Publications by Payel Das

MADE: a spectroscopic mass, age, and distance estimator for red giant stars with Bayesian machine learning

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 484 (2019) 294-304

P Das, JL Sanders

Isochrone ages for ∼3 million stars with the second Gaia data release

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 481 (2018) 4093-4110

JL Sanders, P Das

Cosmic phylogeny: reconstructing the chemical history of the solar neighbourhood with an evolutionary tree

Monthly Notices of the Royal Astronomical Society Oxford University Press 467 (2017) 1140-1153

P Jofré, P Das, J Bertranpetit, R Foley

Using 17 chemical elements as a proxy for stellar DNA, we present a full phylogenetic study of stars in the solar neighbourhood. This entails applying a clustering technique that is widely used in molecular biology to construct an evolutionary tree from which three branches emerge. These are interpreted as stellar populations that separate in age and kinematics and can be thus attributed to the thin disc, the thick disc and an intermediate population of probable distinct origin. We further find six lone stars of intermediate age that could not be assigned to any population with enough statistical significance. Combining the ages of the stars with their position on the tree, we are able to quantify the mean rate of chemical enrichment of each of the populations, and thus show in a purely empirical way that the star formation rate in the thick disc is much higher than that in the thin disc. We are also able to estimate the relative contribution of dynamical processes such as radial migration and disc heating to the distribution of chemical elements in the solar neighbourhood. Our method offers an alternative approach to chemical tagging methods with the advantage of visualizing the behaviour of chemical elements in evolutionary trees. This offers a new way to search for ‘common ancestors’ that can reveal the origin of solar neighbourhood stars.

Designing uncertain optimization schemes for the economic assessment of stock energy-efficiency measures

Journal of Building Performance Simulation Informa UK Limited 10 (2017) 3-16

P Das, L Van Gelder, H Janssen, S Roels

Mapping indoor overheating and air pollution risk modification across Great Britain: A modelling study


J Taylor, M Davies, A Mavrogianni, C Shrubsole, I Hamilton, P Das, B Jones, E Oikonomou, P Biddulph

Impacts of energy efficiency retrofitting measures on indoor PM<inf>2.5</inf> concentrations across different income groups in England: a modelling study

Advances in Building Energy Research 10 (2016) 69-83

C Shrubsole, J Taylor, P Das, IG Hamilton, E Oikonomou, M Davies

© 2015 Taylor & Francis. As part of an effort to reduce carbon emissions in the UK, policies encouraging the energy-efficient retrofit of domestic properties are being implemented. Typical retrofits, including installation of insulation and double glazing can cause tightening of the building envelope which may affect indoor air quality (IAQ) impacting occupant health. Using the example of PM2.5 (an airborne pollutant with known health impacts), this study considers the influence of energy-efficient retrofits on indoor PM2.5 concentrations in domestic properties both above and below the low-income threshold (LIT) for a range of tenancies across England. Simulations using EnergyPlus and its integrated Generic Contaminant model are employed to predict indoor PM2.5 exposures from both indoor and outdoor sources in building archetypes representative of (i) the existing housing stock and (ii) a retrofitted English housing stock. The exposures of occupants for buildings occupied by groups above and below the LIT are then estimated under current conditions and following retrofits. One-way ANOVA tests were applied to clarify results and investigate differences between the various income and tenure groups. Results indicate that all tenures below the LIT experience greater indoor PM2.5 concentrations than those above, suggesting possible social inequalities driven by housing, leading to consequences for health.

Characterizing stellar halo populations II: the age gradient in blue horizontal-branch stars

Monthly Notices of the Royal Astronomical Society Oxford University Press 463 (2016) 3169-3185

P Das, A Williams, J Binney

The distribution of Milky Way halo blue horizontal-branch (BHB) stars is examined using action-based extended distribution functions (EDFs) that describe the locations of stars in phase space, metallicity, and age. The parameters of the EDFs are fitted using stars observed in the Sloan Extension for Galactic Understanding and Exploration-II (SEGUE-II) survey that traces the phase-space kinematics and chemistry out to ∼70 kpc. A maximum a posteriori probability (MAP) estimate method and a Markov Chain Monte Carlo method are applied, taking into account the selection function in positions, distance, and metallicity for the survey. The best-fitting EDF declines with actions less steeply at actions characteristic of the inner halo than at the larger actions characteristic of the outer halo, and older ages are found at smaller actions than at larger actions. In real space, the radial density profile steepens smoothly from −2 at ∼2 kpc to −4 in the outer halo, with an axis ratio ∼0.7 throughout. There is no indication for rotation in the BHBs, although this is highly uncertain. A moderate level of radial anisotropy is detected, with βs varying from isotropic to between ∼0.1 and ∼0.3 in the outer halo depending on latitude. The BHB data are consistent with an age gradient of −0.03 Gyr kpc−1, with some uncertainty in the distribution of the larger ages. These results are consistent with a scenario in which older, larger systems contribute to the inner halo, whilst the outer halo primarily comprises younger, smaller systems.

