χ² contour plot of the density versus temperature obtained by the non-LTE LVG analysis of the two detected HOCO⁺ lines for two different source sizes and HOCO⁺ column densities: 30′′ and 6 × 10¹¹ cm^-2 (dashed line), 100′′ and 2.5 × 10¹¹ cm^-2 (solid line). The thick grey line shows the density-temperature structure of L1544 as derived by Caselli et al. (2012). The ticks along this line show the distance from the L1544 core center, in angular diameter.