pages, 12 Figures. Comments welcom

Interstellar communication. V. Introduction to photon information efficiency (in bits per photon)

How many bits of information can a single photon carry? Intuition says "one", but this is incorrect. With an alphabet based on the photon's time of arrival, energy, and polarization, several bits can be encoded. In this introduction to photon information efficiency, we explain how to calculate the maximum number of bits per photon depending on the number of encoding modes, noise, and losses.Comment: 3 pages, 1 figure. Useful introduction for the previous parts of this series: arXiv:1706.03795, arXiv:1706.05570, arXiv:1711.05761, arXiv:1711.0796

Interstellar communication. II. Application to the solar gravitational lens

We have shown in paper I of this series (arXiv:1706.03795) that interstellar communication to nearby (pc) stars is possible at data rates of bits per second per Watt between a 1 m sized probe and a large receiving telescope (E-ELT, 39 m), when optimizing all parameters such as frequency at 300-400 nm. We now apply our framework of interstellar extinction and quantum state calculations for photon encoding to the solar gravitational lens (SGL), which enlarges the aperture (and thus the photon flux) of the receiving telescope by a factor of $>10^9$. For the first time, we show that the use of the SGL for communication purposes is possible. This was previously unclear because the Einstein ring is placed inside the solar coronal noise, and contributing factors are difficult to determine. We calculate point-spread functions, aperture sizes, heliocentric distance, and optimum communication frequency. The best wavelength for nearby (<100 pc) interstellar communication is limited by current technology to the UV and optical band. To suppress coronal noise, an advanced coronograph is required, alternatively an occulter could be used which would require a second spacecraft in formation flight 78 km from the receiver, and ~10 m in size. Data rates scale approximately linear with the SGL telescope size and with heliocentric distance. Achievable (receiving) data rates from Alpha Cen are 1-10 Mbits per second per Watt for a pair of meter-sized telescopes, an improvement of $10^6$ compared to using the same receiving telescope without the SGL. A 1 m telescope in the SGL can receive data at rates comparable to a km-class "normal" telescope.Comment: 14 pages, 11 figures. Accepted for publication in Acta Astronautic

On the detection of Exomoons: A search in Kepler data for the orbital sampling effect and the scatter peak

Despite the discovery of thousands of exoplanets, no exomoons have been detected so far. We test a recently developed method for exomoon search, the orbital sampling effect (OSE), using the full exoplanet photometry from the Kepler Space Telescope. The OSE is applied to phase-folded transits, for which we present a framework to detect false positives, and discuss four candidates which pass several of our tests. Using numerical simulations, we inject exomoon signals into real Kepler data and retrieve them, showing that under favorable conditions, exomoons can be found with Kepler and the OSE method. In addition, we super-stack a large sample of Kepler planets to search for the average exomoon OSE and the accompanying increase in noise, the scatter peak. We find a significant OSE-like signal, which might indicate the presence of moons, for planets with 35d<P<80d, having an average dip per planet of 6+-2ppm, corresponding to a moon radius of 2120 +330 -370km for the average star radius of 1.24R_Sun in this sample.Comment: ApJ accepted. 24 pages, 26 Figures. Supplementary material at this http URL http://jaekle.info/ose

Interstellar Communication. VIII. Hard limits on the number of bits per photon

A photon can encode several bits of information based on an alphabet of its time of arrival, energy, and polarization. Heisenberg's uncertainty principle places a limit on measuring pairs of physical properties of a particle, limiting the maximal information efficiency to <59 bits per photon in practice, and <171 bits per photon at Planck energy, at a data rate of one photon per second.Comment: 3 pages, 1 figur

Spaceflight from Super-Earths is difficult

Many rocky exoplanets are heavier and larger than the Earth, and have higher surface gravity. This makes space-flight on these worlds very challenging, because the required fuel mass for a given payload is an exponential function of planetary surface gravity. We find that chemical rockets still allow for escape velocities on Super-Earths up to 10x Earth mass. More massive rocky worlds, if they exist, would require other means to leave the planet, such as nuclear propulsion.Comment: Accepted for publication in the International Journal of Astrobiology. arXiv admin note: substantial text overlap with arXiv:1803.1138

Interstellar communication. VI. Searching X-ray spectra for narrowband communication

We have previously argued that targeted interstellar communication has a physical optimum at narrowband X-ray wavelengths $\lambda\approx1\,$nm, limited by the surface roughness of focusing devices at the atomic level (arXiv:1711.05761). We search 24,247 archival X-ray spectra (of 6,454 unique objects) for such features and present 19 sources with monochromatic signals. Close examination reveals that these are most likely of natural origin. The ratio of artificial to natural sources must be <0.01%. This first limit can be improved in future X-ray surveys.Comment: 3 pages, 2 figures, 1 tabl

A statistical search for a population of Exo-Trojans in the Kepler dataset

Trojans are small bodies in planetary Lagrangian points. In our solar system, Jupiter has the largest number of such companions. Their existence is assumed for exoplanetary systems as well, but none has been found so far. We present an analysis by super-stacking $\sim4\times10^4$ Kepler planets with a total of $\sim9\times10^5$ transits, searching for an average trojan transit dip. Our result gives an upper limit to the average Trojan transiting area (per planet) corresponding to one body of radius $<460$km at $2\sigma$ confidence. We find a significant Trojan-like signal in a sub-sample for planets with more (or larger) Trojans for periods $>$60 days. Our tentative results can and should be checked with improved data from future missions like PLATO2.0, and can guide planetary formation theories.Comment: ApJ accepte

Photometry's bright future: Detecting Solar System analogues with future space telescopes

Time-series transit photometry from the Kepler space telescope has allowed for the discovery of thousands of exoplanets. We explore the potential of yet improved future missions such as PLATO 2.0 in detecting solar system analogues. We use real-world solar data and end-to-end simulations to explore the stellar and instrumental noise properties. By injecting and retrieving planets, rings and moons of our own solar system, we show that the discovery of Venus- and Earth-analogues transiting G-dwarfs like our Sun is feasible at high S/N after collecting 6yrs of data, but Mars and Mercury will be difficult to detect due to stellar noise. In the best cases, Saturn's rings and Jupiter's moons will be detectable even in single transit observations. Through the high number (>1 bn) of observed stars by PLATO 2.0, it will become possible to detect thousands of single-transit events by cold gas giants, analogue to our Jupiter, Saturn, Uranus and Neptune. Our own solar system aside, we also show, through signal injection and retrieval, that PLATO 2.0-class photometry will allow for the secure detection of exomoons transiting quiet M-dwarfs. This is the first study analyzing in-depth the potential of future missions, and the ultimate limits of photometry, using realistic case examples.Comment: ApJ accepte