Characterizing stellar halo populations - I. An extended distribution function for halo K giants


P Das, J Binney

Understanding and mitigating overheating and indoor PM2.5 risks using coupled temperature and indoor air quality models


J Taylor, A Mavrogianni, M Davies, P Das, C Shrubsole, P Biddulph, E Oikonomou

Health effects of home energy efficiency interventions in England: a modelling study.

BMJ open 5 (2015) e007298-

I Hamilton, J Milner, Z Chalabi, P Das, B Jones, C Shrubsole, M Davies, P Wilkinson

OBJECTIVE:To assess potential public health impacts of changes to indoor air quality and temperature due to energy efficiency retrofits in English dwellings to meet 2030 carbon reduction targets. DESIGN:Health impact modelling study. SETTING:England. PARTICIPANTS:English household population. INTERVENTION:Three retrofit scenarios were modelled: (1) fabric and ventilation retrofits installed assuming building regulations are met; (2) as with scenario (1) but with additional ventilation for homes at risk of poor ventilation; (3) as with scenario (1) but with no additional ventilation to illustrate the potential risk of weak regulations and non-compliance. MAIN OUTCOME:Primary outcomes were changes in quality adjusted life years (QALYs) over 50 years from cardiorespiratory diseases, lung cancer, asthma and common mental disorders due to changes in indoor air pollutants, including secondhand tobacco smoke, PM2.5 from indoor and outdoor sources, radon, mould, and indoor winter temperatures. RESULTS:The modelling study estimates showed that scenario (1) resulted in positive effects on net mortality and morbidity of 2241 (95% credible intervals (CI) 2085 to 2397) QALYs per 10,000 persons over 50 years follow-up due to improved temperatures and reduced exposure to indoor pollutants, despite an increase in exposure to outdoor-generated particulate matter with a diameter of 2.5 μm or less (PM₂.₅). Scenario (2) resulted in a negative impact of -728 (95% CI -864 to -592) QALYs per 10,000 persons over 50 years due to an overall increase in indoor pollutant exposures. Scenario (3) resulted in -539 (95% CI -678 to -399) QALYs per 10,000 persons over 50 years follow-up due to an increase in indoor exposures despite the targeting of pollutants. CONCLUSIONS:If properly implemented alongside ventilation, energy efficiency retrofits in housing can improve health by reducing exposure to cold and air pollutants. Maximising the health benefits requires careful understanding of the balance of changes in pollutant exposures, highlighting the importance of ventilation to mitigate the risk of poor indoor air quality.

A tale of two cities: Comparison of impacts on CO2 emissions, the indoor environment and health of home energy efficiency strategies in London and Milton Keynes


C Shrubsole, P Das, J Milner, IG Hamilton, JV Spadaro, E Oikonomou, M Davies, P Wilkinson

Employing a multi-Objective robust optimisation method for healthy and low-energy dwelling design in Delhi, India

14th International Conference of IBPSA - Building Simulation 2015, BS 2015, Conference Proceedings (2015) 2093-2100

E Nix, P Das, J Taylor, M Davies

Dwelling design needs to consider multiple objectives and uncertainties to achieve effective and robust performance. A multi-objective robust optimisation method is outlined and then applied with the aim to optimise a one-story archetype in Delhi to achieve a healthy low-energy design. EnergyPlus is used to model a sample of selected design and uncertainty inputs. Sensitivity analysis identifies significant parameters and a meta-model is constructed to replicate input-output relationships. The meta-model is employed in a hybrid multi-objective optimisation algorithm that accounts for uncertainty. Results demonstrate the complexities of achieving a low energy consumption and healthy indoor environmental quality.

Assessing uncertainty in housing stock infiltration rates and associated heat loss: English and UK case studies


B Jones, P Das, Z Chalabi, M Davies, I Hamilton, R Lowe, A Mavrogianni, D Robinson, J Taylor

Tuberculosis transmission: Modelled impact of air-tightness in dwellings in the UK

Indoor Air 2014 - 13th International Conference on Indoor Air Quality and Climate (2014) 60-67

J Taylor, H Altamirano-Medina, C Shrubsole, P Das, P Biddulph, M Davies, A Mavrogianni, E Oikonomou

High CO2 emissions from the residential sector have forced UK authorities to promote measures to improve energy efficiency through retrofit. Air-tightening can reduce infiltration rates, thereby decreasing ventilation heat losses, but also reducing indoor air quality. This paper presents an initial investigation of the increase in airborne transmission risk of Tuberculosis (TB) due to air-tightening in two of the most commonly-occurring dwelling types in London (purpose-built flat and terraced). EnergyPlus is used to calculate the ventilation rate of the main bedroom over a year for a range of building permeabilities representing the current and air-tightened stock. The Wells-Riley equation is then used to calculate the risk of infection under three different rates of TB generation. Results indicate the potential for increased airborne TB transmission between building occupants following air-tightening, with occupants of flats more susceptible to infection, particularly at high TB generation rates.

