Model of a bow shock in an ISM with a quadratic density gradient. The L′ image of IRS 1W is represented in color and the best-fit gradient model is overplotted as contours. The colored vectors represent the velocity of the star (V_s; green arrow), the relative motion of the star through the ISM (V_s−ISM; blue arrow) and the dust motion of the ISM obtained with our bow-shock model (V_ISM; red arrow).