The space density of late M dwarfs, subtypes M7 to M9.5, is not well determined. We applied the photo-type method to iz photometry from the Sloan Digital Sky Survey and YJHK photometry from UKIRT Infrared Deep Sky Survey, over an effective area of 3070 deg 2 , to produce a new, bright J(Vega) < 17.5, homogeneous sample of 33 665 M7 to M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifications are as precise as good spectroscopic classifications. Sources with large χ 2 > 20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias affects only ∼ 1% of the sources. Therefore the sample is well suited to measure the luminosity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.An independent estimate of the M dwarf LF for spectral types M7 to M9 was made by Cruz et al. (2007), using a sample of 53 stars within 20pc of the Sun. The objects were identified using 2MASS JHK s photometry by application of a colour cut J − K s > 1. 0. Cruz et al. (2007) estimated that 79% of M7 dwarfs are redder than this colour limit and all M8 and M9 dwarfs. Unfortunately this sample is also problematic. Recent analysis of the 2MASS colours of M dwarfs by Schmidt et al. (2015) provided median colours J − K s = 0.96, 1.03 for M7, M8. These results suggest that only ∼ 50% of M7 and M8 dwarfs satisfy the above colour cut, meaning their space densities have been substantially underestimated.These questions motivate the compilation of a new sample of late M dwarfs, hereafter M7−M9.5, in order to obtain an improved measurement of the LF. We now briefly consider the issues involved in making an accurate measurement of the LF. Various studies of the LF may be distinguished by whether distances are measured by trigonometric, spectroscopic (absolute magni-Article number, page 1 of 8

The European Space Agency's Gaia spacecraft was launched in December 2013 and has been in orbit at the Earth-Sun Lagrange point 2 (L2) for over 6 years. The spacecraft measures the positions, distances, space motions and many other physical characteristics of around one billion stars in the Milky Way and beyond. It has a focal plane of 106 Charge-Coupled Devices (CCDs) which have all been performing well but have been measuring a small but quantifiable degradation in performance in time due to Non-Ionizing Energy Loss (NIEL) damage from interstellar radiation. This NIEL damage produces trap defects which can capture charge from signals and reduces the quality of the data. Gaia's original mission lifetime was planned to be around 5 years and the pre-flight testing and radiation damage analysis was tailored around those timescales as well as with the projected solar activity before launch. Closer to the time of launch and during Gaia's years of orbit, it has been noted that the solar activity was lower than what was initially predicted. From the previous analysis of in-flight data in 2016, it was calculated that Gaia was experiencing an order of magnitude less radiation damage than was predicted. This paper describes the analysis of charge calibration data and corresponding Charge Transfer Inefficiency (CTI) measurements from the in-flight CCDs, both near the beginning of the mission and after more than 5 years in orbit to quantify the radiation damage impact. These sets of results can be compared with those from the pre-flight tests which can be used to evaluate and understand the differences between the on-ground and in-flight results.

The European Space Agency's Gaia spacecraft has been operating in L2 ever since its launch in December 2013 with a payload that includes 106 scientific charge-coupled devices (CCDs). Due to the predicted radiation environment at the pre-flight testing stage in addition to the high level of accuracy demanded by the science objectives, the non-ionizing energy loss (NIEL) damage on the detectors was identified as a major factor that could affect the science goals of the mission. Here, we present the analysis of an extended set of charge calibration data, taken up to almost six years after launch. It is found that the rate of radiation damage accumulation by the CCDs has not differed significantly from previous results. While the parallel and serial CTI measure an increase in time, the trap defect landscape is still dominated by the preflight defects rather than the radiation-induced traps. CCD devices that were predicted to have a lower NIEL dose measure comparatively larger rates of CTI increase. In addition to this, thicker devices have been measured to have lower serial CTI values compared to thinner devices. The initial parallel CTI values have also been found to be dependent on manufacture year.

We investigate the form of the local vertical density profile of the stars in the Galactic disk, close to the Galactic plane. We use a homogeneous sample of 34 000 ultracool dwarfs M7 to L2.5 that all lie within 350 pc of the plane. We fit a profile of the form sech α , where α = 2 is the theoretically preferred isothermal profile and α = 0 is the exponential function. Larger values of α correspond to greater flattening of the profile towards the plane. We employ a likelihood analysis that accounts in a direct way for unresolved binaries in the sample, as well as for the spread in absolute magnitude M J within each spectral sub-type (Malmquist bias). We measure α = 0.29 +0.12 −0.13 . The α = 1 (sech) and flatter profiles are ruled out at high confidence for this sample, while α = 0 (exponential) is included in the 95% credible interval. Any flattening relative to exponential is modest, and is confined to within 50 pc of the plane. The measured value of α is consistent with the results of the recent analysis by Xiang et al. Our value for α is also similar to that determined for nearby spiral galaxies by de Grijs et al., measured from photometry of galaxies viewed edge on. The measured profile allows an accurate determination of the local space density of ultracool dwarfs M7 to L2.5, and we use this to make a new determination of the luminosity function at the bottom of the main sequence. Our results for the luminosity function are a factor two to three lower than the recent measurement by Bardalez Gagliuffi et al., that uses stars in the local 25 pc radius bubble, but agree well with the older study by Cruz et al.

The European Space Agency’s Gaia spacecraft was launched in 2013 and has been in operation ever since. It has a focal plane of 106 Charge-Coupled Devices (CCDs) which are of the CCD91-72 variant, custom designed by Teledyne e2v. The detectors have been making measurements of parallaxes, positions, velocities, and other physical properties of over one billion stars and other astronomical objects in the Milky Way. Whilst operating in space, CCDs undergo non-ionizing displacement damage from incoming radiation. This causes radiation induced trap defects to form in the silicon lattice which can trap electrons during readout and increase the charge transfer inefficiency (CTI) of the devices significantly. From analysis of in-flight charge calibration data, Gaia’s CTI values have been measured to be lower than what was expected based on the on-ground pre-flight tests. In this study, the CTI and trap landscape in both in-flight and irradiated on-ground devices are modelled to fit the new datasets. This was done thanks to the help of a detector simulation toolkit called Pyxel which implemented a version of a CTI model developed for Gaia called the Charge Distortion Model (CDM). These results provide more insights into the nature of radiation damage and the resulting trap landscapes, both in space and from on-ground irradiations.