The impact of occupancy patterns, occupant-controlled ventilation and shading on indoor overheating risk in domestic environments


A Mavrogianni, M Davies, J Taylor, Z Chalabi, P Biddulph, E Oikonomou, P Das, B Jones

The relative importance of input weather data for indoor overheating risk assessment in dwellings


J Taylor, M Davies, A Mavrogianni, Z Chalabi, P Biddulph, E Oikonomou, P Das, B Jones

Using probabilistic sampling-based sensitivity analyses for indoor air quality modelling


P Das, C Shrubsole, B Jones, I Hamilton, Z Chalabi, M Davies, A Mavrogianni, J Taylor

Home energy efficiency and radon related risk of lung cancer: modelling study.

BMJ (Clinical research ed.) 348 (2014) f7493-

J Milner, C Shrubsole, P Das, B Jones, I Ridley, Z Chalabi, I Hamilton, B Armstrong, M Davies, P Wilkinson

OBJECTIVE: To investigate the effect of reducing home ventilation as part of household energy efficiency measures on deaths from radon related lung cancer. DESIGN: Modelling study. SETTING: England. INTERVENTION: Home energy efficiency interventions, motivated in part by targets for reducing greenhouse gases, which entail reduction in uncontrolled ventilation in keeping with good practice guidance. MAIN OUTCOME MEASURES: Modelled current and future distributions of indoor radon levels for the English housing stock and associated changes in life years due to lung cancer mortality, estimated using life tables. RESULTS: Increasing the air tightness of dwellings (without compensatory purpose-provided ventilation) increased mean indoor radon concentrations by an estimated 56.6%, from 21.2 becquerels per cubic metre (Bq/m(3)) to 33.2 Bq/m(3). After the lag in lung cancer onset, this would result in an additional annual burden of 4700 life years lost and (at peak) 278 deaths. The increases in radon levels for the millions of homes that would contribute most of the additional burden are below the threshold at which radon remediation measures are cost effective. Fitting extraction fans and trickle ventilators to restore ventilation will help offset the additional burden but only if the ventilation related energy efficiency gains are lost. Mechanical ventilation systems with heat recovery may lower radon levels and the risk of cancer while maintaining the advantage of energy efficiency for the most airtight dwellings but there is potential for a major adverse impact on health if such systems fail. CONCLUSION: Unless specific remediation is used, reducing the ventilation of dwellings will improve energy efficiency only at the expense of population wide adverse impact on indoor exposure to radon and risk of lung cancer. The implications of this and other consequences of changes to ventilation need to be carefully evaluated to ensure that the desirable health and environmental benefits of home energy efficiency are not compromised by avoidable negative impacts on indoor air quality.

Comparative study of metamodelling techniques in building energy simulation: Guidelines for practitioners

Simulation Modelling Practice and Theory Elsevier BV 49 (2014) 245-257

L Van Gelder, P Das, H Janssen, S Roels

The modifying effect of the building envelope on population exposure to PM2.5 from outdoor sources.

Indoor air 24 (2014) 639-651

J Taylor, C Shrubsole, M Davies, P Biddulph, P Das, I Hamilton, S Vardoulakis, A Mavrogianni, B Jones, E Oikonomou

A number of studies have estimated population exposure to PM2.5 by examining modeled or measured outdoor PM2.5 levels. However, few have taken into account the mediating effects of building characteristics on the ingress of PM2.5 from outdoor sources and its impact on population exposure in the indoor domestic environment. This study describes how building simulation can be used to determine the indoor concentration of outdoor-sourced pollution for different housing typologies and how the results can be mapped using building stock models and Geographical Information Systems software to demonstrate the modifying effect of dwellings on occupant exposure to PM2.5 across London. Building archetypes broadly representative of those in the Greater London Authority were simulated for pollution infiltration using EnergyPlus. In addition, the influence of occupant behavior on indoor levels of PM2.5 from outdoor sources was examined using a temperature-dependent window-opening scenario. Results demonstrate a range of I/O ratios of PM2.5 , with detached and semi-detached dwellings most vulnerable to high levels of infiltration. When the results are mapped, central London shows lower I/O ratios of PM2.5 compared with outer London, an apparent inversion of exposure most likely caused by the prevalence of flats rather than detached or semi-detached properties.Population exposure to air pollution is typically evaluated using the outdoor concentration of pollutants and does not account for the fact that people in London spend over 80% of their time indoors. In this article, building simulation is used to model the infiltration of outdoor PM2.5 into the domestic indoor environment for dwellings in a London building stock model, and the results mapped. The results show the variation in relative vulnerability of dwellings to pollution infiltration, as well as an estimated absolute indoor concentration across the Greater London Authority (GLA) scaled by local outdoor levels. The practical application of this work is a better understanding of the modifying effect of the building geometry and envelope design on pollution exposure, and how the London building stock may alter exposure. The results will be used to inform population exposure to PM2.5 in future environmental epidemiological studies.