Critical comments on publications by S. Hoffmann and N. Vogt on historical novae/supernovae and their candidates

We critically discuss recent articles by S. Hoffmann and N. Vogt on historical novae and supernovae (SNe) as well as their list of “24 most promising events” “with rather high probability to be a nova” (Hoffmann et al., AN , 2020, 341, 79 (P3)). Their alleged positional accuracy of previously suggested historical nova/SN records is based on inhomogeneous datasets (Vogt et al.), but then used for the nova search in Hoffmann et al., AN , 2020, 341, 79 (P3). Their claim that previously only “point coordinates” for nova/SN candidates were published, is fabricated. Their estimate of expected nova detection rates is off by a factor of 10 due to mis‐calculation. They accept counterparts down to 4–7 mag at peak, which is against the consensus for the typical limit of naked‐eye discovery. When they discuss previously suggested identifications of historical novae, which they all doubt, they do not present new facts (Hoffmann, MNRAS , 2019, 490, 4194 (P2)). Their catalog of “24 most promising events” for novae (Hoffmann et al., AN , 2020, 341, 79 (P3)) neglects important recent literature (e.g. Pankenier et al., Archeoastronomy in East Asia , New York, Cambria, 2008 and Stephenson and Green, JHA , 2009, 40, 31), the claimed methods are not followed, etc. At least half of their short‐list candidates were and are to be considered comets. For many of the others, duration of more than one night and/or a precise position is missing and/or the sources were treated mistakenly. Two “highlights,” a fabricated SN AD 667–8 and a presumable recurrent nova in AD 891, are already rejected in detail in Neuhäuser et al., MNRAS, 2021a, 501, L1—in both cases, all evidence speaks in favor of comets. There remains only one reliable case, where close to one (possible) historically reported position, a nova shell was already found (AD 1437, Shara et al., Nature , 2017b, 548, 558). Since the proposed positional search areas are not justified due to unfounded textual interpretations (e.g. in fact comets), misunderstandings of historical Chinese astronomy (e.g. incorrect asterism), follow‐up observations cannot be recommended

Search for extrasolar planets with high-precision relative astrometry

We present our search program for substellar companions using high-precision relative astronomy. Due to its orbital motion around the star, an unseen substellar companion would produce a periodic "wobble" of the host star, which is the astrometric signal of the unseen companion. By measuring the separation between the components of stellar double and triple systems, we want to measure this astrometric signal of a possible unseen companion indirectly as a relative and periodic change of these separations. Using a new observation mode (the "cube-mode") where the frames were directly saved in cubes with nearly no loss of time during the readout, an adaptive optics system to correct for atmospheric noise and an infrared narrow band filter in the near infrared to suppress differential chromatic refraction (DCR) effects we achive for our first target (the double star HD 19994) a relative precision for the separation measurements of about 100...150 micro-arsecond per epoch. To reach a precision in the micro-arcsec-regime, we use a statistical approach. We take several thousand frames per target and epoche and after a verification of a Gaussian distribution the measurement precision can be calculated as the standard deviation of our measurements divided by the square root of the number of Gaussian distributed measurements. Our first observed target is the stellar binary HD 19994 A & B, where the A component has a known radial velocity planet candidate.Comment: 4 pages, IAUS 249 conferenc

Direct Imaging of Extra-Solar Planets – Homogeneous Comparison of Detected Planets and Candidates

Searching the literature, we found 25 stars with directly imaged planets and candidates. We gathered photometric and spectral information for all these objects to derive their luminosities in a homogeneous way, taking a bolometric correction into account. Using theoretical evolutionary models, one can then estimate the mass from luminosity, temperature, and age. According to our mass estimates, all of them can have a mass below 25 Jup masses, so that they are considered as planets.Comment: Invited review, accepted by InTech for the Book "Topics in Adaptive Optics", 28 pages, 5 figures, in pres

Multiplicity Study of Exoplanet Host Stars

We present recent results of our ongoing multiplicity study of exoplanet host stars.Comment: 3 pages, 3 figure

Finding new sub-stellar co-moving companion candidates - the case of CT Cha

We have searched for close and faint companions around T Tauri stars in the Chamaeleon star forming region. Two epochs of direct imaging data were taken with the VLT Adaptive Optics instrument NaCo in February 2006 and March 2007 in Ks band for the classical T Tauri star CT Cha together with a Hipparcos binary for astrometric calibration. Moreover a J band image was taken in March 2007 to get color information. We found CT Cha to have a very faint companion (Ks{0} = 14.6 mag) of ~ 2.67" separation corresponding to ~ 440 AU. We show that CT Cha A and the faint object form a common proper motion pair and that the companion is not a non-moving background object (with 4 sigma significance).Comment: 5 pages, 2 figures, Proceedings of the IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, Suzhou - China, 22-26 October 200